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Sample records for josephson charge qubits

  1. Implementing Shor's algorithm on Josephson charge qubits

    SciTech Connect

    Vartiainen, Juha J.; Salomaa, Martti M.; Niskanen, Antti O.; Nakahara, Mikio

    2004-07-01

    We investigate the physical implementation of Shor's factorization algorithm on a Josephson charge qubit register. While we pursue a universal method to factor a composite integer of any size, the scheme is demonstrated for the number 21. We consider both the physical and algorithmic requirements for an optimal implementation when only a small number of qubits are available. These aspects of quantum computation are usually the topics of separate research communities; we present a unifying discussion of both of these fundamental features bridging Shor's algorithm to its physical realization using Josephson junction qubits. In order to meet the stringent requirements set by a short decoherence time, we accelerate the algorithm by decomposing the quantum circuit into tailored two- and three-qubit gates and we find their physical realizations through numerical optimization.

  2. Decoherence of Josephson charge qubit in non-Markovian environment

    NASA Astrophysics Data System (ADS)

    Qiu, Qing-Qian; Zhou, Xing-Fei; Liang, Xian-Ting

    2016-05-01

    In this paper we investigate the decoherence of Josephson charge qubit (JCQ) by using a time-nonlocal (TNL) dynamical method. Three kinds of environmental models, described with Ohmic, super-Ohmic, and sub-Ohmic spectral density functions are considered. It is shown that the TNL method can effectively include the non-Markovian effects in the dynamical solutions. In particular, it is shown that the sub-Ohmic environment has longer correlation time than the Ohmic and super-Ohmic ones. And the Markovian and non-Markovian dynamics are obviously different for the qubit in sub-Ohmic environment.

  3. Controllable Quantum State Transfer Between a Josephson Charge Qubit and an Electronic Spin Ensemble

    NASA Astrophysics Data System (ADS)

    Yan, Run-Ying; Wang, Hong-Ling; Feng, Zhi-Bo

    2016-01-01

    We propose a theoretical scheme to implement controllable quantum state transfer between a superconducting charge qubit and an electronic spin ensemble of nitrogen-vacancy centers. By an electro-mechanical resonator acting as a quantum data bus, an effective interaction between the charge qubit and the spin ensemble can be achieved in the dispersive regime, by which state transfers are switchable due to the adjustable electrical coupling. With the accessible experimental parameters, we further numerically analyze the feasibility and robustness. The present scheme could provide a potential approach for transferring quantum states controllably with the hybrid system.

  4. Quantum metamaterials: Electromagnetic waves in a Josephson qubit line

    NASA Astrophysics Data System (ADS)

    Rakhmanov, A. L.; Zagoskin, A. M.; Savel'Ev, Sergey; Nori, Franco

    2008-04-01

    We consider the propagation of a classical electromagnetic wave through a transmission line, formed by identical superconducting charge qubits inside a superconducting resonator. Since the qubits can be in a coherent superposition of quantum states, we show that such a system demonstrates interesting effects, such as a “breathing” photonic crystal with an oscillating band gap and a “quantum Archimedean screw” that transports, at an arbitrary controlled velocity, Josephson plasma waves through a transmission line. The key ingredient of these effects is that the optical properties of the Josephson transmission line are controlled by the quantum coherent state of the qubits.

  5. Qubit readout with the Josephson Photomultiplier

    NASA Astrophysics Data System (ADS)

    Ribeill, Guilhem

    Recent demonstrations of error correction in many qubit circuits, as well as efforts to build a logical qubit, have shown the need for a simple and scalable superconducting quantum bit (qubit) readout. Current solutions based on heterodyne detection and cryogenic amplification of microwave readout tones may prove difficult to scale, while photon counting presents an attractive alternative. However, the development of counters operating at these frequencies has proved technically challenging. In this thesis, we describe the development of the Josephson Photomultiplier (JPM), a microwave photon counting circuit. We discuss the JPM theoretically, and describe the fabrication of the JPM using standard thin film lithography techniques. We measure its properties as a microwave photon counter using a qubit as an in-situ calibrated source of photons. We measure a JPM quantum efficiency at the few percent level. We then use the JPM to perform readout of a transmon qubit in both the dispersive and bright regimes. We observe raw measurement fidelities of 35% and 62% respectively. We discuss how the JPM and measurement protocol could be further optimized to achieve fidelities in excess of 90%.

  6. Superconducting qubits with semiconductor nanowire Josephson junctions

    NASA Astrophysics Data System (ADS)

    Petersson, K. D.; Larsen, T. W.; Kuemmeth, F.; Jespersen, T. S.; Krogstrup, P.; Nygård, J.; Marcus, C. M.

    2015-03-01

    Superconducting transmon qubits are a promising basis for a scalable quantum information processor. The recent development of semiconducting InAs nanowires with in situ molecular beam epitaxy-grown Al contacts presents new possibilities for building hybrid superconductor/semiconductor devices using precise bottom up fabrication techniques. Here, we take advantage of these high quality materials to develop superconducting qubits with superconductor-normal-superconductor Josephson junctions (JJs) where the normal element is an InAs semiconductor nanowire. We have fabricated transmon qubits in which the conventional Al-Al2O3-Al JJs are replaced by a single gate-tunable nanowire JJ. Using spectroscopy to probe the qubit we observe fluctuations in its level splitting with gate voltage that are consistent with universal conductance fluctuations in the nanowire's normal state conductance. Our gate-tunable nanowire transmons may enable new means of control for large scale qubit architectures and hybrid topological quantum computing schemes. Research supported by Microsoft Station Q, Danish National Research Foundation, Villum Foundation, Lundbeck Foundation and the European Commission.

  7. Isolation of a Josephson qubit from the electromagnetic environment

    NASA Astrophysics Data System (ADS)

    Shnyrkov, V. I.; Korolev, A. M.; Turutanov, O. G.; Shulga, V. M.; Lyakhno, V. Yu.; Serebrovsky, V. V.

    2015-11-01

    We consider two aspects of isolation of a Josephson flux (charge-flux) qubit from the external dissipative electromagnetic environment: (i) selecting an optimal topology of the superconducting qubit circuit and (ii) passive filtering of Planck radiation at the input of the qubit-state detection circuit. When reading the state of a macroscopic quantum object ("Schrödinger's cat") with the weak continuous measurement technique, the coupling to the environment, both direct and through the connected circuits, is the cause of the rapid loss of coherence of the superposition states. The coefficients of coupling to the external electromagnetic environment are discussed, as well as the problem of their minimization for flat (2D) and bulk (3D) designs of the qubit quantization loops. The analysis of the characteristics of low-temperature combined broadband filters designed to effectively reduce the electromagnetic noise in the control and measurement circuits is carried out. It is shown experimentally that a cryogenic Cu-CuO powder filter installed directly into the measuring circuit results in a significant suppression of the back action of a cooled HEMT amplifier to the qubit.

  8. Josephson Phase Qubit with a Distributed Reactance: Theory

    NASA Astrophysics Data System (ADS)

    Ferguson, David; Przybysz, Anthony; Naaman, Ofer; Strand, Joel; Medford, James; Pesetski, Aaron

    2014-03-01

    Recently, the Northrop Grumman's superconducting systems team designed, fabricated, and measured a novel phase qubit in which the shunt capacitance across the Josephson junction and the inductance of the SQUID are provided by a microstrip resonator. To account theoretically for this novel design we treat the superconducting phase drop along the microstrip's length as a continuous field. We present an analysis of this model, describing how the normal modes of the coupled system are influenced by both the applied flux and the microstrip's termination impedance, and how the non-linear coupling of the fundamental ``qubit mode'' to higher modes generates significant renormalizations of mode frequencies and anharmonicities.

  9. Charge-qubit-atom hybrid

    NASA Astrophysics Data System (ADS)

    Yu, Deshui; Valado, María Martínez; Hufnagel, Christoph; Kwek, Leong Chuan; Amico, Luigi; Dumke, Rainer

    2016-04-01

    We investigate a hybrid system of a superconducting charge qubit interacting directly with a single neutral atom via electric dipole coupling. Interfacing of the macroscopic superconducting circuit with the microscopic atomic system is accomplished by varying the gate capacitance of the charge qubit. To achieve a strong interaction, we employ two Rydberg states with an electric-dipole-allowed transition, which alters the polarizability of the dielectric medium of the gate capacitor. Sweeping the gate voltage with different rates leads to a precise control of hybrid quantum states. Furthermore, we show a possible implementation of a universal two-qubit gate.

  10. Multi-qubit measurements with a Josephson Photomultiplier

    NASA Astrophysics Data System (ADS)

    Howington, Caleb; Hutchings, M.; Ribeill, Guilhem; Pechenezhskiy, Ivan; Vavilov, Maxim G.; Wilhelm, Frank K.; McDermott, R.; Plourde, Blt

    The ability to measure multi-qubit parity is critical for the realization of a fault-tolerant quantum information processor. For a system of transmon qubits coupled to a superconducting cavity, a threshold photon detector can provide an efficient path towards the digital readout of qubit parity after the parity information is mapped onto the cavity photon occupation. We will describe progress towards the implementation of such a scheme for measuring the parity of two transmon qubits. On-chip flux bias lines allow us to tune the dispersive cavity shifts related to the state of the two qubits and an appropriately shaped pulse driven to the cavity results in a bright state for one parity but not the other. A Josephson Photomultiplier then serves as a phase-insensitive digital detector of the microwave photons that leak out of the cavity. Future improvements and various technical difficulties will be discussed. We acknowledge support from ARO under Contract W911NF-14-1-0080.

  11. Josephson traveling-wave parametric amplifier for superconducting qubit readout

    NASA Astrophysics Data System (ADS)

    Macklin, Chris; Slichter, D. H.; Yaakobi, O.; Friedland, L.; Bolkhovsky, V.; Braje, D. A.; Fitch, G.; Oliver, W. D.; Siddiqi, I.

    2014-03-01

    Superconducting parametric amplifiers (paramps) have successfully demonstrated near quantum limited sensitivity, enabling single-shot qubit readout, feedback, and state tracking. However, these amplifiers are commonly limited to narrow bandwidth and modest dynamic range, and most require microwave circulators to separate input and output modes. These limitations stem from the use of a resonant non-linearity to achieve mixing between a signal and pump mode. Our traveling-wave parametric amplifier (TWPA) is based on a superconducting nonlinear Josephson junction transmission line, thereby inherently sidestepping the limitations associated with a cavity structure. We present theoretical predictions and experimental results, including improved gain and noise performance. We discuss transmon qubit readout in the circuit QED architecture using a TWPA. We also comment on promising architectures for chip-level integration and multiplexing. Work supported by IARPA.

  12. Multiple Resonators as a Multi-Channel Bus for Coupling Josephson Junction Qubits

    NASA Astrophysics Data System (ADS)

    Thrailkill, Zechariah; Lambert, Joseph; Ramos, Roberto

    2010-03-01

    Josephson junction-based qubits have been shown to be promising components for a future quantum computer. A network of these superconducting qubits will require quantum information to be stored in and transferred among them. Resonators made of superconducting metal strips are useful elements for this purpose because they have long coherence times and can dispersively couple qubits. We explore the use of multiple resonators with different resonant frequencies to couple qubits. We find that an array of resonators with different frequencies can be individually addressed to store and retrieve information, while coupling qubits dispersively. We show that a control qubit can be used to effectively isolate an active qubit from an array of resonators so that it can function within the same frequency range used by the resonators.

  13. Measurements of the 1/f Noise in Josephson Junctions for Potential Use as QUbits

    NASA Astrophysics Data System (ADS)

    Mugford, Chas; Kycia, Jan; Korn, Matthias; Mueck, Michael; Clarke, John

    2004-03-01

    Critical current fluctuations can be a major source of intrinsic decoherence of qubits based on Josephson junctions. We have measured the 1/f noise due to critical current fluctuations in macroscopic ( area ≈ 2 × 2 μ m^2 ) Josephson junctions. We directly measure changes δ Ic in the critical current Ic of a voltage biased junction and find the magnitude of the critical current fluctuations to be δ I_c/Ic ≈ 10-5 at a frequency of 1 Hz.^ A second way in which we determine 1/f flux noise due to critical current fluctuations is by measuring the flux noise of either dc or rf SQUIDs. In order to not exceed the critical current of the Josephson junction, we operate the rf SQUID in the dispersive mode. By using the same device as dc or rf SQUID, we can compare the 1/f noise of voltage biased and non-voltage biased Josephson junctions.

  14. Temperature dependence of coherent oscillations in Josephson phase qubits.

    PubMed

    Lisenfeld, J; Lukashenko, A; Ansmann, M; Martinis, J M; Ustinov, A V

    2007-10-26

    We experimentally investigate the temperature dependence of Rabi oscillations and Ramsey fringes in superconducting phase qubits. In a wide range of temperatures, we find that both the decay time and the amplitude of these coherent oscillations remain nearly unaffected by thermal fluctuations. In the two-level limit, coherent qubit response rapidly vanishes as soon as the energy of thermal fluctuations k(B)T becomes larger than the energy level spacing variant Planck's over h omega of the qubit. In contrast, a sample of much shorter coherence times displayed semiclassical oscillations very similar to Rabi oscillation, but showing a qualitatively different temperature dependence. Our observations shed new light on the origin of decoherence in superconducting qubits. The experimental data suggest that, without degrading already achieved coherence times, phase qubits can be operated at temperatures much higher than those reported till now. PMID:17995313

  15. Charge state manipulation of qubits in diamond

    PubMed Central

    Grotz, Bernhard; Hauf, Moritz V.; Dankerl, Markus; Naydenov, Boris; Pezzagna, Sébastien; Meijer, Jan; Jelezko, Fedor; Wrachtrup, Jörg; Stutzmann, Martin; Reinhard, Friedemann; Garrido, Jose A.

    2012-01-01

    The nitrogen-vacancy (NV) centre in diamond is a promising candidate for a solid-state qubit. However, its charge state is known to be unstable, discharging from the qubit state NV− into the neutral state NV0 under various circumstances. Here we demonstrate that the charge state can be controlled by an electrolytic gate electrode. This way, single centres can be switched from an unknown non-fluorescent state into the neutral charge state NV0, and the population of an ensemble of centres can be shifted from NV0 to NV−. Numerical simulations confirm the manipulation of the charge state to be induced by the gate-controlled shift of the Fermi level at the diamond surface. This result opens the way to a dynamic control of transitions between charge states and to explore hitherto inaccessible states, such as NV+. PMID:22395620

  16. Coherent control of a transmon qubit with a nanowire-based Josephson junction

    NASA Astrophysics Data System (ADS)

    Larsen, T. W.; Petersson, K. D.; Kuemmeth, F.; Jespersen, T. S.; Krogstrup, P.; Nygård, J.; Marcus, C. M.

    2015-03-01

    Transmon qubits have taken great leaps towards realizing a quantum processor. Here we present measurements on a novel, gateable transmon. By tuning the electron density in a semiconducting nanowire Josephson junction we can control the qubit frequency from ~3 GHz to ~8 GHz. The transmon was embedded into an aluminum coplanar waveguide cavity for readout and qubit control. In the resonant regime we observe strong cavity-qubit coupling. In the dispersive regime we demonstrate coherent control on the Bloch sphere. The life- and coherence times were measured to T2* ~ 2T1 ~ 1 μ s. The coherence time was measured to almost 1 μs. Fast gate operations facilitate z-rotations as well as promising fast two-qubit operations in future multiple-qubit devices. These measurements open new possibilities for gateable superconducting qubits and promise a plausible system for Majorana hybrid devices. Research supported by Microsoft Station Q, Danish National Research Foundation, Villum Foundation, Lundbeck Foundation, and the European Commission.

  17. Sensitivity and Back-action of Josephson-Junction Comparators as Fast Qubit Readout Devices

    NASA Astrophysics Data System (ADS)

    Walls, Thomas; Averin, Dmitri; Likharev, Konstantin

    2004-03-01

    Experimental [1] and theoretical [2] studies have shown that balanced Josephson-junction comparators may have quantum-limited sensitivity for sampling dc current from a high-impedance source. In this work, we have calculated sensitivity of this device for measurement of signal from a finite-inductance source, e.g., a flux qubit. In contrast with our previous work [2] based on the Feynman-Vernon-Caldeira-Legget formalism, we show that the calculations may be simplified considerably using the equivalent Langevin-Heisenberg-Lax approach. We have calculated two figures of merit of the device: flux sensitivity ɛ_Φ and the product of the variance of the flux fluctuations with the variance of the charge fluctuations characterizing comparator back-action on the measured system. Calculations show that the former measure (important for a single-shot projective measurements) may be much lower than a value following from a naive quantum noise estimates. However, the product of fluctuation variances is larger than hbar ^2/4, showing that the back-action of the comparator is larger than that of the fully quantum-limited detectors. 1. Semenov, V.K. et al, IEEE Trans. Appl. Supercond., 7, 3617 (1997). 2. Walls, T.J., et al, Phys. Rev. Lett., 89, 217004 (2002).

  18. Phase-matched Josephson traveling-wave parametric amplifier for superconducting qubit readout - theory

    NASA Astrophysics Data System (ADS)

    O'Brien, Kevin; Macklin, Chris; Siddiqi, Irfan; Zhang, Xiang

    2015-03-01

    Josephson parametric amplifiers approach quantum-noise-limited performance and are used in experiments requiring high-fidelity detection of single-photon-level microwave signals. Current Josephson parametric amplifiers couple the Josephson junction (a nonlinear inductor) to a resonant cavity, achieving high gain at the expense of limited instantaneous bandwidth. In contrast, Josephson traveling wave parametric amplifiers (JTWPAs) avoid this gain-bandwidth trade-off by employing long propagation lengths rather than a resonant cavity. A major challenge in JTWPA design is that optimum parametric gain is only achieved when the four-wave mixing process is phase matched. We show that by adding a series of resonant elements to the transmission line, phase matching and exponential gain can be achieved. Generation of higher harmonics is automatically suppressed due to the junction plasma resonance. We present the theory and selected results, including the gain, bandwidth, and dynamic range of the amplifier. The simultaneous achievement of high gain (greater than 20 dB), large instantaneous bandwidth (greater than 2 GHz), and high dynamic range make the JTWPA a promising device for the simultaneous readout of frequency-multiplexed superconducting qubits.

  19. Quantum state engineering with flux-biased Josephson phase qubits by rapid adiabatic passages

    SciTech Connect

    Nie, W.; Huang, J. S.; Shi, X.; Wei, L. F.

    2010-09-15

    In this article, the scheme of quantum computing based on the Stark-chirped rapid adiabatic passage (SCRAP) technique [L. F. Wei, J. R. Johansson, L. X. Cen, S. Ashhab, and F. Nori, Phys. Rev. Lett. 100, 113601 (2008)] is extensively applied to implement quantum state manipulations in flux-biased Josephson phase qubits. The broken-parity symmetries of bound states in flux-biased Josephson junctions are utilized to conveniently generate the desirable Stark shifts. Then, assisted by various transition pulses, universal quantum logic gates as well as arbitrary quantum state preparations can be implemented. Compared with the usual {pi}-pulse operations widely used in experiments, the adiabatic population passages proposed here are insensitive to the details of the applied pulses and thus the desirable population transfers can be satisfyingly implemented. The experimental feasibility of the proposal is also discussed.

  20. Single-shot read-out of a superconducting qubit using a Josephson parametric oscillator

    NASA Astrophysics Data System (ADS)

    Krantz, Philip; Bengtsson, Andreas; Simoen, Michaël; Gustavsson, Simon; Shumeiko, Vitaly; Oliver, W. D.; Wilson, C. M.; Delsing, Per; Bylander, Jonas

    2016-05-01

    We propose and demonstrate a read-out technique for a superconducting qubit by dispersively coupling it with a Josephson parametric oscillator. We employ a tunable quarter wavelength superconducting resonator and modulate its resonant frequency at twice its value with an amplitude surpassing the threshold for parametric instability. We map the qubit states onto two distinct states of classical parametric oscillation: one oscillating state, with 185+/-15 photons in the resonator, and one with zero oscillation amplitude. This high contrast obviates a following quantum-limited amplifier. We demonstrate proof-of-principle, single-shot read-out performance, and present an error budget indicating that this method can surpass the fidelity threshold required for quantum computing.

  1. Single-shot Readout of a Superconducting Qubit using a Josephson Parametric Oscillator

    NASA Astrophysics Data System (ADS)

    Krantz, Philip; Bengtsson, Andreas; Simoen, Michael; Gustavsson, Simon; Shumeiko, Vitaly; Oliver, W. D.; Wilson, C. M.; Delsing, Per; Bylander, Jonas

    We propose and demonstrate a new read-out technique for a superconducting qubit by dispersively coupling it to a Josephson parametric oscillator. We employ a tunable quarter-wavelength superconducting resonator and modulate its resonant frequency at twice its value with an amplitude surpassing the threshold for parametric instability. We map the qubit states onto two distinct states of classical parametric oscillation: one oscillating state, with 185 +/- ∑ 15 photons in the resonator, and one with zero oscillation amplitude. This high contrast obviates a following quantum-limited amplifier. We demonstrate proof-of-principle, single-shot readout performance, and present an error budget indicating that this method can surpass the fidelity threshold required for quantum computing. Support came from the Wallenberg foundation, the European Research Council (ERC), the Royal Swedish Academy of Science (KVA), the European project ScaleQIT, STINT, and Marie Curie CIG.

  2. Single-shot read-out of a superconducting qubit using a Josephson parametric oscillator.

    PubMed

    Krantz, Philip; Bengtsson, Andreas; Simoen, Michaël; Gustavsson, Simon; Shumeiko, Vitaly; Oliver, W D; Wilson, C M; Delsing, Per; Bylander, Jonas

    2016-01-01

    We propose and demonstrate a read-out technique for a superconducting qubit by dispersively coupling it with a Josephson parametric oscillator. We employ a tunable quarter wavelength superconducting resonator and modulate its resonant frequency at twice its value with an amplitude surpassing the threshold for parametric instability. We map the qubit states onto two distinct states of classical parametric oscillation: one oscillating state, with 185±15 photons in the resonator, and one with zero oscillation amplitude. This high contrast obviates a following quantum-limited amplifier. We demonstrate proof-of-principle, single-shot read-out performance, and present an error budget indicating that this method can surpass the fidelity threshold required for quantum computing. PMID:27156732

  3. Single-shot read-out of a superconducting qubit using a Josephson parametric oscillator

    PubMed Central

    Krantz, Philip; Bengtsson, Andreas; Simoen, Michaël; Gustavsson, Simon; Shumeiko, Vitaly; Oliver, W. D.; Wilson, C. M.; Delsing, Per; Bylander, Jonas

    2016-01-01

    We propose and demonstrate a read-out technique for a superconducting qubit by dispersively coupling it with a Josephson parametric oscillator. We employ a tunable quarter wavelength superconducting resonator and modulate its resonant frequency at twice its value with an amplitude surpassing the threshold for parametric instability. We map the qubit states onto two distinct states of classical parametric oscillation: one oscillating state, with 185±15 photons in the resonator, and one with zero oscillation amplitude. This high contrast obviates a following quantum-limited amplifier. We demonstrate proof-of-principle, single-shot read-out performance, and present an error budget indicating that this method can surpass the fidelity threshold required for quantum computing. PMID:27156732

  4. Conditional rotation of two strongly coupled semiconductor charge qubits.

    PubMed

    Li, Hai-Ou; Cao, Gang; Yu, Guo-Dong; Xiao, Ming; Guo, Guang-Can; Jiang, Hong-Wen; Guo, Guo-Ping

    2015-01-01

    Universal multiple-qubit gates can be implemented by a set of universal single-qubit gates and any one kind of entangling two-qubit gate, such as a controlled-NOT gate. For semiconductor quantum dot qubits, two-qubit gate operations have so far only been demonstrated in individual electron spin-based quantum dot systems. Here we demonstrate the conditional rotation of two capacitively coupled charge qubits, each consisting of an electron confined in a GaAs/AlGaAs double quantum dot. Owing to the strong inter-qubit coupling strength, gate operations with a clock speed up to 6 GHz have been realized. A truth table measurement for controlled-NOT operation shows comparable fidelities to that of spin-based two-qubit gates, although phase coherence is not explicitly measured. Our results suggest that semiconductor charge qubits have a considerable potential for scalable quantum computing and may stimulate the use of long-range Coulomb interaction for coherent quantum control in other devices. PMID:26184756

  5. Conditional rotation of two strongly coupled semiconductor charge qubits

    PubMed Central

    Li, Hai-Ou; Cao, Gang; Yu, Guo-Dong; Xiao, Ming; Guo, Guang-Can; Jiang, Hong-Wen; Guo, Guo-Ping

    2015-01-01

    Universal multiple-qubit gates can be implemented by a set of universal single-qubit gates and any one kind of entangling two-qubit gate, such as a controlled-NOT gate. For semiconductor quantum dot qubits, two-qubit gate operations have so far only been demonstrated in individual electron spin-based quantum dot systems. Here we demonstrate the conditional rotation of two capacitively coupled charge qubits, each consisting of an electron confined in a GaAs/AlGaAs double quantum dot. Owing to the strong inter-qubit coupling strength, gate operations with a clock speed up to 6 GHz have been realized. A truth table measurement for controlled-NOT operation shows comparable fidelities to that of spin-based two-qubit gates, although phase coherence is not explicitly measured. Our results suggest that semiconductor charge qubits have a considerable potential for scalable quantum computing and may stimulate the use of long-range Coulomb interaction for coherent quantum control in other devices. PMID:26184756

  6. Effects of intrinsic decoherence on various correlations and quantum dense coding in a two superconducting charge qubit system

    NASA Astrophysics Data System (ADS)

    Wang, Fei; Maimaitiyiming-Tusun; Parouke-Paerhati; Ahmad-Abliz

    2015-09-01

    The influence of intrinsic decoherence on various correlations and dense coding in a model which consists of two identical superconducting charge qubits coupled by a fixed capacitor is investigated. The results show that, despite the intrinsic decoherence, the correlations as well as the dense coding channel capacity can be effectively increased via the combination of system parameters, i.e., the mutual coupling energy between the two charge qubits is larger than the Josephson energy of the qubit. The bigger the difference between them is, the better the effect is. Project supported by the Project to Develop Outstanding Young Scientific Talents of China (Grant No. 2013711019), the Natural Science Foundation of Xinjiang Province, China (Grant No. 2012211A052), the Foundation for Key Program of Ministry of Education of China (Grant No. 212193), and the Innovative Foundation for Graduate Students Granted by the Key Subjects of Theoretical Physics of Xinjiang Province, China (Grant No. LLWLL201301).

  7. Fabrication and measurements of hybrid Nb/Al Josephson junctions and flux qubits with π-shifters

    NASA Astrophysics Data System (ADS)

    Shcherbakova, A. V.; Fedorov, K. G.; Shulga, K. V.; Ryazanov, V. V.; Bolginov, V. V.; Oboznov, V. A.; Egorov, S. V.; Shkolnikov, V. O.; Wolf, M. J.; Beckmann, D.; Ustinov, A. V.

    2015-02-01

    We describe fabrication and testing of composite flux qubits combining Nb- and Al-based superconducting circuit technology. This hybrid approach to making qubits allows for employing π-phase shifters fabricated using well-established Nb-based technology of superconductor-ferromagnet-superconductor Josephson junctions. The important feature here is to obtain high interface transparency between Nb and Al layers without degrading sub-micron shadow mask. We achieve this by in situ Ar etching using e-beam gun. Shadow-evaporated Al/AlOx/Al Josephson junctions with Nb bias pads show the expected current-voltage characteristics with reproducible critical currents. Using this technique, we fabricated composite Nb/Al flux qubits with Nb/CuNi/Nb π-shifters and measured their magnetic field response. The observed offset between the field responses of the qubits with and without π-junction is attributed to the π phase shift. The reported approach can be used for implementing a variety of hybrid Nb/Al superconducting quantum circuits.

  8. Josephson parametric phase-locked oscillator and its application to dispersive readout of superconducting qubits.

    PubMed

    Lin, Z R; Inomata, K; Koshino, K; Oliver, W D; Nakamura, Y; Tsai, J S; Yamamoto, T

    2014-01-01

    The parametric phase-locked oscillator (PPLO) is a class of frequency-conversion device, originally based on a nonlinear element such as a ferrite ring, that served as a fundamental logic element for digital computers more than 50 years ago. Although it has long since been overtaken by the transistor, there have been numerous efforts more recently to realize PPLOs in different physical systems such as optical photons, trapped atoms, and electromechanical resonators. This renewed interest is based not only on the fundamental physics of nonlinear systems, but also on the realization of new, high-performance computing devices with unprecedented capabilities. Here we realize a PPLO with Josephson-junction circuitry and operate it as a sensitive phase detector. Using a PPLO, we demonstrate the demodulation of a weak binary phase-shift keying microwave signal of the order of a femtowatt. We apply PPLO to dispersive readout of a superconducting qubit, and achieved high-fidelity, single-shot and non-destructive readout with Rabi-oscillation contrast exceeding 90%. PMID:25059992

  9. Composite multi-qubit gates dynamically corrected against charge noise and magnetic field noise for singlet-triplet qubits

    NASA Astrophysics Data System (ADS)

    Kestner, Jason; Barnes, Edwin; Wang, Xin; Bishop, Lev; Das Sarma, Sankar

    2013-03-01

    We use previously described single-qubit SUPCODE pulses on both intra-qubit and inter-qubit exchange couplings, integrated with existing strategies such as BB1, to theoretically construct a CNOT gate that is robust against both charge noise and magnetic field gradient fluctuations. We show how this allows scalable, high-fidelity implementation of arbitrary multi-qubit operations using singlet-triplet spin qubits in the presence of experimentally realistic noise. This work is supported by LPS-NSA-CMTC, IARPA-MQCO and CNAM.

  10. Protected Flux Pairing Qubit

    NASA Astrophysics Data System (ADS)

    Bell, Matthew; Zhang, Wenyuan; Ioffe, Lev; Gershenson, Michael

    2014-03-01

    We have studied the coherent flux tunneling in a qubit containing two submicron Josephson junctions shunted by a superinductor (a dissipationless inductor with an impedance much greater than the resistance quantum). The two low energy quantum states of this device, 0 and 1, are represented by even and odd number of fluxes in the loop, respectively. This device is dual to the charge pairing Josephson rhombi qubit. The spectrum of the device, studied by microwave spectroscopy, reflects the interference between coherent quantum phase slips in the two junctions (the Aharonov-Casher effect). The time domain measurements demonstrate the suppression of the qubit's energy relaxation in the protected regime, which illustrates the potential of this flux pairing device as a protected quantum circuit. Templeton Foundation, NSF, and ARO.

  11. Nonequilibrium work by charge control in a Josephson junction.

    PubMed

    Yi, Su Do; Kim, Beom Jun; Yi, Juyeon

    2013-08-01

    We consider a single Josephson junction in the presence of time varying gate charge, and examine the nonequilibrium work done by the charge control in the framework of fluctuation theorems. Assuming first a high quality junction with negligible Ohmic current, we obtain the probability distribution functions of the work and confirm the Crooks relation to give the estimation of the free energy changes ΔF=0. The reliability of ΔF estimated from the Jarzynksi equality is crucially dependent on protocol parameters, while the Bennett's acceptance ratio method yields consistently ΔF=0. We examine the behaviors of the work average and point out its relation to heat and entropy production associated with the circuit control. Finally considering finite tunnel resistance we discuss dissipation effects on the work statistics. PMID:24032811

  12. Coherent Manipulation of a Silicon Spin-Charge Hybrid Qubit

    NASA Astrophysics Data System (ADS)

    Kim, Dohun; Shi, Zhan; Simmons, Christie B.; Ward, Daniel R.; Prance, Jon R.; Koh, Teck Seng; Gamble, John King; Savage, Donald E.; Lagally, Max G.; Friesen, Mark; Coppersmith, Susan N.; Eriksson, Mark A.

    2014-03-01

    The recently proposed quantum dot hybrid qubit enables fast, coherent quantum operations. We demonstrate rotations of a hybrid qubit in a three-electron Si/SiGe double quantum dot about two axes of the Bloch sphere (X and Z). We perform Larmor oscillations (x-rotations on the Bloch sphere) between the 0 and 1 hybrid states, demonstrating a T2* time of 2.1 ns at the charge degeneracy point. Using tailored pulse gating sequences, we perform fast (>10GHz) phase (z-axis) rotations of the hybrid qubit states. We measure a lower bound of the coherence time T2* of 10 ns and high figure of merit >150. This work was supported in part by ARO (W911NF-12-0607) and the United States Department of Defense. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the US Government.

  13. Charge noise mitigation in triple-dot encoded spin qubits

    NASA Astrophysics Data System (ADS)

    Pritchett, Emily

    The immediate scalability of electrons confined to semiconductor quantum dots makes them one of the most attractive platforms for quantum information processing; however, 1/f charge noise associated with electrical confinement has been a leading source of noise in quantum dot systems. Recently, there has been a surge of experimental and theoretical work aimed at charge noise mitigation in quantum dot systems implementing AC- or DC- control of triple dots at ''sweet spots''. In this talk, we compare the symmetric operation point (SOP) DC control technique implemented in Reed, et al. [arXiv:1508.01223] to the resonant exchange (RX) AC control technique [Medford, et al., PRL 111, 050501 (2013), Taylor, et al., PRL 111, 050502 (2013), Russ, et al., Phys. Rev. B 91, 235411 (2015)] . Numerical results suggest that both DC and AC triple-dot control can offer a comparably substantial reduction in charge noise however, the validity of the rotating wave approximation forces a trade-off between speed and accuracy for RX qubits, while the performance of SOP qubits actually improves at shorter gate times.

  14. Microwave-driven coherent operation of a semiconductor quantum dot charge qubit

    DOE PAGESBeta

    Kim, Dohun; Ward, D. R.; Simmons, C. B.; Gamble, John King; Blume-Kohout, Robin; Nielsen, Erik; Savage, D. E.; Lagally, M. G.; Friesen, Mark; Coppersmith, S. N.; et al

    2015-02-16

    An intuitive realization of a qubit is an electron charge at two well-defined positions of a double quantum dot. The qubit is simple and has the potential for high-speed operation because of its strong coupling to electric fields. But, charge noise also couples strongly to this qubit, resulting in rapid dephasing at all but one special operating point called the ‘sweet spot’. In previous studies d.c. voltage pulses have been used to manipulate semiconductor charge qubits but did not achieve high-fidelity control, because d.c. gating requires excursions away from the sweet spot. Here, by using resonant a.c. microwave driving wemore » achieve fast (greater than gigahertz) and universal single qubit rotations of a semiconductor charge qubit. The Z-axis rotations of the qubit are well protected at the sweet spot, and we demonstrate the same protection for rotations about arbitrary axes in the X–Y plane of the qubit Bloch sphere. We characterize the qubit operation using two tomographic approaches: standard process tomography and gate set tomography. Moreover, both methods consistently yield process fidelities greater than 86% with respect to a universal set of unitary single-qubit operations.« less

  15. Microwave-driven coherent operation of a semiconductor quantum dot charge qubit

    SciTech Connect

    Kim, Dohun; Ward, D. R.; Simmons, C. B.; Gamble, John King; Blume-Kohout, Robin; Nielsen, Erik; Savage, D. E.; Lagally, M. G.; Friesen, Mark; Coppersmith, S. N.; Eriksson, M. A.

    2015-02-16

    An intuitive realization of a qubit is an electron charge at two well-defined positions of a double quantum dot. The qubit is simple and has the potential for high-speed operation because of its strong coupling to electric fields. But, charge noise also couples strongly to this qubit, resulting in rapid dephasing at all but one special operating point called the ‘sweet spot’. In previous studies d.c. voltage pulses have been used to manipulate semiconductor charge qubits but did not achieve high-fidelity control, because d.c. gating requires excursions away from the sweet spot. Here, by using resonant a.c. microwave driving we achieve fast (greater than gigahertz) and universal single qubit rotations of a semiconductor charge qubit. The Z-axis rotations of the qubit are well protected at the sweet spot, and we demonstrate the same protection for rotations about arbitrary axes in the X–Y plane of the qubit Bloch sphere. We characterize the qubit operation using two tomographic approaches: standard process tomography and gate set tomography. Moreover, both methods consistently yield process fidelities greater than 86% with respect to a universal set of unitary single-qubit operations.

  16. Microwave-driven coherent operation of a semiconductor quantum dot charge qubit

    NASA Astrophysics Data System (ADS)

    Kim, Dohun; Ward, D. R.; Simmons, C. B.; Gamble, John King; Blume-Kohout, Robin; Nielsen, Erik; Savage, D. E.; Lagally, M. G.; Friesen, Mark; Coppersmith, S. N.; Eriksson, M. A.

    2015-03-01

    An intuitive realization of a qubit is an electron charge at two well-defined positions of a double quantum dot. This qubit is simple and has the potential for high-speed operation because of its strong coupling to electric fields. However, charge noise also couples strongly to this qubit, resulting in rapid dephasing at all but one special operating point called the ‘sweet spot’. In previous studies d.c. voltage pulses have been used to manipulate semiconductor charge qubits but did not achieve high-fidelity control, because d.c. gating requires excursions away from the sweet spot. Here, by using resonant a.c. microwave driving we achieve fast (greater than gigahertz) and universal single qubit rotations of a semiconductor charge qubit. The Z-axis rotations of the qubit are well protected at the sweet spot, and we demonstrate the same protection for rotations about arbitrary axes in the X-Y plane of the qubit Bloch sphere. We characterize the qubit operation using two tomographic approaches: standard process tomography and gate set tomography. Both methods consistently yield process fidelities greater than 86% with respect to a universal set of unitary single-qubit operations.

  17. Observable Berry Phase for Charge Qubit in a Dissipative Environment

    NASA Astrophysics Data System (ADS)

    Wang, Z. S.; Li, Yuan-hua; Xu, Hualan; Hu, Li-yun; Nie, Yi-you; Guo, L. P.; Huang, Min; Pan, Hui

    2012-09-01

    A geometric phase of open system is directly obtained from Schrödinger equation with a hermitian Hamiltonian of a two-level atomic system interacting with its reservoirs. We find that the dynamical phases are proportional to the geometric phases in terms of Weisskopf-Wigner theory in the rotational frame. Thus an effective scheme to measure the Berry phase in a charge qubit dissipative system is proposed by coherently controlling the macroscopic quantum states formed in superconducting circuits. Our approach does not need any operations to cancel the dynamical phases so as to reduce the experimental errors. Furthermore, we find that the dissipative effects can be overcome by choosing adapted parameters of the superconducting circuit.

  18. Enhanced Coherence and High Figure of Merit in a Silicon Charge qubit

    NASA Astrophysics Data System (ADS)

    Shi, Zhan; Simmons, Christie; Ward, Daniel; Prance, Jonathan; Koh, Teck Seng; Gamble, John; Wu, Xian; Savage, Donald; Lagally, Max; Friesen, Mark; Coppersmith, Susan; Eriksson, Mark

    2013-03-01

    Coherent manipulation of a charge qubit is an essential step in the use of pulsed gate voltages to manipulate a quantum dot hybrid spin qubit. Here, we demonstrate coherent manipulation of a charge qubit in Si/SiGe double quantum dot. We perform Larmor oscillations (x-rotations on the Bloch sphere) between the (2,1) and (1,2) charge states, measuring a T2* time of 2.1 ns at the charge degeneracy point. We find an increased coherence time (3.7 ns) and higher figure of merit (37) away from the charge degeneracy point, arising from a second charge anti-crossing involving a low lying excited state in the right dot - the desired structure for a hybrid spin qubit. We also observe Ramsey fringes (z-rotations on the Bloch sphere) and measure a T2* of 179 ps at detunings away from any protective energy level structures. Now work at Massachusetts Institute of Technology

  19. Collective modes in the fluxonium qubit

    NASA Astrophysics Data System (ADS)

    Catelani, Gianluigi; Viola, Giovanni

    2015-03-01

    In the fluxonium qubit, an array comprising a large number of identical Josephson junctions form a so-called superinductance. The superinductance is connected to a junction - the phase slip element - with a smaller Josephson energy and a different charging energy. We investigate the effects of unavoidable capacitive couplings to ground as well as non-linearities of the superinductance: they both introduce interactions between the low-energy qubit degree of freedom and higher-energy collective modes of the circuit. We also consider the role of the additional capacitances that are used to couple the qubit to a resonator for driving and read-out. We show that the interactions with the collective modes can affect not only the spectrum of the qubit but also its coherence. Work supported in part by the EU under REA Grant Agreement No. CIG-618258.

  20. Tunable capacitive coupling between two semiconductor charge qubits.

    PubMed

    Yu, Guo-Dong; Li, Hai-Ou; Cao, Gang; Xiao, Ming; Jiang, Hong-Wen; Guo, Guo-Ping

    2016-08-12

    Strong coupling between two qubits is one of the main requirements for high fidelity two-qubit logic operations. Here we experimentally investigate the capacitive coupling between two double quantum dots. A pair of open slot confinement gates is used to enhance the coupling. We find that the coupling energy J can be conveniently tuned in a broad range. Through numerical simulations, we study the effect of J on two-qubit operations. The analysis shows that our experimentally obtained J is adequate to achieve high fidelity two-qubit entanglement and logic gates. PMID:27354414

  1. Tunable capacitive coupling between two semiconductor charge qubits

    NASA Astrophysics Data System (ADS)

    Yu, Guo-Dong; Li, Hai-Ou; Cao, Gang; Xiao, Ming; Jiang, Hong-Wen; Guo, Guo-Ping

    2016-08-01

    Strong coupling between two qubits is one of the main requirements for high fidelity two-qubit logic operations. Here we experimentally investigate the capacitive coupling between two double quantum dots. A pair of open slot confinement gates is used to enhance the coupling. We find that the coupling energy J can be conveniently tuned in a broad range. Through numerical simulations, we study the effect of J on two-qubit operations. The analysis shows that our experimentally obtained J is adequate to achieve high fidelity two-qubit entanglement and logic gates.

  2. Charge and current beats in T-shaped qubit-detector systems

    NASA Astrophysics Data System (ADS)

    Taranko, R.; Kwapiński, T.

    2015-06-01

    The time evolution of a charge qubit coupled electrostatically with different detectors in the forms of single, double and triple quantum dot linear systems in the T-shaped configuration between two reservoirs is theoretically considered. The correspondence between the qubit quantum dot oscillations and the detector current is studied for different values of the inter-dot tunneling amplitudes and the qubit-detector interaction strength. We have found that even for a qubit coupled with a single QD detector, the coherent beat patterns appear in the oscillations of the qubit charge. This effect is more evident for a qubit coupled with double or triple-QD detectors. The beats can be also observed in both the detector current and the detector quantum dot occupations. Moreover, in the presence of beats the qubit oscillations hold longer in time in comparison with the beats-free systems with monotonously decaying oscillations. The dependence of the qubit dynamics on different initial occupations of the detector sites (memory effect) is also analyzed.

  3. Quantum information transfer between topological and conventional charge qubits

    NASA Astrophysics Data System (ADS)

    Jun, Li; Yan, Zou

    2016-02-01

    We propose a scheme to realize coherent quantum information transfer between topological and conventional charge qubits. We first consider a hybrid system where a quantum dot (QD) is tunnel-coupled to a semiconductor Majorana-hosted nanowire (MNW) via using gated control as a switch, the information encoded in the superposition state of electron empty and occupied state can be transferred to each other through choosing the proper interaction time to make measurements. Then we consider another system including a double QDs and a pair of parallel MNWs, it is shown that the entanglement information transfer can be realized between the two kinds of systems. We also realize long distance quantum information transfer between two quantum dots separated by an MNW, by making use of the nonlocal fermionic level formed with the pared Majorana feimions (MFs) emerging at the two ends of the MNW. Furthermore, we analyze the teleportationlike electron transfer phenomenon predicted by Tewari et al. [Phys. Rev. Lett. 100, 027001 (2008)] in our considered system. Interestingly, we find that this phenomenon exactly corresponds to the case that the information encoded in one QD just returns back to its original place during the dynamical evolution of the combined system from the perspective of quantum state transfer. Project supported by the National Natural Science Foundation of China (Grant No. 11304031).

  4. Qubit lattice coherence induced by electromagnetic pulses in superconducting metamaterials

    NASA Astrophysics Data System (ADS)

    Ivić, Z.; Lazarides, N.; Tsironis, G. P.

    2016-07-01

    Quantum bits (qubits) are at the heart of quantum information processing schemes. Currently, solid-state qubits, and in particular the superconducting ones, seem to satisfy the requirements for being the building blocks of viable quantum computers, since they exhibit relatively long coherence times, extremely low dissipation, and scalability. The possibility of achieving quantum coherence in macroscopic circuits comprising Josephson junctions, envisioned by Legett in the 1980’s, was demonstrated for the first time in a charge qubit; since then, the exploitation of macroscopic quantum effects in low-capacitance Josephson junction circuits allowed for the realization of several kinds of superconducting qubits. Furthermore, coupling between qubits has been successfully achieved that was followed by the construction of multiple-qubit logic gates and the implementation of several algorithms. Here it is demonstrated that induced qubit lattice coherence as well as two remarkable quantum coherent optical phenomena, i.e., self-induced transparency and Dicke-type superradiance, may occur during light-pulse propagation in quantum metamaterials comprising superconducting charge qubits. The generated qubit lattice pulse forms a compound ”quantum breather” that propagates in synchrony with the electromagnetic pulse. The experimental confirmation of such effects in superconducting quantum metamaterials may open a new pathway to potentially powerful quantum computing.

  5. Qubit lattice coherence induced by electromagnetic pulses in superconducting metamaterials

    PubMed Central

    Ivić, Z.; Lazarides, N.; Tsironis, G. P.

    2016-01-01

    Quantum bits (qubits) are at the heart of quantum information processing schemes. Currently, solid-state qubits, and in particular the superconducting ones, seem to satisfy the requirements for being the building blocks of viable quantum computers, since they exhibit relatively long coherence times, extremely low dissipation, and scalability. The possibility of achieving quantum coherence in macroscopic circuits comprising Josephson junctions, envisioned by Legett in the 1980’s, was demonstrated for the first time in a charge qubit; since then, the exploitation of macroscopic quantum effects in low-capacitance Josephson junction circuits allowed for the realization of several kinds of superconducting qubits. Furthermore, coupling between qubits has been successfully achieved that was followed by the construction of multiple-qubit logic gates and the implementation of several algorithms. Here it is demonstrated that induced qubit lattice coherence as well as two remarkable quantum coherent optical phenomena, i.e., self-induced transparency and Dicke-type superradiance, may occur during light-pulse propagation in quantum metamaterials comprising superconducting charge qubits. The generated qubit lattice pulse forms a compound ”quantum breather” that propagates in synchrony with the electromagnetic pulse. The experimental confirmation of such effects in superconducting quantum metamaterials may open a new pathway to potentially powerful quantum computing. PMID:27403780

  6. Qubit lattice coherence induced by electromagnetic pulses in superconducting metamaterials.

    PubMed

    Ivić, Z; Lazarides, N; Tsironis, G P

    2016-01-01

    Quantum bits (qubits) are at the heart of quantum information processing schemes. Currently, solid-state qubits, and in particular the superconducting ones, seem to satisfy the requirements for being the building blocks of viable quantum computers, since they exhibit relatively long coherence times, extremely low dissipation, and scalability. The possibility of achieving quantum coherence in macroscopic circuits comprising Josephson junctions, envisioned by Legett in the 1980's, was demonstrated for the first time in a charge qubit; since then, the exploitation of macroscopic quantum effects in low-capacitance Josephson junction circuits allowed for the realization of several kinds of superconducting qubits. Furthermore, coupling between qubits has been successfully achieved that was followed by the construction of multiple-qubit logic gates and the implementation of several algorithms. Here it is demonstrated that induced qubit lattice coherence as well as two remarkable quantum coherent optical phenomena, i.e., self-induced transparency and Dicke-type superradiance, may occur during light-pulse propagation in quantum metamaterials comprising superconducting charge qubits. The generated qubit lattice pulse forms a compound "quantum breather" that propagates in synchrony with the electromagnetic pulse. The experimental confirmation of such effects in superconducting quantum metamaterials may open a new pathway to potentially powerful quantum computing. PMID:27403780

  7. Decoherence dynamics of two charge qubits in vertically coupled quantum dots

    SciTech Connect

    Ben Chouikha, W.; Bennaceur, R.; Jaziri, S.

    2007-12-15

    The decoherence dynamics of two charge qubits in a double quantum dot is investigated theoretically. We consider the quantum dynamics of two interacting electrons in a vertically coupled quantum dot driven by an external electric field. We derive the equations of motion for the density matrix, in which the presence of an electron confined in the double dot represents one qubit. A Markovian approach to the dynamical evolution of the reduced density matrix is adopted. We evaluate the concurrence of two qubits in order to study the effect of acoustic phonons on the entanglement. We also show that the disentanglement effect depends on the double dot parameters and increases with the temperature.

  8. Charge creation and nucleation of the longitudinal plasma wave in coupled Josephson junctions

    NASA Astrophysics Data System (ADS)

    Shukrinov, Yu. M.; Hamdipour, M.

    2010-11-01

    We study the phase dynamics in coupled Josephson junctions described by a system of nonlinear differential equations. Results of detailed numerical simulations of charge creation in the superconducting layers and the longitudinal plasma wave (LPW) nucleation are presented. We demonstrate the different time stages in the development of the LPW and present the results of FFT analysis at different values of bias current. The correspondence between the breakpoint position on the outermost branch of current voltage characteristics (CVC) and the growing region in time dependence of the electric charge in the superconducting layer is established. The effects of noise in the bias current and the external microwave radiation on the charge dynamics of the coupled Josephson junctions are found. These effects introduce a way to regulate the process of LPW nucleation in the stack of IJJ.

  9. Coherent quantum oscillations and echo measurements of a Si charge qubit

    NASA Astrophysics Data System (ADS)

    Shi, Zhan; Simmons, C. B.; Ward, Daniel R.; Prance, J. R.; Mohr, R. T.; Koh, Teck Seng; Gamble, John King; Wu, Xian; Savage, D. E.; Lagally, M. G.; Friesen, Mark; Coppersmith, S. N.; Eriksson, M. A.

    2013-08-01

    Fast quantum oscillations of a charge qubit in a double quantum dot fabricated in a Si/SiGe heterostructure are demonstrated and characterized experimentally. The measured inhomogeneous dephasing time T2* ranges from 127 ps to 2.1 ns; it depends substantially on how the energy difference of the two qubit states varies with external voltages, consistent with a decoherence process that is dominated by detuning noise (charge noise that changes the asymmetry of the qubit's double-well potential). In the regime with the shortest T2*, applying a charge-echo pulse sequence increases the measured inhomogeneous decoherence time from 127 to 760 ps, demonstrating that low-frequency noise processes are an important dephasing mechanism.

  10. Study of charge-phase diagrams for coupled system of Josephson junctions

    NASA Astrophysics Data System (ADS)

    Hamdipour, M.; Shukrinov, Y. U. M.

    2010-11-01

    Dynamics of stacked intrinsic Josephson junctions (IJJ) in the high-Tc superconductors is theoretically investigated. We calculate the current-voltage characteristics (CVC) of IJJ and study the breakpoint region on the outermost branch of the CVC for the stacks with 9 IJJ. A method for investigation of the fine structure in CVC of IJJ based on the recording the "phase-charge" diagrams is suggested. It is demonstrated that this method reflects the main features of the breakpoint region.

  11. Dephasing of two-spin qubits due to their charge and nuclear environments

    NASA Astrophysics Data System (ADS)

    Ramon, Guy

    2011-03-01

    We consider dephasing of qubits encoded in the singlet and unpolarized triplet states of pairs of spins localized in biased double quantum dots. The charge environment is modeled by both two-center charge traps in the insulator (where electrons tunnel between the two centers), and single charge traps located near the gate electrodes and QPCs (where electrons charge and empty the trap). The couplings of these trapped charges to the qubits are calculated by considering their charge distributions within a multipole expansion. It is demonstrated that the summation over these random telegraph processes in mesoscopic devices results in non-Markovian and non-Gaussian noise. For the nuclear environment we consider hyperfine-induced electron-spin dephasing in a nuclear spin bath with narrowed distribution. Nuclear state preparation using dynamical polarization cycles was experimentally achieved recently, and it is also essential to enable X -rotations for two-spin qubits. Our analysis is performed for both free induction and echo signals. The scaling of these dephasing mechanisms with the number of qubits is also discussed. Supported by Research Corporation.

  12. Non-Gaussian signatures and collective effects in charge noise affecting a dynamically decoupled qubit

    NASA Astrophysics Data System (ADS)

    Ramon, Guy

    2015-10-01

    The effects of a collection of classical two-level charge fluctuators on the coherence of a dynamically decoupled qubit are studied. Distinct dynamics is found at different qubit working positions. Exact analytical formulas are derived at pure dephasing and approximate solutions are found at the general working position, for weakly and strongly coupled fluctuators. Analysis of these solutions, combined with numerical simulations of the multiple random telegraph processes, reveal the scaling of the noise with the number of fluctuators and the number of control pulses, as well as dependence on other parameters of the qubit-fluctuators system. These results can be used to determine potential microscopic models for the charge environment by performing noise spectroscopy.

  13. Role of metastable charge states in a quantum-dot spin-qubit readout

    NASA Astrophysics Data System (ADS)

    Mason, J. D.; Studenikin, S. A.; Kam, A.; Wasilewski, Z. R.; Sachrajda, A. S.; Kycia, J. B.

    2015-09-01

    Readout of a spin qubit in a lateral gate-defined quantum-dot device typically involves a charge detector and a spin-to-charge conversion technique employing spin blockade. We investigate alternative mechanisms for spin-to-charge conversion involving metastable excited charge states made possible by an asymmetry in the tunneling rates to the leads. This technique is used to observe Landau-Zener-Stückelberg oscillations of the S -T+ qubit within the (1,0) ground state region of the charge stability diagram. The oscillations are π phase shifted relative to those detected using the standard technique and display a nonsinusoidal waveform due to the increased relaxation time from the metastable state.

  14. Macroscopic quantum tunneling and qubit design parameters of Bi2Sr2CaCu2O8+δ surface intrinsic Josephson junctions

    NASA Astrophysics Data System (ADS)

    Cui, D. J.; Yu, H. F.; Peng, Z. H.; Cao, W. H.; Zhu, X. B.; Tian, Ye; Chen, G. H.; Lin, D. H.; Gu, C. Z.; Zheng, D. N.; Jing, X. N.; Lu, Li; Zhao, S. P.

    2008-12-01

    Macroscopic quantum tunneling (MQT) has been demonstrated recently in a Bi2Sr2CaCu2O8+δ surface intrinsic Josephson junction (SIJJ) with its critical current density Jc below 100 A cm-2 and its size below 1 µm. In this work, we present a study of the switching current distributions of SIJJs fabricated on the same crystal, with Jc>500 A cm-2 and size of 0.8 and 1.6 µm. MQT is clearly observed, and the crossover from MQT to thermal activation (TA) is seen. Our analysis shows that the data agree well with the theoretical predictions of MQT and TA for different-sized SIJJs when parameters that roughly scale with the SIJJ size are used. In the crossover regime, the data are found to be better fitted by considering quantum corrections to TA. We discuss the realistic design of phase- and flux-type qubits using the experimentally attainable SIJJ parameters, which shows that the SIJJs, with their controllable Jc and size (or junction capacitance), are feasible for qubit applications in the future.

  15. Reduced Sensitivity to Charge Noise in Semiconductor Spin Qubits via Symmetric Operation

    NASA Astrophysics Data System (ADS)

    Reed, M. D.; Maune, B. M.; Andrews, R. W.; Borselli, M. G.; Eng, K.; Jura, M. P.; Kiselev, A. A.; Ladd, T. D.; Merkel, S. T.; Milosavljevic, I.; Pritchett, E. J.; Rakher, M. T.; Ross, R. S.; Schmitz, A. E.; Smith, A.; Wright, J. A.; Gyure, M. F.; Hunter, A. T.

    2016-03-01

    We demonstrate improved operation of exchange-coupled semiconductor quantum dots by substantially reducing the sensitivity of exchange operations to charge noise. The method involves biasing a double dot symmetrically between the charge-state anticrossings, where the derivative of the exchange energy with respect to gate voltages is minimized. Exchange remains highly tunable by adjusting the tunnel coupling. We find that this method reduces the dephasing effect of charge noise by more than a factor of 5 in comparison to operation near a charge-state anticrossing, increasing the number of observable exchange oscillations in our qubit by a similar factor. Performance also improves with exchange rate, favoring fast quantum operations.

  16. Capacitively coupled singlet-triplet qubits in the double charge resonant regime

    NASA Astrophysics Data System (ADS)

    Srinivasa, V.; Taylor, J. M.

    2015-12-01

    We investigate a method for entangling two singlet-triplet qubits in adjacent double quantum dots via capacitive interactions. In contrast to prior work, here we focus on a regime with strong interactions between the qubits. The interplay of the interaction energy and simultaneous large detunings for both double dots gives rise to the "double charge resonant" regime, in which the unpolarized (1111) and fully polarized (0202) four-electron states in the absence of interqubit tunneling are near degeneracy, while being energetically well separated from the partially polarized (0211 and 1102) states. A rapid controlled-phase gate may be realized by combining time evolution in this regime in the presence of intraqubit tunneling and the interqubit Coulomb interaction with refocusing π pulses that swap the singly occupied singlet and triplet states of the two qubits via, e.g., magnetic gradients. We calculate the fidelity of this entangling gate, incorporating models for two types of noise—charge fluctuations in the single-qubit detunings and charge relaxation within the low-energy subspace via electron-phonon interaction—and identify parameter regimes that optimize the fidelity. The rates of phonon-induced decay for pairs of GaAs or Si double quantum dots vary with the sizes of the dipolar and quadrupolar contributions and are several orders of magnitude smaller for Si, leading to high theoretical gate fidelities for coupled singlet-triplet qubits in Si dots. We also consider the dependence of the capacitive coupling on the relative orientation of the double dots and find that a linear geometry provides the fastest potential gate.

  17. Charge noise, spin-orbit coupling, and dephasing of single-spin qubits

    SciTech Connect

    Bermeister, Adam; Keith, Daniel; Culcer, Dimitrie

    2014-11-10

    Quantum dot quantum computing architectures rely on systems in which inversion symmetry is broken, and spin-orbit coupling is present, causing even single-spin qubits to be susceptible to charge noise. We derive an effective Hamiltonian for the combined action of noise and spin-orbit coupling on a single-spin qubit, identify the mechanisms behind dephasing, and estimate the free induction decay dephasing times T{sub 2}{sup *} for common materials such as Si and GaAs. Dephasing is driven by noise matrix elements that cause relative fluctuations between orbital levels, which are dominated by screened whole charge defects and unscreened dipole defects in the substrate. Dephasing times T{sub 2}{sup *} differ markedly between materials and can be enhanced by increasing gate fields, choosing materials with weak spin-orbit, making dots narrower, or using accumulation dots.

  18. Microwave Emission from Hybridized States in a Semiconductor Charge Qubit

    NASA Astrophysics Data System (ADS)

    Stockklauser, A.; Maisi, V. F.; Basset, J.; Cujia, K.; Reichl, C.; Wegscheider, W.; Ihn, T.; Wallraff, A.; Ensslin, K.

    2015-07-01

    We explore the microwave radiation emitted from a biased double quantum dot due to the inelastic tunneling of single charges. Radiation is detected over a broad range of detuning configurations between the dot energy levels, with pronounced maxima occurring in resonance with a capacitively coupled transmission line resonator. The power emitted for forward and reverse resonant detuning is found to be in good agreement with a rate equation model, which considers the hybridization of the individual dot charge states.

  19. Microwave Emission from Hybridized States in a Semiconductor Charge Qubit.

    PubMed

    Stockklauser, A; Maisi, V F; Basset, J; Cujia, K; Reichl, C; Wegscheider, W; Ihn, T; Wallraff, A; Ensslin, K

    2015-07-24

    We explore the microwave radiation emitted from a biased double quantum dot due to the inelastic tunneling of single charges. Radiation is detected over a broad range of detuning configurations between the dot energy levels, with pronounced maxima occurring in resonance with a capacitively coupled transmission line resonator. The power emitted for forward and reverse resonant detuning is found to be in good agreement with a rate equation model, which considers the hybridization of the individual dot charge states. PMID:26252704

  20. Decoherence Dynamics of a Superconducting Charge Qubit Coupled to a Boson Bath and a Spin Bath

    NASA Astrophysics Data System (ADS)

    Yang, Qin-Ying; Liang, Bao-Long; Wang, Ji-Suo

    2014-01-01

    By virtue of a superconducting charge qubit, we derive the off-diagonal matrix operator and investigate the decoherence of the system in different regimes coupled to, respectively, the boson bath and the spin bath. It is found that the two different baths make a bit of difference on the decay of the system at low but finite temperature and the decoherence of the system is most closely linked with the regime as well as the coupling strength. Therefore, by optimizing some reasonable parameters, we can suppress appropriately the decoherence of a given quantum system.

  1. Decoherence dynamics of a charge qubit coupled to the noise bath

    NASA Astrophysics Data System (ADS)

    Yang, Qin-Ying; Liang, Bao-Long; Wang, Ji-Suo

    2013-07-01

    By virtue of the canonical quantization method, we present a quantization scheme for a charge qubit based on the superconducting quantum interference device (SQUID), taking the self-inductance of the loop into account. Under reasonable short-time approximation, we study the effect of decoherence in the ohmic case by employing the response function and the norm. It is confirmed that the decoherence time, which depends on the parameters of the circuit components, the coupling strength, and the temperature, can be as low as several picoseconds, so there is enough time to record the information.

  2. Ultrafast universal quantum control of a quantum-dot charge qubit using Landau-Zener-Stückelberg interference.

    PubMed

    Cao, Gang; Li, Hai-Ou; Tu, Tao; Wang, Li; Zhou, Cheng; Xiao, Ming; Guo, Guang-Can; Jiang, Hong-Wen; Guo, Guo-Ping

    2013-01-01

    A basic requirement for quantum information processing is the ability to universally control the state of a single qubit on timescales much shorter than the coherence time. Although ultrafast optical control of a single spin has been achieved in quantum dots, scaling up such methods remains a challenge. Here we demonstrate complete control of the quantum-dot charge qubit on the picosecond scale [corrected], orders of magnitude faster than the previously measured electrically controlled charge- or spin-based qubits. We observe tunable qubit dynamics in a charge-stability diagram, in a time domain, and in a pulse amplitude space of the driven pulse. The observations are well described by Landau-Zener-Stückelberg interference. These results establish the feasibility of a full set of all-electrical single-qubit operations. Although our experiment is carried out in a solid-state architecture, the technique is independent of the physical encoding of the quantum information and has the potential for wider applications. PMID:23360992

  3. Ultrafast universal quantum control of a quantum-dot charge qubit using Landau–Zener–Stückelberg interference

    PubMed Central

    Cao, Gang; Li, Hai-Ou; Tu, Tao; Wang, Li; Zhou, Cheng; Xiao, Ming; Guo, Guang-Can; Jiang, Hong-Wen; Guo, Guo-Ping

    2013-01-01

    A basic requirement for quantum information processing is the ability to universally control the state of a single qubit on timescales much shorter than the coherence time. Although ultrafast optical control of a single spin has been achieved in quantum dots, scaling up such methods remains a challenge. Here we demonstrate complete control of the quantum-dot charge qubit on the picosecond scale, orders of magnitude faster than the previously measured electrically controlled charge- or spin-based qubits. We observe tunable qubit dynamics in a charge-stability diagram, in a time domain, and in a pulse amplitude space of the driven pulse. The observations are well described by Landau–Zener–Stückelberg interference. These results establish the feasibility of a full set of all-electrical single-qubit operations. Although our experiment is carried out in a solid-state architecture, the technique is independent of the physical encoding of the quantum information and has the potential for wider applications. PMID:23360992

  4. Coherent Charge Transport in Ballistic InSb Nanowire Josephson Junctions

    PubMed Central

    Li, S.; Kang, N.; Fan, D. X.; Wang, L. B.; Huang, Y. Q.; Caroff, P.; Xu, H. Q.

    2016-01-01

    Hybrid InSb nanowire-superconductor devices are promising for investigating Majorana modes and topological quantum computation in solid-state devices. An experimental realisation of ballistic, phase-coherent superconductor-nanowire hybrid devices is a necessary step towards engineering topological superconducting electronics. Here, we report on a low-temperature transport study of Josephson junction devices fabricated from InSb nanowires grown by molecular-beam epitaxy and provide a clear evidence for phase-coherent, ballistic charge transport through the nanowires in the junctions. We demonstrate that our devices show gate-tunable proximity-induced supercurrent and clear signatures of multiple Andreev reflections in the differential conductance, indicating phase-coherent transport within the junctions. We also observe periodic modulations of the critical current that can be associated with the Fabry-Pérot interference in the nanowires in the ballistic transport regime. Our work shows that the InSb nanowires grown by molecular-beam epitaxy are of excellent material quality and hybrid superconducting devices made from these nanowires are highly desirable for investigation of the novel physics in topological states of matter and for applications in topological quantum electronics. PMID:27102689

  5. Coherent Charge Transport in Ballistic InSb Nanowire Josephson Junctions.

    PubMed

    Li, S; Kang, N; Fan, D X; Wang, L B; Huang, Y Q; Caroff, P; Xu, H Q

    2016-01-01

    Hybrid InSb nanowire-superconductor devices are promising for investigating Majorana modes and topological quantum computation in solid-state devices. An experimental realisation of ballistic, phase-coherent superconductor-nanowire hybrid devices is a necessary step towards engineering topological superconducting electronics. Here, we report on a low-temperature transport study of Josephson junction devices fabricated from InSb nanowires grown by molecular-beam epitaxy and provide a clear evidence for phase-coherent, ballistic charge transport through the nanowires in the junctions. We demonstrate that our devices show gate-tunable proximity-induced supercurrent and clear signatures of multiple Andreev reflections in the differential conductance, indicating phase-coherent transport within the junctions. We also observe periodic modulations of the critical current that can be associated with the Fabry-Pérot interference in the nanowires in the ballistic transport regime. Our work shows that the InSb nanowires grown by molecular-beam epitaxy are of excellent material quality and hybrid superconducting devices made from these nanowires are highly desirable for investigation of the novel physics in topological states of matter and for applications in topological quantum electronics. PMID:27102689

  6. Coherent Charge Transport in Ballistic InSb Nanowire Josephson Junctions

    NASA Astrophysics Data System (ADS)

    Li, S.; Kang, N.; Fan, D. X.; Wang, L. B.; Huang, Y. Q.; Caroff, P.; Xu, H. Q.

    2016-04-01

    Hybrid InSb nanowire-superconductor devices are promising for investigating Majorana modes and topological quantum computation in solid-state devices. An experimental realisation of ballistic, phase-coherent superconductor-nanowire hybrid devices is a necessary step towards engineering topological superconducting electronics. Here, we report on a low-temperature transport study of Josephson junction devices fabricated from InSb nanowires grown by molecular-beam epitaxy and provide a clear evidence for phase-coherent, ballistic charge transport through the nanowires in the junctions. We demonstrate that our devices show gate-tunable proximity-induced supercurrent and clear signatures of multiple Andreev reflections in the differential conductance, indicating phase-coherent transport within the junctions. We also observe periodic modulations of the critical current that can be associated with the Fabry-Pérot interference in the nanowires in the ballistic transport regime. Our work shows that the InSb nanowires grown by molecular-beam epitaxy are of excellent material quality and hybrid superconducting devices made from these nanowires are highly desirable for investigation of the novel physics in topological states of matter and for applications in topological quantum electronics.

  7. Enhanced electron-phonon coupling for a semiconductor charge qubit in a surface phonon cavity

    PubMed Central

    Chen, J. C. H.; Sato, Y.; Kosaka, R.; Hashisaka, M.; Muraki, K.; Fujisawa, T.

    2015-01-01

    Electron-phonon coupling is a major decoherence mechanism, which often causes scattering and energy dissipation in semiconductor electronic systems. However, this electron-phonon coupling may be used in a positive way for reaching the strong or ultra-strong coupling regime in an acoustic version of the cavity quantum electrodynamic system. Here we propose and demonstrate a phonon cavity for surface acoustic waves, which is made of periodic metal fingers that constitute Bragg reflectors on a GaAs/AlGaAs heterostructure. Phonon band gap and cavity phonon modes are identified by frequency, time and spatially resolved measurements of the piezoelectric potential. Tunneling spectroscopy on a double quantum dot indicates the enhancement of phonon assisted transitions in a charge qubit. This encourages studying of acoustic cavity quantum electrodynamics with surface phonons. PMID:26469629

  8. Enhanced electron-phonon coupling for a semiconductor charge qubit in a surface phonon cavity

    NASA Astrophysics Data System (ADS)

    Chen, J. C. H.; Sato, Y.; Kosaka, R.; Hashisaka, M.; Muraki, K.; Fujisawa, T.

    2015-10-01

    Electron-phonon coupling is a major decoherence mechanism, which often causes scattering and energy dissipation in semiconductor electronic systems. However, this electron-phonon coupling may be used in a positive way for reaching the strong or ultra-strong coupling regime in an acoustic version of the cavity quantum electrodynamic system. Here we propose and demonstrate a phonon cavity for surface acoustic waves, which is made of periodic metal fingers that constitute Bragg reflectors on a GaAs/AlGaAs heterostructure. Phonon band gap and cavity phonon modes are identified by frequency, time and spatially resolved measurements of the piezoelectric potential. Tunneling spectroscopy on a double quantum dot indicates the enhancement of phonon assisted transitions in a charge qubit. This encourages studying of acoustic cavity quantum electrodynamics with surface phonons.

  9. Enhanced electron-phonon coupling for a semiconductor charge qubit in a surface phonon cavity.

    PubMed

    Chen, J C H; Sato, Y; Kosaka, R; Hashisaka, M; Muraki, K; Fujisawa, T

    2015-01-01

    Electron-phonon coupling is a major decoherence mechanism, which often causes scattering and energy dissipation in semiconductor electronic systems. However, this electron-phonon coupling may be used in a positive way for reaching the strong or ultra-strong coupling regime in an acoustic version of the cavity quantum electrodynamic system. Here we propose and demonstrate a phonon cavity for surface acoustic waves, which is made of periodic metal fingers that constitute Bragg reflectors on a GaAs/AlGaAs heterostructure. Phonon band gap and cavity phonon modes are identified by frequency, time and spatially resolved measurements of the piezoelectric potential. Tunneling spectroscopy on a double quantum dot indicates the enhancement of phonon assisted transitions in a charge qubit. This encourages studying of acoustic cavity quantum electrodynamics with surface phonons. PMID:26469629

  10. Fabrication of capacitively-shunted superconducting qubits

    NASA Astrophysics Data System (ADS)

    Yoder, Jonilyn L.; Gudmundsen, Theodore J.; Bolkhovsky, Vladimir; Welander, Paul B.; Gustavsson, Simon; Hover, David; Kerman, Andrew J.; Sears, Adam P.; Oliver, William D.

    2014-03-01

    Improvements in superconducting qubit coherence times and reproducibility have been demonstrated using capacitive shunting. In this study, we present methods for the preparation of both capacitively-shunted charge qubits (transmons) and capacitively-shunted flux qubits. Hybrid fabrication techniques were employed to combine high-quality-factor aluminum capacitive shunts with shadow-evaporated Josephson junctions, and the Josephson junctions were prepared using suspended-bridge germanium masks. We also will describe process testing results that were acquired to assess wafer-to-wafer reproducibility of our fabrication protocols. This research was funded in part by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA); and by the Assistant Secretary of Defense for Research and Engineering under Air Force Contract number FA8721-05-C-0002. All statements of fact, opinion or conclusions contained herein are those of the authors and should not be construed as representing the official views or policies of IARPA, the ODNI, or the U.S. Government.

  11. Charge-Insensitive Single-Atom Spin-Orbit Qubit in Silicon.

    PubMed

    Salfi, Joe; Mol, Jan A; Culcer, Dimitrie; Rogge, Sven

    2016-06-17

    High fidelity entanglement of an on-chip array of spin qubits poses many challenges. Spin-orbit coupling (SOC) can ease some of these challenges by enabling long-ranged entanglement via electric dipole-dipole interactions, microwave photons, or phonons. However, SOC exposes conventional spin qubits to decoherence from electrical noise. Here, we propose an acceptor-based spin-orbit qubit in silicon offering long-range entanglement at a sweet spot where the qubit is protected from electrical noise. The qubit relies on quadrupolar SOC with the interface and gate potentials. As required for surface codes, 10^{5} electrically mediated single-qubit and 10^{4} dipole-dipole mediated two-qubit gates are possible in the predicted spin lifetime. Moreover, circuit quantum electrodynamics with single spins is feasible, including dispersive readout, cavity-mediated entanglement, and spin-photon entanglement. An industrially relevant silicon-based platform is employed. PMID:27367400

  12. Charge-Insensitive Single-Atom Spin-Orbit Qubit in Silicon

    NASA Astrophysics Data System (ADS)

    Salfi, Joe; Mol, Jan A.; Culcer, Dimitrie; Rogge, Sven

    2016-06-01

    High fidelity entanglement of an on-chip array of spin qubits poses many challenges. Spin-orbit coupling (SOC) can ease some of these challenges by enabling long-ranged entanglement via electric dipole-dipole interactions, microwave photons, or phonons. However, SOC exposes conventional spin qubits to decoherence from electrical noise. Here, we propose an acceptor-based spin-orbit qubit in silicon offering long-range entanglement at a sweet spot where the qubit is protected from electrical noise. The qubit relies on quadrupolar SOC with the interface and gate potentials. As required for surface codes, 105 electrically mediated single-qubit and 104 dipole-dipole mediated two-qubit gates are possible in the predicted spin lifetime. Moreover, circuit quantum electrodynamics with single spins is feasible, including dispersive readout, cavity-mediated entanglement, and spin-photon entanglement. An industrially relevant silicon-based platform is employed.

  13. The C-shunt Flux Qubit: A New Generation of Superconducting Flux Qubit

    NASA Astrophysics Data System (ADS)

    Birenbaum, Jeffrey Scott

    While quantum computation has the potential to revolutionize the scientific community, to date no architecture has been developed which offers the necessary combination of high coherence times and massive scalability. Superconducting flux qubits satisfy the second requirement well but to date useful devices are limited to coherence times of typically only a few mus. In this dissertation we examine the possibilities of improving the coherence performance of the flux qubit to the levels required for fault-tolerant quantum computation. We find that coherence times for many devices are limited by photon-induced quasiparticles and mitigation of these quasiparticles increases coherence times by more than a factor of two. Beyond this, however, we find little improvement in flux qubit performance compared to prior results. Despite improved fabrication techniques and varied device designs we find flux qubit coherence times are still typically below 5 mus. Furthermore, wide device-to-device variations are observed which prevent effective scaling of the flux qubit to quantum information circuits. Based on the proposal by You, et al. we develop of a capacitively-shunted version of the flux qubit called the C-shunt flux qubit. With the addition of a capacitive shunt across the small junction of the flux qubit we are able to reduce the amplitude sensitivity to both charge and flux noise by more than a factor of three. The result is a predicted ten-fold enhancement in the coherence times compared to the unshunted flux qubit. At the same time we preserve much of the anharmonicity of the flux qubit resulting in a device with coherence times comparable to modern transmons but with a factor of four better anharmonicity and more flexible coupling configurations. By using a high-quality MBE aluminum shunt process on an annealed sapphire substrate coupled with a more conventional electron-beam-evaporated aluminum Josephson junction process we fabricate hybrid C-shunt flux qubits. We

  14. Fluxon readout of a superconducting qubit.

    PubMed

    Fedorov, Kirill G; Shcherbakova, Anastasia V; Wolf, Michael J; Beckmann, Detlef; Ustinov, Alexey V

    2014-04-25

    An experiment demonstrating a link between classical single-flux quantum digital logic and a superconducting quantum circuit is reported. We implement coupling between a moving Josephson vortex (fluxon) and a flux qubit by reading out of a state of the flux qubit through a frequency shift of the fluxon oscillations in an annular Josephson junction. The energy spectrum of the flux qubit is measured using this technique. The implemented hybrid scheme opens an opportunity to readout quantum states of superconducting qubits with the classical fluxon logic circuits. PMID:24815629

  15. Fluxon Readout of a Superconducting Qubit

    NASA Astrophysics Data System (ADS)

    Fedorov, Kirill G.; Shcherbakova, Anastasia V.; Wolf, Michael J.; Beckmann, Detlef; Ustinov, Alexey V.

    2014-04-01

    An experiment demonstrating a link between classical single-flux quantum digital logic and a superconducting quantum circuit is reported. We implement coupling between a moving Josephson vortex (fluxon) and a flux qubit by reading out of a state of the flux qubit through a frequency shift of the fluxon oscillations in an annular Josephson junction. The energy spectrum of the flux qubit is measured using this technique. The implemented hybrid scheme opens an opportunity to readout quantum states of superconducting qubits with the classical fluxon logic circuits.

  16. Method for observing robust and tunable phonon blockade in a nanomechanical resonator coupled to a charge qubit

    NASA Astrophysics Data System (ADS)

    Wang, Xin; Miranowicz, Adam; Li, Hong-Rong; Nori, Franco

    2016-06-01

    Phonon blockade is a purely quantum phenomenon, analogous to Coulomb and photon blockades, in which a single phonon in an anharmonic mechanical resonator can impede the excitation of a second phonon. We propose an experimental method to realize phonon blockade in a driven harmonic nanomechanical resonator coupled to a qubit, where the coupling is proportional to the second-order nonlinear susceptibility χ(2 ). This is in contrast to the standard realizations of phonon and photon blockade effects in Kerr-type χ(3 ) nonlinear systems. The nonlinear coupling strength can be adjusted conveniently by changing the coherent drive field. As an example, we apply this model to predict and describe phonon blockade in a nanomechanical resonator coupled to a Cooper-pair box (i.e., a charge qubit) with a linear longitudinal coupling. By obtaining the solutions of the steady state for this composite system, we give the conditions for observing strong antibunching and sub-Poissonian phonon-number statistics in this induced second-order nonlinear system. Besides using the qubit to produce phonon blockade states, the qubit itself can also be employed to detect blockade effects by measuring its states. Numerical simulations indicate that the robustness of the phonon blockade, and the sensitivity of detecting it, will benefit from this strong induced nonlinear coupling.

  17. Superconducting nanocircuits for topologically protected qubits

    NASA Astrophysics Data System (ADS)

    Gladchenko, Sergey; Olaya, David; Dupont-Ferrier, Eva; Douçot, Benoit; Ioffe, Lev B.; Gershenson, Michael E.

    2009-01-01

    For successful realization of a quantum computer, its building blocks-the individual qubits-should be simultaneously scalable and sufficiently protected from environmental noise. Recently, a novel approach to the protection of superconducting qubits has been proposed. The idea is to prevent errors at the hardware level, by building a fault-free logical qubit from `faulty' physical qubits with properly engineered interactions between them. The decoupling of such a topologically protected logical qubit from local noises is expected to grow exponentially with the number of physical qubits. Here, we report on proof-of-concept experiments with a prototype device that consists of twelve physical qubits made of nanoscale Josephson junctions. We observed that owing to properly tuned quantum fluctuations, this qubit is protected against magnetic flux variations well beyond linear order, in agreement with theoretical predictions. These results suggest that topologically protected superconducting qubits are feasible.

  18. Charge Qubit Coupled to an Intense Microwave Electromagnetic Field in a Superconducting Nb Device: Evidence for Photon-Assisted Quasiparticle Tunneling

    NASA Astrophysics Data System (ADS)

    de Graaf, S. E.; Leppäkangas, J.; Adamyan, A.; Danilov, A. V.; Lindström, T.; Fogelström, M.; Bauch, T.; Johansson, G.; Kubatkin, S. E.

    2013-09-01

    We study a superconducting charge qubit coupled to an intensive electromagnetic field and probe changes in the resonance frequency of the formed dressed states. At large driving strengths, exceeding the qubit energy-level splitting, this reveals the well known Landau-Zener-Stückelberg interference structure of a longitudinally driven two-level system. For even stronger drives, we observe a significant change in the Landau-Zener-Stückelberg pattern and contrast. We attribute this to photon-assisted quasiparticle tunneling in the qubit. This results in the recovery of the qubit parity, eliminating effects of quasiparticle poisoning, and leads to an enhanced interferometric response. The interference pattern becomes robust to quasiparticle poisoning and has a good potential for accurate charge sensing.

  19. Charge qubit coupled to an intense microwave electromagnetic field in a superconducting Nb device: evidence for photon-assisted quasiparticle tunneling.

    PubMed

    de Graaf, S E; Leppäkangas, J; Adamyan, A; Danilov, A V; Lindström, T; Fogelström, M; Bauch, T; Johansson, G; Kubatkin, S E

    2013-09-27

    We study a superconducting charge qubit coupled to an intensive electromagnetic field and probe changes in the resonance frequency of the formed dressed states. At large driving strengths, exceeding the qubit energy-level splitting, this reveals the well known Landau-Zener-Stückelberg interference structure of a longitudinally driven two-level system. For even stronger drives, we observe a significant change in the Landau-Zener-Stückelberg pattern and contrast. We attribute this to photon-assisted quasiparticle tunneling in the qubit. This results in the recovery of the qubit parity, eliminating effects of quasiparticle poisoning, and leads to an enhanced interferometric response. The interference pattern becomes robust to quasiparticle poisoning and has a good potential for accurate charge sensing. PMID:24116809

  20. Qubit readout with a directional parametric amplifier

    NASA Astrophysics Data System (ADS)

    Sliwa, K. M.; Abdo, B.; Narla, A.; Shankar, S.; Hatridge, M.; Frunzio, L.; Schoelkopf, R. J.; Devoret, M. H.

    2014-03-01

    Josephson junction based quantum limited parametric amplifiers play an essential role in superconducting qubit measurements. These measurements necessitate circulators and isolators between the amplifier and qubit to add directionality and/or isolation. Unfortunately, this extra hardware limits both quantum measurement efficiency and experimental scalability. Here we present a quantum-limited Josephson-junction-based directional amplifier (JDA) based on a novel coupling between two nominally identical Josephson parametric converters (JPCs). The device achieves a forward gain of 11 dB with a 15 MHz dynamical bandwidth, but higher gains are possible at the expense of bandwidth. We also present measurements of a transmon qubit made with the JDA, and show minimal measurement back-action despite the absence of any isolator or circulator before the amplifier. These results provide a first step toward realizing on-chip integration of qubits and parametric amplifiers. Work supported by: IARPA, ARO, and NSF.

  1. Gatemon Benchmarking and Two-Qubit Operations

    NASA Astrophysics Data System (ADS)

    Casparis, L.; Larsen, T. W.; Olsen, M. S.; Kuemmeth, F.; Krogstrup, P.; Nygârd, J.; Petersson, K. D.; Marcus, C. M.

    2016-04-01

    Recent experiments have demonstrated superconducting transmon qubits with semiconductor nanowire Josephson junctions. These hybrid gatemon qubits utilize field effect tunability characteristic of semiconductors to allow complete qubit control using gate voltages, potentially a technological advantage over conventional flux-controlled transmons. Here, we present experiments with a two-qubit gatemon circuit. We characterize qubit coherence and stability and use randomized benchmarking to demonstrate single-qubit gate errors below 0.7% for all gates, including voltage-controlled Z rotations. We show coherent capacitive coupling between two gatemons and coherent swap operations. Finally, we perform a two-qubit controlled-phase gate with an estimated fidelity of 91%, demonstrating the potential of gatemon qubits for building scalable quantum processors.

  2. Gatemon Benchmarking and Two-Qubit Operation

    NASA Astrophysics Data System (ADS)

    Casparis, Lucas; Larsen, Thorvald; Olsen, Michael; Petersson, Karl; Kuemmeth, Ferdinand; Krogstrup, Peter; Nygard, Jesper; Marcus, Charles

    Recent experiments have demonstrated superconducting transmon qubits with semiconductor nanowire Josephson junctions. These hybrid gatemon qubits utilize field effect tunability singular to semiconductors to allow complete qubit control using gate voltages, potentially a technological advantage over conventional flux-controlled transmons. Here, we present experiments with a two-qubit gatemon circuit. We characterize qubit coherence and stability and use randomized benchmarking to demonstrate single-qubit gate errors of ~0.5 % for all gates, including voltage-controlled Z rotations. We show coherent capacitive coupling between two gatemons and coherent SWAP operations. Finally, we perform a two-qubit controlled-phase gate with an estimated fidelity of ~91 %, demonstrating the potential of gatemon qubits for building scalable quantum processors. We acknowledge financial support from Microsoft Project Q and the Danish National Research Foundation.

  3. Gatemon Benchmarking and Two-Qubit Operations.

    PubMed

    Casparis, L; Larsen, T W; Olsen, M S; Kuemmeth, F; Krogstrup, P; Nygård, J; Petersson, K D; Marcus, C M

    2016-04-15

    Recent experiments have demonstrated superconducting transmon qubits with semiconductor nanowire Josephson junctions. These hybrid gatemon qubits utilize field effect tunability characteristic of semiconductors to allow complete qubit control using gate voltages, potentially a technological advantage over conventional flux-controlled transmons. Here, we present experiments with a two-qubit gatemon circuit. We characterize qubit coherence and stability and use randomized benchmarking to demonstrate single-qubit gate errors below 0.7% for all gates, including voltage-controlled Z rotations. We show coherent capacitive coupling between two gatemons and coherent swap operations. Finally, we perform a two-qubit controlled-phase gate with an estimated fidelity of 91%, demonstrating the potential of gatemon qubits for building scalable quantum processors. PMID:27127949

  4. Composite pulses robust against charge noise and magnetic field noise for universal control of a singlet-triplet qubit

    NASA Astrophysics Data System (ADS)

    Wang, Xin; Barnes, Edwin; Kestner, Jason P.; Bishop, Lev S.; Das Sarma, Sankar

    2013-03-01

    We generalize our SUPCODE pulse sequences for singlet-triplet qubits to correct errors from imperfect control. This yields gates that are simultaneously corrected for both charge noise and magnetic field gradient fluctuations, addressing the two dominant T2* processes. By using this more efficient version of SUPCODE, we are able to introduce this capability while also substantially reducing the overall pulse time compared to the previous sequence. We show that our sequence remains realistic under experimental constraints such as finite bandwidth. This work is supported by LPS-NSA-CMTC, IARPA-MQCO and CNAM.

  5. Direct measurement of the concurrence for two-qubit electron spin entangled pure state based on charge detection

    NASA Astrophysics Data System (ADS)

    Liu, Jiong; Zhou, Lan; Sheng, Yu-Bo

    2015-07-01

    We propose a protocol for directly measuring the concurrence of a two-qubit electronic pure entangled state. To complete this task, we first design a parity-check measurement (PCM) which is constructed by two polarization beam splitters (PBSs) and a charge detector. By using the PCM for three rounds, we can achieve the concurrence by calculating the total probability of picking up the odd parity states from the initial states. Since the conduction electron may be a good candidate for the realization of quantum computation, this protocol may be useful in future solid quantum computation. Project supported by the National Natural Science Foundation of China (Grant Nos. 11474168 and 61401222), the Qing Lan Project in Jiangsu Province, China, and the Priority Academic Development Program of Jiangsu Higher Education Institutions, China.

  6. Restoring interlayer Josephson coupling in La1.885Ba0.115CuO4 by charge transfer melting of stripe order

    NASA Astrophysics Data System (ADS)

    Khanna, V.; Mankowsky, R.; Petrich, M.; Bromberger, H.; Cavill, S. A.; Möhr-Vorobeva, E.; Nicoletti, D.; Laplace, Y.; Gu, G. D.; Hill, J. P.; Först, M.; Cavalleri, A.; Dhesi, S. S.

    2016-06-01

    We show that disruption of charge-density-wave (stripe) order by charge transfer excitation, enhances the superconducting phase rigidity in La1.885Ba0.115CuO4 . Time-resolved resonant soft x-ray diffraction demonstrates that charge order melting is prompt following near-infrared photoexcitation whereas the crystal structure remains intact for moderate fluences. THz time-domain spectroscopy reveals that, for the first 2 ps following photoexcitation, a new Josephson plasma resonance edge, at higher frequency with respect to the equilibrium edge, is induced indicating enhanced superconducting interlayer coupling. The fluence dependence of the charge-order melting and the enhanced superconducting interlayer coupling are correlated with a saturation limit of ˜0.5 mJ /cm2 . Using a combination of x-ray and optical spectroscopies we establish a hierarchy of timescales between enhanced superconductivity, melting of charge order, and rearrangement of the crystal structure.

  7. Measurement of Quantum Phase-Slips in Josephson Junction Chains

    NASA Astrophysics Data System (ADS)

    Guichard, Wiebke

    2011-03-01

    Quantum phase-slip dynamics in Josephson junction chains could provide the basis for the realization of a new type of topologically protected qubit or for the implementation of a new current standard. I will present measurements of the effect of quantum phase-slips on the ground state of a Josephson junction chain. We can tune in situ the strength of the phase-slips. These phase-slips are the result of fluctuations induced by the finite charging energy of each junction in the chain. Our measurements demonstrate that a Josephson junction chain under phase bias constraint behaves in a collective way. I will also show evidence of coherent phase-slip interference, the so called Aharonov-Casher effect. This phenomenon is the dual of the well known Aharonov-Bohm interference. In collaboration with I.M. Pop, Institut Neel, C.N.R.S. and Universite Joseph Fourier, BP 166, 38042 Grenoble, France; I. Protopopov, L. D. Landau Institute for Theoretical Physics, Kosygin str. 2, Moscow 119334, Russia and Institut fuer Nanotechnologie, Karlsruher Institut fuer Technologie, 76021 Karlsruhe, Germany; and F. Lecocq, Z. Peng, B. Pannetier, O. Buisson, Institut Neel, C.N.R.S. and Universite Joseph Fourier. European STREP MIDAS, ANR QUANTJO.

  8. Fast Charge Sensing of a Cavity-Coupled Double Quantum Dot Using a Josephson Parametric Amplifier

    NASA Astrophysics Data System (ADS)

    Stehlik, J.; Liu, Y.-Y.; Quintana, C. M.; Eichler, C.; Hartke, T. R.; Petta, J. R.

    2015-07-01

    We demonstrate fast readout of a double quantum dot (DQD) that is coupled to a superconducting resonator. Utilizing a parametric amplifier beyond its range of linear amplification, we improve the signal-to-noise ratio (SNR) by a factor of 2000 compared to the situation with the parametric amplifier turned off. With an integration time of 400 ns comparable to the inverse effective bandwidth, we achieve a SNR of 76. By measuring the SNR as a function of the integration time, we extract an equivalent charge sensitivity of 8 ×10-5 e /√{Hz } . The high SNR allows us to acquire a DQD charge-stability diagram in just 20 ms. At such a high data rate, it is possible to acquire charge-stability diagrams in a live "video mode," enabling real-time tuning of the DQD confinement potential.

  9. Josephson junction

    DOEpatents

    Wendt, Joel R.; Plut, Thomas A.; Martens, Jon S.

    1995-01-01

    A novel method for fabricating nanometer geometry electronic devices is described. Such Josephson junctions can be accurately and reproducibly manufactured employing photolithographic and direct write electron beam lithography techniques in combination with aqueous etchants. In particular, a method is described for manufacturing planar Josephson junctions from high temperature superconducting material.

  10. Josephson junction

    DOEpatents

    Wendt, J.R.; Plut, T.A.; Martens, J.S.

    1995-05-02

    A novel method for fabricating nanometer geometry electronic devices is described. Such Josephson junctions can be accurately and reproducibly manufactured employing photolithographic and direct write electron beam lithography techniques in combination with aqueous etchants. In particular, a method is described for manufacturing planar Josephson junctions from high temperature superconducting material. 10 figs.

  11. Simultaneous monitoring of fluxonium qubits in a waveguide

    NASA Astrophysics Data System (ADS)

    Kou, A.; Smith, W. C.; Vool, U.; Pop, I. M.; Sliwa, K. M.; Hatridge, M.; Schoelkopf, R. J.; Devoret, M. H.

    Building quantum computers and quantum simulators requires separate control and readout of multiple qubits. We present an architecture for multiplexed readout of fluxonium qubits. We measured lifetimes in excess of 100 us for such artificial atoms placed in a wide-bandwidth electromagnetic environment. We use cascaded Josephson parametric converters to measure the quantum jumps of two fluxonium qubits simultaneously. Our method can access correlations between different qubits and can easily be scaled to read out larger numbers of qubits. Work supported by: ARO, ONR, AFOSR, and YINQE.

  12. Qubit dephasing due to quasiparticle tunneling

    NASA Astrophysics Data System (ADS)

    Zanker, Sebastian; Marthaler, Michael

    2015-05-01

    We study dephasing of a superconducting qubit due to quasiparticle tunneling through a Josephson junction. While qubit decay due to tunneling processes is well understood within a golden rule approximation, pure dephasing due to BCS quasiparticles gives rise to a divergent golden rule rate. We calculate qubit dephasing due to quasiparticle tunneling beyond lowest-order approximation in coupling between qubit and quasiparticles. Summing up a certain class of diagrams, we show that qubit dephasing due to purely longitudinal coupling to quasiparticles leads to dephasing ˜exp[-x (t )] where x (t ) ∝t3 /2 for short-time scales and x (t )∝t ln(t ) for long-time scales.

  13. Design of a ballistic fluxon qubit readout

    NASA Astrophysics Data System (ADS)

    Herr Kidiyarova-Shevchenko, Anna; Fedorov, Arkady; Shnirman, Alexander; Il'ichev, Evgeny; Schön, Gerd

    2007-11-01

    A detailed design is given for a flux qubit readout using ballistic fluxons. In this scheme, fluxons propagate through an underdamped Josephson transmission line (JTL) coupled to the qubit, whose state affects the fluxon propagation time. For strong qubit-JTL coupling, and far from the symmetry point, a qubit can be measured with fidelity greater than 99% and measurement time of 4 ns. The readout circuit requires additional rapid single flux quantum (RSFQ) interface circuitry to launch and receive the delayed flux solitons. The parameters of this driver and receiver have been optimized to produce low fluxon speed at launch and impedance matching at the receiver. The delayed solitons are compared to a reference line using a detector with time resolution of better than 16 ps. Both the JTL and RSFQ interface were designed for the Nb 30 A cm-2 process developed at VTT, Finland, with postdeposition of the Al qubit at IPHT, Germany.

  14. Reduced sensitivity to charge noise in semiconductor spin qubits via symmetric operation

    NASA Astrophysics Data System (ADS)

    Reed, Matthew

    Gated semiconductor quantum dots controlled with the exchange interaction are attractive candidates for quantum information processing because of their long coherence time and electrical controllability. Exchange is conventionally modulated by detuning the chemical potentials of neighboring dots over a fixed tunnel barrier, an approach whose precision is limited by charge noise. In this talk we demonstrate a ''symmetric'' mode of operation which substantially reduces the sensitivity of exchange operations to gate fluctuations. The method involves biasing a double-dot symmetrically between the charge-state anti-crossings, where the derivative of the exchange energy with respect to gate voltages is minimized. Exchange remains highly tunable by adjusting the tunnel coupling. We propose a metric, insensitivity, to quantify the technique's improvement and find that it increases by at least a factor of five between operating regimes. We also demonstrate a substantial increase in the number of Rabi fringes observed.

  15. Disordered graphene Josephson junctions

    NASA Astrophysics Data System (ADS)

    Muñoz, W. A.; Covaci, L.; Peeters, F. M.

    2015-02-01

    A tight-binding approach based on the Chebyshev-Bogoliubov-de Gennes method is used to describe disordered single-layer graphene Josephson junctions. Scattering by vacancies, ripples, or charged impurities is included. We compute the Josephson current and investigate the nature of multiple Andreev reflections, which induce bound states appearing as peaks in the density of states for energies below the superconducting gap. In the presence of single-atom vacancies, we observe a strong suppression of the supercurrent, which is a consequence of strong intervalley scattering. Although lattice deformations should not induce intervalley scattering, we find that the supercurrent is still suppressed, which is due to the presence of pseudomagnetic barriers. For charged impurities, we consider two cases depending on whether the average doping is zero, i.e., existence of electron-hole puddles, or finite. In both cases, short-range impurities strongly affect the supercurrent, similar to the vacancies scenario.

  16. Tunable current-phase relation in double-dot Josephson junctions

    NASA Astrophysics Data System (ADS)

    Koch, Jens; Le Hur, Karyn

    2008-03-01

    The current-phase relation I() for a Josephson junction contains information about the microscopic nature of the Cooper pair transfer. In particular, junctions more complicated than the single tunnel junction exhibit characteristic non-sinusoidal forms. Here, we investigate the Josephson effect in a superconducting double dot device, similar to the devices studied experimentally by Y. A. Pashkin et al. [1] and E. Bibow et al. [2]. In the vicinity of a charge degeneracy line, the system reduces to a two-level system equivalent to a charge qubit. In this regime, we find that the interplay between sequential tunneling and cotunneling of Cooper pairs leads to a strongly non-sinusoidal current- phase relation, tunable via gate electrodes. We propose the measurement of I() in a SQUID configuration, analyze the implications of flux noise, and compare our results to different types of Josephson junctions such as single-dot systems and microbridges. [1] Y. A. Pashkin et al., Nature (London) 421 (2003), 823 [2] E. Bibow, P. Lafarge, L. L'evy, Phys. Rev. Lett. 88 (2002), 017003

  17. Wrapped branes as qubits.

    PubMed

    Borsten, L; Dahanayake, D; Duff, M J; Ebrahim, H; Rubens, W

    2008-06-27

    Recent work has established a correspondence between the tripartite entanglement measure of three qubits and the macroscopic entropy of the four-dimensional 8-charge STU black hole of supergravity. Here we consider the configurations of intersecting D3-branes, whose wrapping around the six compact dimensions T6 provides the microscopic string-theoretic interpretation of the charges, and associate the three-qubit basis vectors |ABC>, (A, B, C=0 or 1) with the corresponding 8 wrapping cycles. In particular, we relate a well-known fact of quantum information theory, that the most general real three-qubit state can be parameterized by four real numbers and an angle, to a well-known fact of string theory, that the most general STU black hole can be described by four D3-branes intersecting at an angle. PMID:18643650

  18. dc SQUID Phase Qubit with Sub-Micron Junction and Interdigitated Capacitor

    NASA Astrophysics Data System (ADS)

    Przybysz, Anthony; Kwon, H.; Crowe, E.; Cooper, B. K.; Budoyo, R.; Mitra, K.; Anderson, J. R.; Lobb, C. J.; Dragt, A. J.; Wellstood, F. C.; Gladchenko, S.; Zaretsky, V.; Kim, Z.; Palmer, B.; Osborn, K.

    2010-03-01

    We have designed an Al/AlOx/Al dc SQUID phase qubit on sapphire that minimizes the effects from sources of loss and dephasing, with the goal of reaching a coherence time of 10 micro-seconds. Loss from the Josephson junction's tunnel barrier and other neighboring dielectric layers is believed to be the dominant sources of decoherence in most phase qubits. To minimize the number of charge two-level systems in the barrier and the effect of dielectric loss, our phase qubit employs a 300 x 300 nm junction with a designed critical current of 150 nA and a 1 pF interdigitated capacitor that is added in parallel. The capacitor is made on the sapphire substrate and has 100 fingers that are about 1.2 microns wide with a 1.2 micron spacing between them. To minimize loss from the bias leads, the qubit is isolated from the leads by a tunable inductive isolation network and an on-chip LC filter. We will discuss the design as well as on-going research into the effect that these parameters have on the coherence time of such a device.

  19. SCB Quantum Computers Using iSWAP and 1-Qubit Rotations

    NASA Technical Reports Server (NTRS)

    Williams, Colin; Echtemach, Pierre

    2005-01-01

    Units of superconducting circuitry that exploit the concept of the single- Cooper-pair box (SCB) have been built and are undergoing testing as prototypes of logic gates that could, in principle, constitute building blocks of clocked quantum computers. These units utilize quantized charge states as the quantum information-bearing degrees of freedom. An SCB is an artificial two-level quantum system that comprises a nanoscale superconducting electrode connected to a reservoir of Cooper-pair charges via a Josephson junction. The logical quantum states of the device, .0. and .1., are implemented physically as a pair of charge-number states that differ by 2e (where e is the charge of an electron). Typically, some 109 Cooper pairs are involved. Transitions between the logical states are accomplished by tunneling of Cooper pairs through the Josephson junction. Although the two-level system contains a macroscopic number of charges, in the superconducting regime, they behave collectively, as a Bose-Einstein condensate, making possible a coherent superposition of the two logical states. This possibility makes the SCB a candidate for the physical implementation of a qubit. A set of quantum logic operations and the gates that implement them is characterized as universal if, in principle, one can form combinations of the operations in the set to implement any desired quantum computation. To be able to design a practical quantum computer, one must first specify how to decompose any valid quantum computation into a sequence of elementary 1- and 2-qubit quantum gates that are universal and that can be realized in hardware that is feasible to fabricate. Traditionally, the set of universal gates has been taken to be the set of all 1-qubit quantum gates in conjunction with the controlled-NOT (CNOT) gate, which is a 2-qubit gate. Also, it has been known for some time that the SWAP gate, which implements square root of the simple 2-qubit exchange interaction, is as computationally

  20. Solitons in Josephson junctions

    NASA Astrophysics Data System (ADS)

    Ustinov, A. V.

    1998-11-01

    Magnetic flux quanta in Josephson junctions, often called fluxons, in many cases behave as solitons. A review of recent experiments and modelling of fluxon dynamics in Josephson circuits is presented. Classic quasi-one-dimensional junctions, stacked junctions (Josephson superlattices), and discrete Josephson transmission lines (JTLs) are discussed. Applications of fluxon devices as high-frequency oscillators and digital circuits are also addressed.

  1. Superfluid qubit systems with ring shaped optical lattices

    PubMed Central

    Amico, Luigi; Aghamalyan, Davit; Auksztol, Filip; Crepaz, Herbert; Dumke, Rainer; Kwek, Leong Chuan

    2014-01-01

    We study an experimentally feasible qubit system employing neutral atomic currents. Our system is based on bosonic cold atoms trapped in ring-shaped optical lattice potentials. The lattice makes the system strictly one dimensional and it provides the infrastructure to realize a tunable ring-ring interaction. Our implementation combines the low decoherence rates of neutral cold atoms systems, overcoming single site addressing, with the robustness of topologically protected solid state Josephson flux qubits. Characteristic fluctuations in the magnetic fields affecting Josephson junction based flux qubits are expected to be minimized employing neutral atoms as flux carriers. By breaking the Galilean invariance we demonstrate how atomic currents through the lattice provide an implementation of a qubit. This is realized either by artificially creating a phase slip in a single ring, or by tunnel coupling of two homogeneous ring lattices. The single qubit infrastructure is experimentally investigated with tailored optical potentials. Indeed, we have experimentally realized scaled ring-lattice potentials that could host, in principle, n ~ 10 of such ring-qubits, arranged in a stack configuration, along the laser beam propagation axis. An experimentally viable scheme of the two-ring-qubit is discussed, as well. Based on our analysis, we provide protocols to initialize, address, and read-out the qubit. PMID:24599096

  2. Silicon based quantum dot hybrid qubits

    NASA Astrophysics Data System (ADS)

    Kim, Dohun

    2015-03-01

    The charge and spin degrees of freedom of an electron constitute natural bases for constructing quantum two level systems, or qubits, in semiconductor quantum dots. The quantum dot charge qubit offers a simple architecture and high-speed operation, but generally suffers from fast dephasing due to strong coupling of the environment to the electron's charge. On the other hand, quantum dot spin qubits have demonstrated long coherence times, but their manipulation is often slower than desired for important future applications. This talk will present experimental progress of a `hybrid' qubit, formed by three electrons in a Si/SiGe double quantum dot, which combines desirable characteristics (speed and coherence) in the past found separately in qubits based on either charge or spin degrees of freedom. Using resonant microwaves, we first discuss qubit operations near the `sweet spot' for charge qubit operation. Along with fast (>GHz) manipulation rates for any rotation axis on the Bloch sphere, we implement two independent tomographic characterization schemes in the charge qubit regime: traditional quantum process tomography (QPT) and gate set tomography (GST). We also present resonant qubit operations of the hybrid qubit performed on the same device, DC pulsed gate operations of which were recently demonstrated. We demonstrate three-axis control and the implementation of dynamic decoupling pulse sequences. Performing QPT on the hybrid qubit, we show that AC gating yields π rotation process fidelities higher than 93% for X-axis and 96% for Z-axis rotations, which demonstrates efficient quantum control of semiconductor qubits using resonant microwaves. We discuss a path forward for achieving fidelities better than the threshold for quantum error correction using surface codes. This work was supported in part by ARO (W911NF-12-0607), NSF (PHY-1104660), DOE (DE-FG02-03ER46028), and by the Laboratory Directed Research and Development program at Sandia National Laboratories

  3. Manipulating the Majorana qubit with Landau-Zener-Stückelberg interference

    NASA Astrophysics Data System (ADS)

    Huang, Wen-Chao; Liang, Qi-Feng; Yao, Dao-Xin; Wang, Zhi

    2015-07-01

    Constructing a universal operation scheme for Majorana qubits remains a central issue for the topological quantum computation. We study the Landau-Zener-Stückelberg interference in a Majorana qubit and show that this interference can be used to achieve controllable operations. The Majorana qubit consists of an rf superconducting quantum interference device (SQUID) with a topological nanowire Josephson junction which hosts Majorana bound states. In the SQUID, a magnetic flux pulse can drive the quantum evolution of the Majorana qubit. The qubit experiences two Landau-Zener transitions when the amplitude of the pulse is tuned around the superconducting flux quanta h /2 e . The Landau-Zener-Stückelberg interference between the two transitions rotates the Majorana qubit, with the angle controlled by the time scale of the pulse. This rotation operation implements a high-speed single-qubit gate on the Majorana qubit, which is a necessary ingredient for the topological quantum computation.

  4. Qubit-Based Memcapacitors and Meminductors

    NASA Astrophysics Data System (ADS)

    Shevchenko, Sergey N.; Pershin, Yuriy V.; Nori, Franco

    2016-07-01

    It is shown that superconducting charge and flux quantum bits (qubits) can be classified as memory capacitive and inductive systems, respectively. We demonstrate that such memcapacitive and meminductive devices offer remarkable and rich response functionalities. In particular, when subjected to periodic input, qubit-based memcapacitors and meminductors exhibit unusual hysteresis curves. Our work not only extends the set of known memcapacitive and meminductive systems to qubit-based devices, but also highlights their unique properties potentially useful for future technological applications.

  5. Semiconductor-Nanowire-Based Superconducting Qubit.

    PubMed

    Larsen, T W; Petersson, K D; Kuemmeth, F; Jespersen, T S; Krogstrup, P; Nygård, J; Marcus, C M

    2015-09-18

    We introduce a hybrid qubit based on a semiconductor nanowire with an epitaxially grown superconductor layer. Josephson energy of the transmonlike device ("gatemon") is controlled by an electrostatic gate that depletes carriers in a semiconducting weak link region. Strong coupling to an on-chip microwave cavity and coherent qubit control via gate voltage pulses is demonstrated, yielding reasonably long relaxation times (~0.8 μs) and dephasing times (~1 μs), exceeding gate operation times by 2 orders of magnitude, in these first-generation devices. Because qubit control relies on voltages rather than fluxes, dissipation in resistive control lines is reduced, screening reduces cross talk, and the absence of flux control allows operation in a magnetic field, relevant for topological quantum information. PMID:26431009

  6. Parametric amplification by coupled flux qubits

    SciTech Connect

    Rehák, M.; Neilinger, P.; Grajcar, M.; Oelsner, G.; Hübner, U.; Meyer, H.-G.; Il'ichev, E.

    2014-04-21

    We report parametric amplification of a microwave signal in a Kerr medium formed from superconducting qubits. Two mutually coupled flux qubits, embedded in the current antinode of a superconducting coplanar waveguide resonator, are used as a nonlinear element. Shared Josephson junctions provide the qubit-resonator coupling, resulting in a device with a tunable Kerr constant (up to 3 × 10{sup −3}) and a measured gain of about 20 dB. This arrangement represents a unit cell which can be straightforwardly extended to a quasi one-dimensional quantum metamaterial with large tunable Kerr nonlinearity, providing a basis for implementation of wide-band travelling wave parametric amplifiers.

  7. Normal Metal Quasiparticle Traps in 3D-Transmon Qubits

    NASA Astrophysics Data System (ADS)

    Burkhart, Luke D.; Gao, Yvonne Y.; Wang, Chen; Serniak, Kyle; de Lange, Gijs; Chu, Yiwen; Vool, Uri; Frunzio, Luigi; Devoret, Michel H.; Catelani, Gianluigi; Glazman, Leonid I.; Schoelkopf, Robert J.

    Quasiparticles are a known source of decoherence in Josephson-junction based superconducting qubits. While equilibrium quasiparticles should not be present in devices operated at dilution refrigeration temperatures well below the superconducting energy gap, non-thermal quasiparticles have been observed in many different superconducting qubits, including 3D-transmons and fluxonium qubits. Vortices induced by applied magnetic fields have been shown to improve non-equilibrium quasiparticle decay rates and improve coherence times by creating regions of the superconductor with vanishing energy gap, which act as quasiparticle traps. We aim to further mitigate quasiparticle-induced limits on coherence by engineering strong trapping via the introduction of normal metal to the superconducting qubit. In this talk, we present recent results regarding normal metal quasiparticle traps in 3D-transmon qubits. Work supported by ARO, A*STAR.

  8. High-Fidelity Measurements of Long-Lived Flux Qubits

    NASA Astrophysics Data System (ADS)

    Hover, David; Macklin, Chris; O'Brien, Kevin; Sears, Adam; Yoder, Jonilyn; Gudmundsen, Ted; Kerman, Jamie; Bolkhovsky, Vladimir; Tolpygo, Sergey; Fitch, George; Weir, Terry; Kamal, Archana; Gustavsson, Simon; Yan, Fei; Birenbaum, Jeff; Siddiqi, Irfan; Orlando, Terry; Clarke, John; Oliver, Will

    2015-03-01

    We report on high-fidelity dispersive measurements of a long-lived flux qubit using a Josephson superconducting traveling wave parametric amplifier (JTWPA). A capacitively shunted flux qubit that incorporates high-Q MBE aluminum will have longer relaxation and dephasing times when compared to a conventional flux qubit, while also maintaining the large anharmonicity necessary for complex gate operations. The JTWPA relies on a Josephson junction embedded transmission line to deliver broadband, nonreciprocal gain with large dynamic range. This research was funded in part by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA); and by the Assistant Secretary of Defense for Research & Engineering under Air Force Contract number FA8721-05-C-0002. All statements of fact, opinion or conclusions contained herein are those of the authors and should not be construed as representing the official views or policies of

  9. Optimization and characterization of protected Josephson circuits

    NASA Astrophysics Data System (ADS)

    Dupont-Ferrier, Eva; Gladchenko, Sergey; Ioffe, Lev; Gershenson, Michael

    2009-03-01

    Recently, it was proposed that small Josephson arrays can operate as superconducting qubits protected from local noises [1,2]. Here we present measurements of several optimized array designs. The read-out circuit for these arrays consists of an inductively-coupled DC SQUID, which helps to minimize perturbations of the system during measurement. We will discuss the current-phase characteristics of these arrays and their response to microwave radiation. Our results indicate that the scattering of Josephson junction parameters can be made small enough to implement the symmetry-protected superconducting qubits; our theoretical model [1] captures all essential features of real devices. 1. see e.g., B. Doucot and L.B. Ioffe, Phys. Rev. B 76, 214507 (2007) and references therein. 2. S. Gladchenko, D. Olaya, E. Dupont-Ferrier, B. Doucot, L.B. Ioffe, and M.E. Gershenson, ``Superconducting Nanocircuits for Topologically Protected Qubits'', arXiv:cond-mat/0802.2295, to be published in Nature Physics.

  10. Characterization of an exchange-based two-qubit gate for resonant exchange qubits

    NASA Astrophysics Data System (ADS)

    Wardrop, Matthew P.; Doherty, Andrew C.

    2016-02-01

    Resonant exchange qubits are a promising addition to the family of experimentally implemented encodings of single qubits using semiconductor quantum dots. We have shown previously that it ought to be straightforward to perform a CPHASE gate between two resonant exchange qubits with a single exchange pulse. This approach uses energy gaps to suppress leakage rather than conventional pulse sequences. In this paper we present analysis and simulations of our proposed two-qubit gate subject to charge and Overhauser field noise at levels observed in current experiments. Our main result is that we expect implementations of our two-qubit gate to achieve high fidelities, with errors at the percent level and gate times comparable to single-qubit operations. As such, exchange-coupled resonant exchange qubits remain an attractive approach for quantum computing.

  11. Noise Charactoristics of the Josephson Amplifiers by Stochastic Calulus

    NASA Astrophysics Data System (ADS)

    Liu, Wenshuo; McDermott, Robert; Vavilov, Maxim

    We present theoretical studies of the noise performance of non-reciprocal gain elements based on Josephson junctions including the SQUID and the SLUG. We develop a perturbative approach by means of stochastic calculus which combines both analytical and numerical methods, and calculate the noise characteristics of the amplifiers in the thermal regime. We show that noise in the amplifiers originates mainly from the diffusive behavior of phase slips. This new method could help with the optimization of Josephson amplifiers for high-fidelity multiplexed qubit readout. Supported by the Army Research Office under Contract W911NF-14-1-0080.

  12. Fabrication and characterization of low loss and high inductance Josephson tunnel junction chains for quantum circuits

    NASA Astrophysics Data System (ADS)

    Grabon, Nicholas; Solovyeva, Natalya; Nguyen, Long; Lin, Yen-Hsiang; Manucharyan, Vladimir

    Linear chains of tightly packed Josephson junctions can realize a very high kinetic inductance circuit element, superinductance, with minimal losses. Superinductance is used in a conventional fluxonium qubit, but it has also been put forward as a key element of a fault-tolerant quantum circuits toolbox. We report fabrication and microwave characterization of linear Al/AlOx/Al Josephson tunnel junction chains and discuss their advantages and limitations as superinductors

  13. Quantum Interference between Three Spin Qubits

    NASA Astrophysics Data System (ADS)

    Sachrajda, Andrew; Poulin-Lamarre, Gabriel; Thorgrimson, Joelle; Studenikin, Sergei; Aers, Geof; Kam, Alicia; Zawadzki, Piotr; Wasilewski, Zbigniew

    2014-03-01

    Recently both hyperfine and exchange based qubits based on three spin states in triple quantum dot circuits have been individually demonstrated. The effective targeting of a specific qubit species required a carefully designed pulse shape and measurement sequence. We discuss results where pulses are chosen to activate both three spin qubit species simultaneously. In our results two novel coherent behaviors have been identified which are related to quantum interference effects involving an interplay between the two qubits types. Such experiments are important to gain an understanding of critical leakage paths which drive the system away from the intended qubit states. Certain features of the data are analyzed in terms of a breakdown of the usual spin blockade spin to charge conversion technique for three spin experiments and the consequences of charge noise on the measurements.

  14. Josephson junction array protected from local noises.

    NASA Astrophysics Data System (ADS)

    Gladchenko, Sergey; Olaya, David; Dupont-Ferrier, Eva; Doucot, Benoit; Ioffe, Lev; Gershenson, Michael

    2009-03-01

    We have developed small arrays of Josephson junctions (JJs) that can be viewed as prototypes of superconducting qubits protected from local noises [1]. The array consists of twelve superconducting loops interrupted by four sub-micron JJs. The protected state is realized when each loop is threaded by half of the magnetic flux quantum. It has been observed that the array with the optimized amplitude of quantum fluctuations is protected against magnetic flux variations well beyond linear order, in agreement with theoretical predictions [2]. 1. S. Gladchenko et al., ``Superconducting Nanocircuits for Topologically Protected Qubits'', arXiv:cond-mat/0802.2295, to be published in Nature Physics. 2. L.B. Ioffe and M.V. Feigelman, Phys. Rev. B 66, 224503 (2002); B. Doucot et al., Phys. Rev. B 71, 024505 (2005); B. Doucot and L.B. Ioffe, Phys. Rev. B 76, 214507 (2007).

  15. Improving Qubit Quality Factors Through Exotic Materials

    NASA Astrophysics Data System (ADS)

    Norman, Victoria

    In the time since the first qubits were successfully fabricated, the coherence times of superconducting Josephson junction qubits have improved by several orders of magnitude. Yet as the quantum information and computation field moves forward, these coherence times still need further improvement. We are now finding that in some superconducting systems, non-thermal equilibrium quasiparticles are becoming the limiting factor in qubit lifetimes. For SIS superconducting qubits, the T1 and T2* values may be improved by the use of materials with higher superconducting band gap, EG, for which low values may allow for quasiparticles to break up cooper pairs more easily, leading to a shorter lifetime. At this time, Al-Al2Ox3-Al transmons are very well characterized and understood and will therefore serve as an appropriate baseline with which to compare the more exotic junction materials. Using tantalum and niobium, which have Eg values of 3 times and 10 times that of aluminum respectively, we expect the T1 and T2* values to increase significantly for the Al-Al2Ox3-Nb, Al-Al2Ox3-Ta, and Ta-Ta2Ox5-Nb qubits.

  16. Realization of Microwave Quantum Circuits Using Hybrid Superconducting-Semiconducting Nanowire Josephson Elements.

    PubMed

    de Lange, G; van Heck, B; Bruno, A; van Woerkom, D J; Geresdi, A; Plissard, S R; Bakkers, E P A M; Akhmerov, A R; DiCarlo, L

    2015-09-18

    We report the realization of quantum microwave circuits using hybrid superconductor-semiconductor Josephson elements comprised of InAs nanowires contacted by NbTiN. Capacitively shunted single elements behave as transmon circuits with electrically tunable transition frequencies. Two-element circuits also exhibit transmonlike behavior near zero applied flux but behave as flux qubits at half the flux quantum, where nonsinusoidal current-phase relations in the elements produce a double-well Josephson potential. These hybrid Josephson elements are promising for applications requiring microwave superconducting circuits operating in a magnetic field. PMID:26431010

  17. Simulation of supersymmetric quantum mechanics in a Cooper-pair box shunted by a Josephson rhombus

    NASA Astrophysics Data System (ADS)

    Ulrich, Jascha; Otten, Daniel; Hassler, Fabian

    2015-12-01

    Supersymmetries in quantum mechanics offer a way to obtain degeneracies in the excitation spectrum which do not originate from selection rules. The mechanism behind the degeneracies is the same as the one that leads to the miraculous cancellations of divergences in supersymmetric field theories found in the high energy physics context. There is up to now no realistic proposal of nonintegrable systems that show level degeneracies due to a supersymmetric structure. Here, we propose an implementation of a quantum-mechanical supersymmetry in a Cooper-pair box shunted by a Josephson junction rhombus which is effectively π periodic in the superconducting phase difference. For a characteristic ratio between the strength of the 2 π - and the π -periodic junction, we find a twofold degeneracy of all the energy levels all the way from the weak junction/charge qubit limit to the strong junction/transmon regime. We provide explicit experimental values for the parameters of the system and show that tuning in and out of the supersymmetric point is easily achieved by varying an external gate voltage. We furthermore discuss a microwave experiment to detect the supersymmetry and conclude that it can indeed be implemented with currently existing Josephson junction technology.

  18. Macroscopic quantum tunnelling in spin filter ferromagnetic Josephson junctions.

    PubMed

    Massarotti, D; Pal, A; Rotoli, G; Longobardi, L; Blamire, M G; Tafuri, F

    2015-01-01

    The interfacial coupling of two materials with different ordered phases, such as a superconductor (S) and a ferromagnet (F), is driving new fundamental physics and innovative applications. For example, the creation of spin-filter Josephson junctions and the demonstration of triplet supercurrents have suggested the potential of a dissipationless version of spintronics based on unconventional superconductivity. Here we demonstrate evidence for active quantum applications of S-F-S junctions, through the observation of macroscopic quantum tunnelling in Josephson junctions with GdN ferromagnetic insulator barriers. We show a clear transition from thermal to quantum regime at a crossover temperature of about 100 mK at zero magnetic field in junctions, which present clear signatures of unconventional superconductivity. Following previous demonstration of passive S-F-S phase shifters in a phase qubit, our result paves the way to the active use of spin filter Josephson systems in quantum hybrid circuits. PMID:26054495

  19. Macroscopic quantum tunnelling in spin filter ferromagnetic Josephson junctions

    PubMed Central

    Massarotti, D.; Pal, A.; Rotoli, G.; Longobardi, L.; Blamire, M. G.; Tafuri, F.

    2015-01-01

    The interfacial coupling of two materials with different ordered phases, such as a superconductor (S) and a ferromagnet (F), is driving new fundamental physics and innovative applications. For example, the creation of spin-filter Josephson junctions and the demonstration of triplet supercurrents have suggested the potential of a dissipationless version of spintronics based on unconventional superconductivity. Here we demonstrate evidence for active quantum applications of S-F-S junctions, through the observation of macroscopic quantum tunnelling in Josephson junctions with GdN ferromagnetic insulator barriers. We show a clear transition from thermal to quantum regime at a crossover temperature of about 100 mK at zero magnetic field in junctions, which present clear signatures of unconventional superconductivity. Following previous demonstration of passive S-F-S phase shifters in a phase qubit, our result paves the way to the active use of spin filter Josephson systems in quantum hybrid circuits. PMID:26054495

  20. Nondegenerate three-wave mixing with the Josephson ring modulator

    NASA Astrophysics Data System (ADS)

    Abdo, Baleegh; Kamal, Archana; Devoret, Michel

    2013-01-01

    The Josephson ring modulator (JRM) is a device, based on Josephson tunnel junctions, capable of performing nondegenerate mixing in the microwave regime without losses. The generic scattering matrix of the device is calculated by solving coupled quantum Langevin equations. Its form shows that the device can achieve quantum-limited noise performance both as an amplifier and a mixer. Fundamental limitations on simultaneous optimization of performance metrics like gain, bandwidth, and dynamic range (including the effect of pump depletion) are discussed. We also present three possible integrations of the JRM as the active medium in a different electromagnetic environment. The resulting circuits, named Josephson parametric converters (JPC), are discussed in detail, and experimental data on their dynamic range are found to be in good agreement with theoretical predictions. We also discuss future prospects and requisite optimization of JPC as a preamplifier for qubit readout applications.

  1. Long distance coupling of resonant exchange qubits

    NASA Astrophysics Data System (ADS)

    Russ, Maximilian; Burkard, Guido

    2015-11-01

    We investigate the effectiveness of a microwave cavity as a mediator of interactions between two resonant exchange (RX) qubits in semiconductor quantum dots (QDs) over long distances, limited only by the extension of the cavity. Our interaction model includes the orthonormalized Wannier orbitals constructed from Fock-Darwin states under the assumption of a harmonic QD confinement potential. We calculate the qubit-cavity coupling strength in a Jaynes-Cummings Hamiltonian and find that dipole transitions between two states with an asymmetric charge configuration constitute the relevant RX qubit-cavity coupling mechanism. The effective coupling between two RX qubits in a shared cavity yields a universal two-qubit iswap gate with gate times on the order of nanoseconds over distances on the order of up to a millimeter.

  2. Long distance coupling of resonant exchange qubits

    NASA Astrophysics Data System (ADS)

    Russ, Maximilian; Burkard, Guido

    We investigate the effectiveness of a microwave cavity as a mediator of interactions between two resonant exchange (RX) qubits in semiconductor quantum dots (QDs) over long distances, limited only by the extension of the cavity. Our interaction model includes the orthonormalized Wannier orbitals constructed from Fock-Darwin states under the assumption of a harmonic QD confinement potential. We calculate the qubit-cavity coupling strength gr in a Jaynes Cummings Hamiltonian, and find that dipole transitions between two states with an asymmetric charge configuration constitute the relevant RX qubit-cavity coupling mechanism. The effective coupling between two RX qubits in a shared cavity yields a universal two-qubit iSWAP-gate with gate times on the order of nanoseconds over distances on the order of up to a millimeter. Funded by ARO through Grant No. W911NF-15-1-0149.

  3. Large Dispersive Shift of Cavity Resonance Induced by a Superconducting Flux Qubit in the Straddling Regime

    NASA Astrophysics Data System (ADS)

    Inomata, Kunihiro; Yamamoto, Tsuyoshi; Billangeon, Pierre-M.; Lin, Zhirong; Nakamura, Yasunobu; Tsai, Jaw-Shen; Koshino, Kazuki

    2013-03-01

    We demonstrate enhancement of the dispersive frequency shift in a coplanar waveguide resonator induced by a capacitively coupled superconducting flux qubit in the straddling regime. The magnitude of the observed shift, 80 MHz for the qubit-resonator detuning of 5 GHz, is quantitatively explained by the generalized Rabi model which takes into account the contribution of the qubit higher energy levels. By applying the enhanced dispersive shift to the qubit readout, we achieved 90 % contrast of the Rabi oscillations which is mainly limited by the energy relaxation of the qubit. We also discuss the qubit readout using a Josephson parametric amplifier. This work was supported by the MEXT Kakenhi ``Quantum Cybernetics'', the JSPS through its FIRST Program, and the NICT Commissioned Research.

  4. 3D Integration for Superconducting Qubits

    NASA Astrophysics Data System (ADS)

    Rosenberg, Danna; Yost, Donna-Ruth; Das, Rabindra; Hover, David; Racz, Livia; Weber, Steven; Yoder, Jonilyn; Kerman, Andrew; Oliver, William

    As the field of superconducting quantum computing advances from the few-qubit stage to large-scale fault-tolerant devices, scalability requirements will necessitate the use of standard 3D packaging and integration processes. While the field of 3D integration is well-developed, relatively little work has been performed to determine the compatibility of the associated processes with superconducting qubits. Qubit coherence time could potentially be affected by required process steps or by the proximity of an interposer that could introduce extra sources of charge or flux noise. As a first step towards a large-scale quantum information processor, we have used a flip-chip process to bond a chip with flux qubits to an interposer containing structures for qubit readout and control. We will present data on the effect of the presence of the interposer on qubit coherence time for various qubit-chip-interposer spacings and discuss the implications for integrated multi-qubit devices. This research was funded by the ODNI and IARPA under Air Force Contract No. FA8721-05-C-0002. The views and conclusions contained herein are those of the authors and should not be interpreted as representing the official policies or endorsements, either expressed or implied, of ODNI, IARPA, or the US Government.

  5. Tunable Hybrid Qubit in a GaAs Double Quantum Dot

    NASA Astrophysics Data System (ADS)

    Cao, Gang; Li, Hai-Ou; Yu, Guo-Dong; Wang, Bao-Chuan; Chen, Bao-Bao; Song, Xiang-Xiang; Xiao, Ming; Guo, Guang-Can; Jiang, Hong-Wen; Hu, Xuedong; Guo, Guo-Ping

    2016-02-01

    We experimentally demonstrate a tunable hybrid qubit in a five-electron GaAs double quantum dot. The qubit is encoded in the (1,4) charge regime of the double dot and can be manipulated completely electrically. More importantly, dot anharmonicity leads to quasiparallel energy levels and a new anticrossing, which help preserve quantum coherence of the qubit and yield a useful working point. We have performed Larmor precession and Ramsey fringe experiments near the new working point and find that the qubit decoherence time is significantly improved over a charge qubit. This work shows a new way to encode a semiconductor qubit that is controllable and coherent.

  6. Josephson junction mixing.

    NASA Technical Reports Server (NTRS)

    Thompson, E. D.

    1973-01-01

    A theory is presented which, though too simple to explain quantitative details in the Josephson junction mixing response, is sufficient for explaining qualitatively the results observed. Crucial to the theory presented, and that which differentiates it from earlier ones, is the inclusion of harmonic voltages across the ideal Josephson element.

  7. Feedback-tuned, noise resilient gates for encoded spin qubits

    NASA Astrophysics Data System (ADS)

    Bluhm, Hendrik

    Spin 1/2 particles form native two level systems and thus lend themselves as a natural qubit implementation. However, encoding a single qubit in several spins entails benefits, such as reducing the resources necessary for qubit control and protection from certain decoherence channels. While several varieties of such encoded spin qubits have been implemented, accurate control remains challenging, and leakage out of the subspace of valid qubit states is a potential issue. Optimal performance typically requires large pulse amplitudes for fast control, which is prone to systematic errors and prohibits standard control approaches based on Rabi flopping. Furthermore, the exchange interaction typically used to electrically manipulate encoded spin qubits is inherently sensitive to charge noise. I will discuss all-electrical, high-fidelity single qubit operations for a spin qubit encoded in two electrons in a GaAs double quantum dot. Starting from a set of numerically optimized control pulses, we employ an iterative tuning procedure based on measured error syndromes to remove systematic errors.Randomized benchmarking yields an average gate fidelity exceeding 98 % and a leakage rate into invalid states of 0.2 %. These gates exhibit a certain degree of resilience to both slow charge and nuclear spin fluctuations due to dynamical correction analogous to a spin echo. Furthermore, the numerical optimization minimizes the impact of fast charge noise. Both types of noise make relevant contributions to gate errors. The general approach is also adaptable to other qubit encodings and exchange based two-qubit gates.

  8. Using Spontaneous Emission of a Qubit as a Resource for Feedback Control

    NASA Astrophysics Data System (ADS)

    Campagne-Ibarcq, P.; Jezouin, S.; Cottet, N.; Six, P.; Bretheau, L.; Mallet, F.; Sarlette, A.; Rouchon, P.; Huard, B.

    2016-08-01

    Persistent control of a transmon qubit is performed by a feedback protocol based on continuous heterodyne measurement of its fluorescence. By driving the qubit and cavity with microwave signals whose amplitudes depend linearly on the instantaneous values of the quadratures of the measured fluorescence field, we show that it is possible to stabilize permanently the qubit in any targeted state. Using a Josephson mixer as a phase-preserving amplifier, it was possible to reach a total measurement efficiency η =35 %, leading to a maximum of 59% of excitation and 44% of coherence for the stabilized states. The experiment demonstrates multiple-input multiple-output analog Markovian feedback in the quantum regime.

  9. Real-time tuning of a double quantum dot using a Josephson parametric amplifier

    NASA Astrophysics Data System (ADS)

    Stehlik, J.; Liu, Y.-Y.; Quintana, C. M.; Eichler, C.; Hartke, T. R.; Petta, J. R.

    Josephson parametric amplifiers (JPAs) have enabled advances in readout of quantum systems. Here we demonstrate JPA-assisted readout of a cavity-coupled double quantum dot (DQD). Utilizing a JPA we improve the signal-to-noise ratio (SNR) by a factor of 2000 compared to the situation with the parametric amplifier turned off. At an interdot charge transition we achieve a SNR of 76 (19 dB) with an integration time τ = 400 ns, which is limited by the linewidth of our cavity. By measuring the SNR as a function of τ we extract an equivalent charge sensitivity of 8 ×10-5 e /√{ Hz} . We develop a dual-gate-voltage rastering scheme that allows us to acquire a DQD charge stability diagram in just 20 ms. Such rapid data acquisition rates enable device tuning in live ``video-mode,'' where the results of parameter changes are immediately displayed. Live tuning allows the DQD confinement potential to be rapidly tuned, a capability that will become increasingly important as semiconductor spin qubits are scaled to a larger number of dots. Research is supported by the Packard Foundation, ARO Grant No. W911NF-15-1-0149, DARPA QuEST Grant No. HR0011-09-1-0007, and the NSF (Grants No. DMR-1409556 and DMR-1420541).

  10. Josephson-vortex Cherenkov radiation

    SciTech Connect

    Mints, R.G.; Snapiro, I.B.

    1995-10-01

    We predict the Josephson-vortex Cherenkov radiation of an electromagnetic wave. We treat a long one-dimensional Josephson junction. We consider the wavelength of the radiated electromagnetic wave to be much less than the Josephson penetration depth. We use for calculations the nonlocal Josephson electrodynamics. We find the expression for the radiated power and for the radiation friction force acting on a Josephson vortex and arising due to the Cherenkov radiation. We calculate the relation between the density of the bias current and the Josephson vortex velocity.

  11. Experiments on Interaction of Quasiparticles with Two-Level-Systems in a Superconducting Phase Qubit

    NASA Astrophysics Data System (ADS)

    Bilmes, Alexander; Lisenfeld, Jürgen; Heimes, Andreas; Zanker, Sebastian; Schön, Gerd; Ustinov, Alexey

    2015-03-01

    Two-Level-Systems (TLS) are one of the main sources of decoherence in superconducting qubits. Some individual and coherent TLS, present in the tunnel barrier of the qubit's Josephson junction, can be coherently operated via the qubit. In the past, experiments on superconducting glasses indicated that quasiparticles may give rise to TLS energy loss similar to Korringa relaxation. We will present experiments in which we use a phase qubit to explore the interaction of single TLS with non-equilibrium quasiparticles. We have implemented in-situ quasiparticle injection by using an on-chip dc-SQUID that is pulse-biased beyond its critical current. The quasiparticle density is calibrated by measuring associated characteristic changes to the qubit resonance frequency and energy relaxation rate. The coherence times of individual TLS is measured in dependence of the non-equilibrium quasiparticle density and compared to thermally generated quasiparticles. PI, KIT, Wolfgang-Gaede-Strasse 1, 76131 Karlsruhe, Germany.

  12. Quasioptical Josephson Oscillator

    NASA Astrophysics Data System (ADS)

    Wengler, Michael J.

    1994-09-01

    The Quasioptical Josephson Oscillator (QJO) is a 2-D array of between 100 and 1,000,000 Josephson junctions, each at the center of a small dipole antenna. HYPRES, Inc. of Elmsford, NY has fabricated test chips which have demonstrated 0.35 microwatts radiation at 190 CHz in one case, and 0.7 microwatts radiation at 345 GHz in another case. A significant understanding of the 2-D oscillators was developed through theoretical and numerical calculations.

  13. High-fidelity qubit measurement with a microwave-photon counter

    NASA Astrophysics Data System (ADS)

    Govia, Luke C. G.; Pritchett, Emily J.; Xu, Canran; Plourde, B. L. T.; Vavilov, Maxim G.; Wilhelm, Frank K.; McDermott, R.

    2014-12-01

    High-fidelity, efficient quantum nondemolition readout of quantum bits is integral to the goal of quantum computation. As superconducting circuits approach the requirements of scalable, universal fault tolerance, qubit readout must also meet the demand of simplicity to scale with growing system size. Here we propose a fast, high-fidelity, scalable measurement scheme based on the state-selective ring-up of a cavity followed by photodetection with the recently introduced Josephson photomultiplier (JPM), a current-biased Josephson junction. This scheme maps qubit state information to the binary digital output of the JPM, circumventing the need for room-temperature heterodyne detection and offering the possibility of a cryogenic interface to superconducting digital control circuitry. Numerics show that measurement contrast in excess of 95% is achievable in a measurement time of 140 ns. We discuss perspectives to scale this scheme to enable readout of multiple qubit channels with a single JPM.

  14. A new microwave resonator readout scheme for superconducting qubits

    NASA Astrophysics Data System (ADS)

    Metcalfe, Michael B.

    Quantum computation is a relatively new field of research, which uses the properties of quantum mechanical systems for information processing. While most proposals for constructing such a quantum computer involve using microscopic degrees of freedom such as those of trapped ions or nuclear spins, this thesis concentrates on using the collective electromagnetic response of a macroscopic electrical circuit to construct the fundamental building block of a quantum computer---a qubit. These macroscopic systems are inherently more difficult to protect from decoherence compared to the microscopic qubit systems because of strong environmental coupling through, for example, the measurement leads. However, superconducting quantum circuits should be easier to scale to large multi qubit systems since they involve simple electrical elements, such as inductors and capacitors for coupling qubits. Furthermore, they can be produced using the highly developed fabrication techniques of integrated circuits. One of the outstanding issues in superconducting qubit circuits is to read out the qubit state without introducing excessive noise. Such a readout scheme requires speed, sensitivity and should minimally disturb the qubit state. To meet these requirements we have developed a new type of dispersive bifurcating amplifier, called the cavity bifurcation amplifier (CBA), which consists of a Josephson junction imbedded in a microwave on-chip resonator. The optimum resonator design is based on a simple coplanar waveguide (CPW), imposing a pre-determined frequency and whose other RF characteristics like the quality factor are easily controlled and optimized. The CBA is sensitive to the susceptibility of the superconducting qubit with respect to an external control parameter (e.g., flux) and hence during both qubit manipulation and readout sequences, the qubit can be biased on a so-called "sweet spot", where it is immune to first order fluctuations in this parameter. This readout has no on

  15. Quantum Phase Slips in Topological Josephson Junction Rings

    NASA Astrophysics Data System (ADS)

    Rodriguez Mota, Rosa; Vishveshwara, Smitha; Pereg-Barnea, Tami

    We study quantum phase slip processes (QPS) in a ring of N topological superconducting islands joined by Josephson junctions and threaded by magnetic flux. In this array, neighboring islands interact through the usual charge 2e Josephson tunneling and the Majorana assisted charge e tunneling. When the charging energy associated with the island's capacitance is zero, the energy vs. flux relation of the system is characterized by parabolas centered around even or odd multiples of the superconducting flux quantum, depending on the parity of the system. For small but non-zero charging energy, quantum fluctuations can lead to tunneling between these classical states. In this work, we calculate the amplitude of these tunneling processes, commonly known as quantum phase slips. We also add gate voltages to our system and study how the amplitude of QPS in these topological Josephson array is modified by Aharanov-Casher interference effects.

  16. Electrically Protected Resonant Exchange Qubits in Triple Quantum Dots

    NASA Astrophysics Data System (ADS)

    Taylor, J. M.; Srinivasa, V.; Medford, J.

    2013-08-01

    We present a modulated microwave approach for quantum computing with qubits comprising three spins in a triple quantum dot. This approach includes single- and two-qubit gates that are protected against low-frequency electrical noise, due to an operating point with a narrowband response to high frequency electric fields. Furthermore, existing double quantum dot advances, including robust preparation and measurement via spin-to-charge conversion, are immediately applicable to the new qubit. Finally, the electric dipole terms implicit in the high frequency coupling enable strong coupling with superconducting microwave resonators, leading to more robust two-qubit gates.

  17. Electrically protected resonant exchange qubits in triple quantum dots.

    PubMed

    Taylor, J M; Srinivasa, V; Medford, J

    2013-08-01

    We present a modulated microwave approach for quantum computing with qubits comprising three spins in a triple quantum dot. This approach includes single- and two-qubit gates that are protected against low-frequency electrical noise, due to an operating point with a narrowband response to high frequency electric fields. Furthermore, existing double quantum dot advances, including robust preparation and measurement via spin-to-charge conversion, are immediately applicable to the new qubit. Finally, the electric dipole terms implicit in the high frequency coupling enable strong coupling with superconducting microwave resonators, leading to more robust two-qubit gates. PMID:23952376

  18. Real-time measurement of quasiparticle tunneling in a single-junction transmon qubit using feedback

    NASA Astrophysics Data System (ADS)

    Ristè, Diego; Bultink, Niels; Tiggelman, Marijn; Schouten, Raymond; Lehnert, Konrad; Dicarlo, Leonardo

    2013-03-01

    With coherence times of superconducting qubits now exceeding 100 μs , the contribution of quasiparticle (QP) tunneling to qubit relaxation and dephasing becomes potentially relevant. We report the real-time measurement of QP tunneling across the single junction of a 3D transmon qubit. We integrate recent developments in projective qubit readout with 99 % fidelity and feedback-based reset to transform the qubit into a charge-parity detector with 6 μs resolution. We detect a symmetric random telegraph signal matching a QP tunneling time of 0 . 8 ms . By measuring the correlation function of charge parity conditioned on specific initial and final qubit states, we determine that most QP tunneling does not induce qubit transitions, in contradiction with recent theory. We extract a QP-induced qubit relaxation time T1qp ~ 3 ms , decidedly not limiting the measured T1 = 0 . 14 ms . Research supported by NWO, FOM, and EU Project SOLID.

  19. Advanced Concepts in Josephson Junction Reflection Amplifiers

    NASA Astrophysics Data System (ADS)

    Lähteenmäki, Pasi; Vesterinen, Visa; Hassel, Juha; Paraoanu, G. S.; Seppä, Heikki; Hakonen, Pertti

    2014-06-01

    Low-noise amplification at microwave frequencies has become increasingly important for the research related to superconducting qubits and nanoelectromechanical systems. The fundamental limit of added noise by a phase-preserving amplifier is the standard quantum limit, often expressed as noise temperature . Towards the goal of the quantum limit, we have developed an amplifier based on intrinsic negative resistance of a selectively damped Josephson junction. Here we present measurement results on previously proposed wide-band microwave amplification and discuss the challenges for improvements on the existing designs. We have also studied flux-pumped metamaterial-based parametric amplifiers, whose operating frequency can be widely tuned by external DC-flux, and demonstrate operation at pumping, in contrast to the typical metamaterial amplifiers pumped via signal lines at.

  20. Simplifying the circuit of Josephson parametric converters

    NASA Astrophysics Data System (ADS)

    Abdo, Baleegh; Brink, Markus; Chavez-Garcia, Jose; Keefe, George

    Josephson parametric converters (JPCs) are quantum-limited three-wave mixing devices that can play various important roles in quantum information processing in the microwave domain, including amplification of quantum signals, transduction of quantum information, remote entanglement of qubits, nonreciprocal amplification, and circulation of signals. However, the input-output and biasing circuit of a state-of-the-art JPC consists of bulky components, i.e. two commercial off-chip broadband 180-degree hybrids, four phase-matched short coax cables, and one superconducting magnetic coil. Such bulky hardware significantly hinders the integration of JPCs in scalable quantum computing architectures. In my talk, I will present ideas on how to simplify the JPC circuit and show preliminary experimental results

  1. Improving the gate fidelity of capacitively coupled spin qubits

    NASA Astrophysics Data System (ADS)

    Wang, Xin; Barnes, Edwin

    2015-03-01

    Precise execution of quantum gates acting on two or multiple qubits is essential to quantum computation. For semiconductor spin qubits coupled via capacitive interaction, the best fidelity for a two-qubit gate demonstrated so far is around 70%, insufficient for fault-tolerant quantum computation. In this talk we present control protocols that may substantially improve the robustness of two-qubit gates against both nuclear noise and charge noise. Our pulse sequences incorporate simultaneous dynamical decoupling protocols and are simple enough for immediate experimental realization. Together with existing control protocols for single-qubit gates, our results constitute an important step toward scalable quantum computation using spin qubits. This work is done in collaboration with Sankar Das Sarma and supported by LPS-NSA-CMTC and IARPA-MQCO.

  2. Critical Josephson current in the dynamical Coulomb blockade regime

    NASA Astrophysics Data System (ADS)

    Jäck, Berthold; Eltschka, Matthias; Assig, Maximilian; Etzkorn, Markus; Ast, Christian R.; Kern, Klaus

    2016-01-01

    The current-voltage characteristics of a voltage-biased Josephson junction in the low conductance regime of an ultra-low temperature scanning tunneling microscope (STM) is dominated by sequential charge tunneling. Using P (E ) theory we show that the Josephson coupling energy, experimentally determined in this regime, is in good agreement with the critical current I0 calculated from the Ambegaokar-Baratoff formula. In this way, we can determine the critical current values of a Josephson junction in an STM. Furthermore, we experimentally determine a range of validity for P (E ) theory, which is in accordance with theoretical predictions. In this way, we establish an optimal parameter range, in which Josephson STM can be performed.

  3. Fractional Solitons in Excitonic Josephson Junctions

    PubMed Central

    Hsu, Ya-Fen; Su, Jung-Jung

    2015-01-01

    The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ϕ0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q = ϕ0/2π is the lowest energy state starting from zero ϕ0 – until ϕ0 > π – then the alternative group of solitons with Q = ϕ0/2π − 1 takes place and switches the polarity of CPR. PMID:26511770

  4. Fractional Solitons in Excitonic Josephson Junctions.

    PubMed

    Hsu, Ya-Fen; Su, Jung-Jung

    2015-01-01

    The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ϕ0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q = ϕ0/2π is the lowest energy state starting from zero ϕ0 - until ϕ0 > π - then the alternative group of solitons with Q = ϕ0/2π - 1 takes place and switches the polarity of CPR. PMID:26511770

  5. Fractional Solitons in Excitonic Josephson Junctions

    NASA Astrophysics Data System (ADS)

    Hsu, Ya-Fen; Su, Jung-Jung

    2015-10-01

    The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ϕ0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q = ϕ0/2π is the lowest energy state starting from zero ϕ0 - until ϕ0 > π - then the alternative group of solitons with Q = ϕ0/2π - 1 takes place and switches the polarity of CPR.

  6. Stochastic master equation approach for analysis of remote entanglement with Josephson parametric converter amplifier

    NASA Astrophysics Data System (ADS)

    Silveri, M.; Zalys-Geller, E.; Hatridge, M.; Leghtas, Z.; Devoret, M. H.; Girvin, S. M.

    2015-03-01

    In the remote entanglement process, two distant stationary qubits are entangled with separate flying qubits and the which-path information is erased from the flying qubits by interference effects. As a result, an observer cannot tell from which of the two sources a signal came and the probabilistic measurement process generates perfect heralded entanglement between the two signal sources. Notably, the two stationary qubits are spatially separated and there is no direct interaction between them. We study two transmon qubits in superconducting cavities connected to a Josephson Parametric Converter (JPC). The qubit information is encoded in the traveling wave leaking out from each cavity. Remarkably, the quantum-limited phase-preserving amplification of two traveling waves provided by the JPC can work as a which-path information eraser. By using a stochastic master approach we demonstrate the probabilistic production of heralded entangled states and that unequal qubit-cavity pairs can be made indistinguishable by simple engineering of driving fields. Additionally, we will derive measurement rates, measurement optimization strategies and discuss the effects of finite amplification gain, cavity losses, and qubit relaxations and dephasing. Work supported by IARPA, ARO and NSF.

  7. Observation of short ballistic Josephson effect in vertical graphene junctions

    NASA Astrophysics Data System (ADS)

    Lee, Gil-Ho; Lee, Hu-Jong

    2014-03-01

    The current-phase relation (CPR) of vertical graphene Josephson junctions (vGJJs) was measured using phase-sensitive dc-SQUID interferometry. A vGJJ, realized by vertically sandwiching a monolayer graphene between two Al electrodes, had an atomically short channel with transparent contacts for the highly coherent junction nature. The measured CPR was almost perfectly skewed, which rigorously confirmed the short ballisticity of the vGJJs. The short ballistic character of a Josephson junction has been predicted since 1970's but has never been realized in scalable hybrid systems. The CPR also provided energy spectrum of Andreev levels formed inside the junction, which offered a promising prospect for scalable quantum information devices such as Andreev-level qubits. This vertical-junction scheme is also readily applicable to the other cleavable materials such as three-dimensional topological insulators or transition metal dichalcogenides, opening a new pathway for uncovering exotic coherence phenomena arising in an atomic scale.

  8. Effects of anharmonicity of current-phase relation in Josephson junctions (Review Article)

    NASA Astrophysics Data System (ADS)

    Askerzade, I. N.

    2015-04-01

    The aim of this review is the analysis of dynamical properties of Josephson junctions (JJ) with anharmonic current-phase relation (CPR). Firstly, discussion of theoretical foundation of anharmonic CPR in different Josephson structures and their experimental observation are presented. The influence of anisotropy and multiband effects on CPR of JJ are analyzed. We present recent theoretical study results of the anharmonic CPR influence on I-V curve, plasma frequency, and dynamics of long JJ. Results of study of Shapiro steps in I-V curve of anharmonic JJ are also presented. Finally, CPR anharmonicity effect on characteristics of JJ-based qubits is discussed.

  9. Josephson magnetic rotary valve

    SciTech Connect

    Soloviev, I. I.; Klenov, N. V.; Bakurskiy, S. V.; Bol'ginov, V. V.; Ryazanov, V. V.; Kupriyanov, M. Yu.; Golubov, A. A.

    2014-12-15

    We propose a control element for a Josephson spin valve. It is a complex Josephson device containing ferromagnetic (F) layer in the weak-link area consisting of two regions, representing 0 and π Josephson junctions, respectively. The valve's state is defined by mutual orientations of the F-layer magnetization vector and boundary line between 0 and π sections of the device. We consider possible implementation of the control element by introduction of a thin normal metal layer in a part of the device area. By means of theoretical simulations, we study properties of the valve's structure as well as its operation, revealing such advantages as simplicity of control, high characteristic frequency, and good legibility of the basic states.

  10. Quantum control and process tomography of a semiconductor quantum dot hybrid qubit

    NASA Astrophysics Data System (ADS)

    Kim, Dohun; Shi, Zhan; Simmons, C. B.; Ward, D. R.; Prance, J. R.; Koh, Teck Seng; Gamble, John King; Savage, D. E.; Lagally, M. G.; Friesen, Mark; Coppersmith, S. N.; Eriksson, Mark A.

    2014-07-01

    The similarities between gated quantum dots and the transistors in modern microelectronics--in fabrication methods, physical structure and voltage scales for manipulation--have led to great interest in the development of quantum bits (qubits) in semiconductor quantum dots. Although quantum dot spin qubits have demonstrated long coherence times, their manipulation is often slower than desired for important future applications, such as factoring. Furthermore, scalability and manufacturability are enhanced when qubits are as simple as possible. Previous work has increased the speed of spin qubit rotations by making use of integrated micromagnets, dynamic pumping of nuclear spins or the addition of a third quantum dot. Here we demonstrate a qubit that is a hybrid of spin and charge. It is simple, requiring neither nuclear-state preparation nor micromagnets. Unlike previous double-dot qubits, the hybrid qubit enables fast rotations about two axes of the Bloch sphere. We demonstrate full control on the Bloch sphere with π-rotation times of less than 100 picoseconds in two orthogonal directions, which is more than an order of magnitude faster than any other double-dot qubit. The speed arises from the qubit's charge-like characteristics, and its spin-like features result in resistance to decoherence over a wide range of gate voltages. We achieve full process tomography in our electrically controlled semiconductor quantum dot qubit, extracting high fidelities of 85 per cent for X rotations (transitions between qubit states) and 94 per cent for Z rotations (phase accumulation between qubit states).

  11. Qubit state discrimination

    SciTech Connect

    Deconinck, Matthieu E.

    2010-06-15

    We show how one can solve the problem of discriminating between qubit states. We use the quantum state discrimination duality theorem and the Bloch sphere representation of qubits, which allows for an easy geometric and analytical representation of the optimal guessing strategies.

  12. Microwave response and photon emission of a voltage baised Josephson junction

    NASA Astrophysics Data System (ADS)

    Jebari, Salha; Grimm, Alexander; Hazra, Dibyendu; Hofheinz, Max

    The readout of superconducting qubits requires amplifiers combining noise close to the quantum limit, high gain, large bandwidth, and sufficient dynamic range. Josephson parametric amplifiers using Josephson junctions in the 0-voltage state, driven by a large microwave signals, begin to perform sufficiently well in all 4 of these aspects to be of practical use, but remain difficult to optimize and use. Recent experiments with superconducting circuits consisting of a DC voltage-biased Josephson junction in series with a resonator, showed that a tunneling Cooper pair can emit one or several photons with a total energy of 2e times the applied voltage. We present microwave reflection measurements on this device indicating that amplification is possible with a simple DC voltage-biased Josephson junction. We compare these measurements with the noise power emitted by the junction and show that, for low Josephson energy, transmission and noise emission can be explained within the framework of P(E) theory of inelastic Cooper pair tunneling. Combined with a theoretical model, our results indicate that voltage-biased Josephson junctions might be useful for amplification near the quantum limit, offering simpler design and a different trade-off between gain, bandwidth and dynamic range.

  13. Non-exponential energy decay and quasi-particle fluctuations in a superconducting flux qubit

    NASA Astrophysics Data System (ADS)

    Gustavsson, Simon; Yan, Fei; Catelani, Gianluigi; Kamal, Archana; Bylander, Jonas; Yoshihara, Fumiki; Nakamura, Yasunobu; Orlando, Terry; Oliver, Wiliam

    2015-03-01

    We measure pronounced non-exponential energy relaxation in a superconducting flux qubit, observing a decay function that exhibits a fast initial decay followed by a much slower decay for long times. When applying a sequence of pi pulses to the qubit and measuring the decay after the last pi pulse, we observe strong modifications to the decay function, including a slow-down of the fast initial decay and a three-fold increase of the 1/e-time. If we attribute the non-exponential decay to quasiparticle number fluctuations, we speculate that the improvements in T1 are due to a qubit-mediated shuffling of quasiparticles between the metallic islands of the device, which will eventually pump them away from the Josephson junctions to a larger ground plane where their contribution to qubit energy relaxation become negligible.

  14. Boson Josephson Junction with Trapped Atoms

    NASA Astrophysics Data System (ADS)

    Raghavan, S.; Smerzi, A.; Fantoni, S.; Shenoy, S. R.

    We consider coherent atomic tunneling between two weakly coupled Bose-Einstein condensates at T=0 in a double-well trap. The condensate dynamics of the macroscopic amplitudes in the two wells is modeled by two Gross-Pitaevskii equations (GPE) coupled by a tunneling matrix element. Analytic elliptic function solutions are obtained for the time evolution of the inter-well fractional population imbalance z(t) (related to the condensate phase difference) of the Boson Josephson junction (BJJ). Surprisingly, the neutral-atom BJJ shows (non-sinusoidal generalizations of) effects seen in charged-electron superconductor Josephson junctions (SJJ). The BJJ elliptic-function behavior has a singular dependence on a GPE parameter ratio Λ at a critical ratio Λ=Λc, beyond which a novel 'macroscopic quantum self-trapping' effect sets in with a non-zero time-averaged imbalance ≠0.

  15. Qubit metrology and decoherence

    SciTech Connect

    Shaji, Anil; Caves, Carlton M.

    2007-09-15

    Quantum properties of the probes used to estimate a classical parameter can be used to attain accuracies that beat the standard quantum limit. When qubits are used to construct a quantum probe, it is known that initializing n qubits in an entangled state, rather than in a separable state, can improve the measurement uncertainty by a factor of 1/{radical}(n). We investigate how the measurement uncertainty is affected when the individual qubits in a probe are subjected to decoherence. In the face of such decoherence, we regard the rate R at which qubits can be generated and the total duration {tau} of a measurement as fixed resources, and we determine the optimal use of entanglement among the qubits and the resulting optimal measurement uncertainty as functions of R and {tau}.

  16. Collective Dynamics of Intrinsic Josephson Junctions in HTSC

    NASA Astrophysics Data System (ADS)

    Shukrinov, Yu M.; Mahfouzi, F.

    2006-06-01

    The dynamics of a stack of intrinsic Josephson junctions (IJJ) in the high-Tc superconductors is theoretically investigated with both the quasineutrality breakdown effect and quasiparticle charge imbalance effect taken into account. The current-voltage characteristics (IVC) of IJJ are numerically calculated in the framework of capacitively coupled Josephson junctions model and charge imbalance model including set of differential equations for phase differences, kinetic equations and generalized Josephson relations. We obtain the branch structure in IVC and investigate it as a function of model parameters such as coupling constant, McCumber parameter and number of junctions in the stack. The dependence of branch slopes and branch endpoints on the coupling and disequilibrium parameters are found. We study the nonequilibrium effects created by current injection and show that the increase in the disequilibrium parameter changes essentially the character of IVC. The new features of the hysteresis behavior of IVC of IJJ are obtained.

  17. Signatures of topological Josephson junctions

    NASA Astrophysics Data System (ADS)

    Peng, Yang; Pientka, Falko; Berg, Erez; Oreg, Yuval; von Oppen, Felix

    2016-08-01

    Quasiparticle poisoning and diabatic transitions may significantly narrow the window for the experimental observation of the 4 π -periodic dc Josephson effect predicted for topological Josephson junctions. Here, we show that switching-current measurements provide accessible and robust signatures for topological superconductivity which persist in the presence of quasiparticle poisoning processes. Such measurements provide access to the phase-dependent subgap spectrum and Josephson currents of the topological junction when incorporating it into an asymmetric SQUID together with a conventional Josephson junction with large critical current. We also argue that pump-probe experiments with multiple current pulses can be used to measure the quasiparticle poisoning rates of the topological junction. The proposed signatures are particularly robust, even in the presence of Zeeman fields and spin-orbit coupling, when focusing on short Josephson junctions. Finally, we also consider microwave excitations of short topological Josephson junctions which may complement switching-current measurements.

  18. Isotopically enhanced triple-quantum-dot qubit

    PubMed Central

    Eng, Kevin; Ladd, Thaddeus D.; Smith, Aaron; Borselli, Matthew G.; Kiselev, Andrey A.; Fong, Bryan H.; Holabird, Kevin S.; Hazard, Thomas M.; Huang, Biqin; Deelman, Peter W.; Milosavljevic, Ivan; Schmitz, Adele E.; Ross, Richard S.; Gyure, Mark F.; Hunter, Andrew T.

    2015-01-01

    Like modern microprocessors today, future processors of quantum information may be implemented using all-electrical control of silicon-based devices. A semiconductor spin qubit may be controlled without the use of magnetic fields by using three electrons in three tunnel-coupled quantum dots. Triple dots have previously been implemented in GaAs, but this material suffers from intrinsic nuclear magnetic noise. Reduction of this noise is possible by fabricating devices using isotopically purified silicon. We demonstrate universal coherent control of a triple-quantum-dot qubit implemented in an isotopically enhanced Si/SiGe heterostructure. Composite pulses are used to implement spin-echo type sequences, and differential charge sensing enables single-shot state readout. These experiments demonstrate sufficient control with sufficiently low noise to enable the long pulse sequences required for exchange-only two-qubit logic and randomized benchmarking. PMID:26601186

  19. Isotopically enhanced triple-quantum-dot qubit.

    PubMed

    Eng, Kevin; Ladd, Thaddeus D; Smith, Aaron; Borselli, Matthew G; Kiselev, Andrey A; Fong, Bryan H; Holabird, Kevin S; Hazard, Thomas M; Huang, Biqin; Deelman, Peter W; Milosavljevic, Ivan; Schmitz, Adele E; Ross, Richard S; Gyure, Mark F; Hunter, Andrew T

    2015-05-01

    Like modern microprocessors today, future processors of quantum information may be implemented using all-electrical control of silicon-based devices. A semiconductor spin qubit may be controlled without the use of magnetic fields by using three electrons in three tunnel-coupled quantum dots. Triple dots have previously been implemented in GaAs, but this material suffers from intrinsic nuclear magnetic noise. Reduction of this noise is possible by fabricating devices using isotopically purified silicon. We demonstrate universal coherent control of a triple-quantum-dot qubit implemented in an isotopically enhanced Si/SiGe heterostructure. Composite pulses are used to implement spin-echo type sequences, and differential charge sensing enables single-shot state readout. These experiments demonstrate sufficient control with sufficiently low noise to enable the long pulse sequences required for exchange-only two-qubit logic and randomized benchmarking. PMID:26601186

  20. A near-quantum-limited Josephson traveling-wave parametric amplifier

    NASA Astrophysics Data System (ADS)

    Macklin, C.; O'Brien, K.; Hover, D.; Schwartz, M. E.; Bolkhovsky, V.; Zhang, X.; Oliver, W. D.; Siddiqi, I.

    2015-10-01

    Detecting single-photon level signals—carriers of both classical and quantum information—is particularly challenging for low-energy microwave frequency excitations. Here we introduce a superconducting amplifier based on a Josephson junction transmission line. Unlike current standing-wave parametric amplifiers, this traveling wave architecture robustly achieves high gain over a bandwidth of several gigahertz with sufficient dynamic range to read out 20 superconducting qubits. To achieve this performance, we introduce a subwavelength resonant phase-matching technique that enables the creation of nonlinear microwave devices with unique dispersion relations. We benchmark the amplifier with weak measurements, obtaining a high quantum efficiency of 75% (70% including noise added by amplifiers following the Josephson amplifier). With a flexible design based on compact lumped elements, this Josephson amplifier has broad applicability to microwave metrology and quantum optics.

  1. A near-quantum-limited Josephson traveling-wave parametric amplifier.

    PubMed

    Macklin, C; O'Brien, K; Hover, D; Schwartz, M E; Bolkhovsky, V; Zhang, X; Oliver, W D; Siddiqi, I

    2015-10-16

    Detecting single-photon level signals—carriers of both classical and quantum information—is particularly challenging for low-energy microwave frequency excitations. Here we introduce a superconducting amplifier based on a Josephson junction transmission line. Unlike current standing-wave parametric amplifiers, this traveling wave architecture robustly achieves high gain over a bandwidth of several gigahertz with sufficient dynamic range to read out 20 superconducting qubits. To achieve this performance, we introduce a subwavelength resonant phase-matching technique that enables the creation of nonlinear microwave devices with unique dispersion relations. We benchmark the amplifier with weak measurements, obtaining a high quantum efficiency of 75% (70% including noise added by amplifiers following the Josephson amplifier). With a flexible design based on compact lumped elements, this Josephson amplifier has broad applicability to microwave metrology and quantum optics. PMID:26338795

  2. Spin qubits with electrically gated polyoxometalate molecules.

    PubMed

    Lehmann, Jörg; Gaita-Arino, Alejandro; Coronado, Eugenio; Loss, Daniel

    2007-05-01

    Spin qubits offer one of the most promising routes to the implementation of quantum computers. Very recent results in semiconductor quantum dots show that electrically-controlled gating schemes are particularly well-suited for the realization of a universal set of quantum logical gates. Scalability to a larger number of qubits, however, remains an issue for such semiconductor quantum dots. In contrast, a chemical bottom-up approach allows one to produce identical units in which localized spins represent the qubits. Molecular magnetism has produced a wide range of systems with properties that can be tailored, but so far, there have been no molecules in which the spin state can be controlled by an electrical gate. Here we propose to use the polyoxometalate [PMo12O40(VO)2]q-, where two localized spins with S = 1/2 can be coupled through the electrons of the central core. Through electrical manipulation of the molecular redox potential, the charge of the core can be changed. With this setup, two-qubit gates and qubit readout can be implemented. PMID:18654290

  3. Josephson junction simulation of neurons

    NASA Astrophysics Data System (ADS)

    Crotty, Patrick; Schult, Dan; Segall, Ken

    2010-07-01

    With the goal of understanding the intricate behavior and dynamics of collections of neurons, we present superconducting circuits containing Josephson junctions that model biologically realistic neurons. These “Josephson junction neurons” reproduce many characteristic behaviors of biological neurons such as action potentials, refractory periods, and firing thresholds. They can be coupled together in ways that mimic electrical and chemical synapses. Using existing fabrication technologies, large interconnected networks of Josephson junction neurons would operate fully in parallel. They would be orders of magnitude faster than both traditional computer simulations and biological neural networks. Josephson junction neurons provide a new tool for exploring long-term large-scale dynamics for networks of neurons.

  4. SLUG Microwave Amplifiers for Scalable Superconducting Qubit Readout

    NASA Astrophysics Data System (ADS)

    Zhu, Shaojiang; Hover, David; Ribeill, Guilhem; Thorbeck, Ted; McDermott, Robert; University of Wisconsin, Madison Team

    2014-03-01

    We describe a phase-insensitive microwave linear amplifier based on the Superconducting Low-inductance Undulatory Galvanometer (SLUG). The amplifier is well suited to the high fidelity quantum nondemolition measurement of superconducting qubits in a circuit quantum electrodynamics architecture. The amplifier has achieved instantaneous bandwidth greater than 400 MHz and system added noise of order one quantum in the GHz frequency range; moreover, the SLUG -1 dB compression point is around -95 dBm, about two orders of magnitude higher than that achieved with typical Josephson parametric amplifiers. We describe efforts to increase instantaneous bandwidth toward 1 GHz and discuss prospects for simultaneous measurement of multiple superconducting qubits using frequency-domain multiplexing with a broadband SLUG amplifier.

  5. Destruction of the Josephson effect by fluctuations

    NASA Astrophysics Data System (ADS)

    Fibich, M.; Kuper, C. G.; Revzen, M.; Ron, Amiram

    1987-12-01

    The Josephson effect occurs when the phases of the complex order parameter of two superconducting bodies separated by an insulating barrier become correlated. Prima facie, this is energetically favorable, since there is a finite coupling energy. However, when fluctuations of charge and phase are taken into account, one has to include the “cost” of these fluctuations in the energy balance. This becomes especially important in very small systems. We use the fluctuation-dissipation theorem to calculate the fluctuation energy and the associated free energy both when the junction is phase-correlated and when it is uncorrelated. In these calculations, we use the “resistively-shunted-junction” model to describe the junction. By comparing the excess free energy in the correlated state (over the uncorrelated one) with the Josephson coupling energy, we find a thermodynamic criterion for destruction of the Josephson characteristic of the junction. In the limiting case where the shunt resistance has a finite zero-temperature limit, we find a nearly universal resistance threshold. In another limit, where the resistance is taken to be the frequency-independent tunneling resistance of the Bogoliubov quasiparticles (in a “semiconductor” picture), our criterion reduces to that of Anderson when the temperature T=0. We predict that for some junction parameters, the system can be reentrant, while for some other values, the transition temperature may be lowered.

  6. Wireless Josephson amplifier

    SciTech Connect

    Narla, A.; Sliwa, K. M.; Hatridge, M.; Shankar, S.; Frunzio, L.; Schoelkopf, R. J.; Devoret, M. H.

    2014-06-09

    Josephson junction parametric amplifiers are playing a crucial role in the readout chain in superconducting quantum information experiments. However, their integration with current 3D cavity implementations poses the problem of transitioning between waveguide, coax cables, and planar circuits. Moreover, Josephson amplifiers require auxiliary microwave components, like directional couplers and/or hybrids, that are sources of spurious losses and impedance mismatches that limit measurement efficiency and amplifier tunability. We have developed a wireless architecture for these parametric amplifiers that eliminates superfluous microwave components and interconnects. This greatly simplifies their assembly and integration into experiments. We present an experimental realization of such a device operating in the 9–11 GHz band with about 100 MHz of amplitude gain-bandwidth product, on par with devices mounted in conventional sample holders. The simpler impedance environment presented to the amplifier also results in increased amplifier tunability.

  7. Theoretical exploration of Josephson Plasma Emission in Intrinsic Josephson Junctions

    SciTech Connect

    Tachiki, M.; Machida, M.

    2000-07-18

    In this paper, the authors theoretically predict the best efficient way for electromagnetic wave emission by Josephson plasma excitation in intrinsic Josephson junctions. First, they briefly derive basic equations describing dynamics of phase differences inside junction sites in intrinsic Josephson junctions, and review the nature of Josephson plasma excitation modes based on the equations. Especially, they make an attention to that Josephson plasma modes have much different dispersion relations depending on the propagating directions and their different modes can be recognized as N standing waves propagating along ah-plane in cases of finite stacked systems composed of N junctions. Second, they consider how to excite their modes and point out that excitations of in-phase mode with the highest propagation velocity among their N modes are the most efficient way for electromagnetic wave emissions. Finally, they clarify that in-phase excitations over all junctions are possible by using Josephson vortex flow states. They show simulation results of Josephson vortex flow states resonating with some Josephson plasma modes and predict that superradiance of electromagnetic field may occur in rectangular vortex flow state in which spatiotemporal oscillations of electromagnetic fields are perfectly in-phase.

  8. Josephson radiation from InSb-nanowire junction

    NASA Astrophysics Data System (ADS)

    van Woerkom, David; Proutski, Alexander; Krivachy, Tamas; Bouman, Daniel; van Gulik, Ruben; Gul, Onder; Cassidy, Maja; Car, Diana; Bakkers, Erik; Kouwenhoven, Leo; Geresdi, Attila

    Semiconducting nanowire Josephson junctions has recently gained interest as building blocks for Majorana circuits and gate-tuneable superconducting qubits . Here we investigate the rich physics of the Andreev bound state spectrum of InSb nanowire junctions utilizing the AC Josephson relation 2eV_bias =hf . We designed and characterized an on-chip microwave circuit coupling the nanowire junction to an Al/AlOx/Al tunnel junction. The DC response of the tunnel junction is affected by photon-assisted quasiparticle current, which gives us the possibility to measure the radiation spectrum of the nanowire junction up to several tens of GHz in frequency. Our circuit design allows for voltage or phase biasing of the Josephson junction enabling direct mapping of Andreev bound states. We discuss our fabrication methods and choice of materials to achieve radiation detection up to a magnetic field of few hundred milliTesla, compatible with Majorana states in spin-orbit coupled nanowires. This work has been supported by the Netherlands Foundations FOM, Abstract NWO and Microsoft Corporation Station Q.

  9. Robust Josephson-Kondo screening cloud in circuit quantum electrodynamics

    NASA Astrophysics Data System (ADS)

    Snyman, Izak; Florens, Serge

    2015-08-01

    We investigate the entanglement properties of a standard circuit-QED setup that consists of a Cooper pair box coupled to a long chain of Josephson junctions. We calculate the static charge polarization at finite distances along the device. Our calculations reveal a deep connection to the Kondo screening cloud, together with robust correlations that are difficult to measure in a condensed matter context. We also find weak sensitivity of these Kondo signatures to the actual parameters and design of the device, demonstrating the universality of the Josephson entanglement cloud.

  10. Genuine four tangle for four qubit states

    SciTech Connect

    Sharma, S. Shelly; Sharma, N. K.

    2014-12-04

    We report a four qubit polynomial invariant that quantifies genuine four-body correlations. The four qubit invariants are obtained from transformation properties of three qubit invariants under a local unitary on the fourth qubit.

  11. Electrically controlled quantum gates for two-spin qubits in two double quantum dots

    NASA Astrophysics Data System (ADS)

    Ramon, Guy

    2011-10-01

    Exchange-coupled singlet-triplet spin qubits in two gate-defined double quantum dots are considered theoretically. Using charge density operators to describe the double-dot orbital states, we calculate the Coulomb couplings between the qubits and identify optimal bias points for single- and two-qubit operations, as well as convenient idle positions. The same intuitive formulation is used to derive dephasing rates of these qubits due to the fluctuating charge environment, thereby providing the main considerations for a quantum computation architecture that is within current experimental capabilities.

  12. Cooper pair splitting in parallel quantum dot Josephson junctions.

    PubMed

    Deacon, R S; Oiwa, A; Sailer, J; Baba, S; Kanai, Y; Shibata, K; Hirakawa, K; Tarucha, S

    2015-01-01

    Devices to generate on-demand non-local spin entangled electron pairs have potential application as solid-state analogues of the entangled photon sources used in quantum optics. Recently, Andreev entanglers that use two quantum dots as filters to adiabatically split and separate the quasi-particles of Cooper pairs have shown efficient splitting through measurements of the transport charge but the spin entanglement has not been directly confirmed. Here we report measurements on parallel quantum dot Josephson junction devices allowing a Josephson current to flow due to the adiabatic splitting and recombination of the Cooper pair between the dots. The evidence for this non-local transport is confirmed through study of the non-dissipative supercurrent while tuning independently the dots with local electrical gates. As the Josephson current arises only from processes that maintain the coherence, we can confirm that a current flows from the spatially separated entangled pair. PMID:26130172

  13. Cooper pair splitting in parallel quantum dot Josephson junctions

    PubMed Central

    Deacon, R. S.; Oiwa, A.; Sailer, J.; Baba, S.; Kanai, Y.; Shibata, K.; Hirakawa, K.; Tarucha, S.

    2015-01-01

    Devices to generate on-demand non-local spin entangled electron pairs have potential application as solid-state analogues of the entangled photon sources used in quantum optics. Recently, Andreev entanglers that use two quantum dots as filters to adiabatically split and separate the quasi-particles of Cooper pairs have shown efficient splitting through measurements of the transport charge but the spin entanglement has not been directly confirmed. Here we report measurements on parallel quantum dot Josephson junction devices allowing a Josephson current to flow due to the adiabatic splitting and recombination of the Cooper pair between the dots. The evidence for this non-local transport is confirmed through study of the non-dissipative supercurrent while tuning independently the dots with local electrical gates. As the Josephson current arises only from processes that maintain the coherence, we can confirm that a current flows from the spatially separated entangled pair. PMID:26130172

  14. Asymmetric resonant exchange qubit under the influence of electrical noise

    NASA Astrophysics Data System (ADS)

    Russ, Maximilian; Burkard, Guido

    2015-06-01

    We investigate the influence of electrical charge noise on a resonant exchange (RX) qubit in a triple quantum dot. This RX qubit is a variation of the exchange-only spin qubit which responds to a narrow-band resonant frequency. Our noise model includes uncorrelated charge noise in each quantum dot giving rise to two independent (noisy) bias parameters ɛ and Δ . We calculate the energy splitting of the two qubit states as a function of these two bias detuning parameters to find "sweet spots," where the qubit is least susceptible to noise. Our investigation shows that such sweet spots exist within the low-bias regime, in which the bias detuning parameters have the same magnitude as the hopping parameters. The location of the sweet spots in the (ɛ ,Δ ) plane depends on the hopping strength and asymmetry between the quantum dots. In the regime of weak charge noise, we identify a new favorable operating regime for the RX qubit based on these sweet spots.

  15. Josephson inplane and tunneling currents in bilayer quantum Hall system

    SciTech Connect

    Ezawa, Z. F.; Tsitsishvili, G.; Sawada, A.

    2013-12-04

    A Bose-Einstein condensation is formed by composite bosons in the quantum Hall state. A composite boson carries the fundamental charge (–e). We investigate Josephson tunneling of such charges in the bilayer quantum Hall system at the total filling ν = 1. We show the existence of the critical current for the tunneling current to be coherent and dissipationless in tunneling experiments with various geometries.

  16. High-fidelity ac gate operations of a three-electron double quantum dot qubit

    NASA Astrophysics Data System (ADS)

    Wong, Clement H.

    2016-01-01

    Semiconductor quantum dots in silicon are promising qubits because of long spin coherence times and their potential for scalability. However, such qubits with complete electrical control and fidelities above the threshold for quantum error correction have not yet been achieved. We show theoretically that the threshold fidelity can be achieved with ac gate operation of the quantum dot hybrid qubit. Formed by three electrons in a double dot, this qubit is electrically controlled, does not require magnetic fields, and runs at gigahertz (GHz) gate speeds. We analyze the decoherence caused by 1 /f charge noise in this qubit, find parameters that minimize the charge noise dependence in the qubit frequency, and determine the optimal working points for ac gate operations that drive the detuning and tunnel coupling.

  17. Josephson Effect in SFNS Josephson Junctions

    NASA Astrophysics Data System (ADS)

    Karminskaya, T. Yu.; Kupriyanov, M. Yu.; Golubov, A. A.; Sidorenko, A. S.

    The critical current, I C, of Josephson junctions both in ramp-type (S-FN-S) and in overlap (SNF-FN-FNS, SN-FN-NS, SNF-N-FNS) geometries has been calculated in the frame of linearized Usadel equations (S-superconductor, F-ferromagnetic, N-normal metal). For the ramp-type structures, in which S electrodes contact directly the end walls of FN bilayer, it is shown that I C may exhibit damping oscillations as a function of both the distance L between superconductors and thicknesses d F,N of ferromagnetic and normal layers. The conditions have been determined under which the decay length and period of oscillation of I C(L) at fixed d F are of the order of decay length of superconducting correlations in the N metal, ξN, that is much larger than in F film. In overlap configurations, in which S films are placed on the top of NF bilayer, the studied junctions have complex SNF or SN electrodes (N or NF bilayer are situated under a superconductor). We demonstrate that in these geometries the critical current can exceed that in ramp-type junctions. Based on these results, the choice of the most practically applicable geometry is discussed.

  18. Entanglement of remote atomic qubits.

    PubMed

    Matsukevich, D N; Chanelière, T; Jenkins, S D; Lan, S-Y; Kennedy, T A B; Kuzmich, A

    2006-01-27

    We report observations of entanglement of two remote atomic qubits, achieved by generating an entangled state of an atomic qubit and a single photon at site , transmitting the photon to site in an adjacent laboratory through an optical fiber, and converting the photon into an atomic qubit. Entanglement of the two remote atomic qubits is inferred by performing, locally, quantum state transfer of each of the atomic qubits onto a photonic qubit and subsequent measurement of polarization correlations in violation of the Bell inequality [EQUATION: SEE TEXT]. We experimentally determine [EQUATION: SEE TEXT]. Entanglement of two remote atomic qubits, each qubit consisting of two independent spin wave excitations, and reversible, coherent transfer of entanglement between matter and light represent important advances in quantum information science. PMID:16486672

  19. Entanglement between static and flying qubits in an Aharonov Bohm double electrometer

    NASA Astrophysics Data System (ADS)

    Schomerus, Henning; Robinson, John P.

    2007-03-01

    We consider the phase-coherent transport of electrons passing through an Aharonov Bohm ring while interacting with a tunnel charge in a double quantum dot (representing a charge qubit) which couples symmetrically to both arms of the ring. For Aharonov Bohm flux ΦAB = h/2e we find that electrons can only be transmitted when they flip the charge qubit's pseudospin parity an odd number of times. The perfect correlations of the dynamics of the pseudospin and individual electronic transmission and reflection events can be used to entangle the charge qubit with an individual passing electron.

  20. Measurements of 1/f noise in Josephson junctions at zero voltage: Implications for decoherence in superconducting quantum bits

    NASA Astrophysics Data System (ADS)

    Mück, Michael; Korn, Matthias; Mugford, C. G. A.; Kycia, J. B.; Clarke, John

    2005-01-01

    Critical current fluctuations with a 1/f spectral density (f is frequency) are potentially a limiting source of intrinsic decoherence in superconducting quantum bits (qubits) based on Josephson tunnel junctions. Prior measurements of this noise were made at nonzero voltages whereas qubits are operated in the zero voltage state. We report measurements of 1/f noise in a dc superconducting quantum interference device first, coupled to a resonant tank circuit and operated in a dispersive mode at zero voltage, and, second, operated conventionally with a current bias in the voltage regime. Both measurements yield essentially the same magnitude of critical current 1/f noise.

  1. Three coupled qubits in a single superconducting quantum circuit

    NASA Astrophysics Data System (ADS)

    Chand, Madhavi; Kundu, Suman; Nehra, N.; Raj, Cosmic; Roy, Tanay; Ranadive, A.; Patankar, Meghan P.; Vijay, R.

    We propose a new design for a 3-qubit system in the 3D circuit QED architecture. Our design exploits the geometrical symmetry of a single superconducting circuit with three degrees of freedom to generate three coupled qubits. However, only one of these is strongly coupled to the environment while the other two are protected from the Purcell effect. Nevertheless, all three qubits can be measured using the standard dispersive technique. We will present preliminary data on this circuit showing evidence of three distinct qubits that retain the essential properties of a 3D transmon, namely insensitivity to charge noise, sufficient anharmonicity and good coherence times. We will also characterize the coupling of the three qubits to each other, to the environment and to a neighboring transmon qubit. Finally, we will compare our design to previous multi-qubit circuits and discuss possible applications in quantum computing and quantum simulations. Funding: Department of Atomic Energy, Govt. of India; Department of Science and Technology, Govt. of India.

  2. Asymptotic geometric phase and purity for phase qubit dispersively coupled to lossy LC circuit

    SciTech Connect

    Mohamed, A.-B.A.; Obada, A.-S.F.

    2011-09-15

    Analytical descriptions of the geometric phases (GPs) for the total system and subsystems are studied for a current biased Josephson phase qubit strongly coupled to a lossy LC circuit in the dispersive limit. It is found that, the GP and purity depend on the damping parameter which leads to the phenomenon of GP death. Coherence parameter delays the phenomenon of a regular sequence of deaths and births of the GP. The asymptotic behavior of the GP and the purity for the qubit-LC resonator state closely follow that for the qubit state, but however, for the LC circuit these asymptotic values are equal to zero. - Highlights: > The model of a current biased Josephson phase qubit, strongly coupled to loss LC circuit, is considered. > Analytical descriptions of the geometric phase (GP) of this model, in the dispersive limit, are studied. > The GP and purity depend on the dissipation which leads to the GP death phenomenon. > Coherence parameter delays the phenomenon of a regular sequence of deaths and births of the GP.

  3. Ultrafast optical control of individual quantum dot spin qubits.

    PubMed

    De Greve, Kristiaan; Press, David; McMahon, Peter L; Yamamoto, Yoshihisa

    2013-09-01

    Single spins in semiconductor quantum dots form a promising platform for solid-state quantum information processing. The spin-up and spin-down states of a single electron or hole, trapped inside a quantum dot, can represent a single qubit with a reasonably long decoherence time. The spin qubit can be optically coupled to excited (charged exciton) states that are also trapped in the quantum dot, which provides a mechanism to quickly initialize, manipulate and measure the spin state with optical pulses, and to interface between a stationary matter qubit and a 'flying' photonic qubit for quantum communication and distributed quantum information processing. The interaction of the spin qubit with light may be enhanced by placing the quantum dot inside a monolithic microcavity. An entire system, consisting of a two-dimensional array of quantum dots and a planar microcavity, may plausibly be constructed by modern semiconductor nano-fabrication technology and could offer a path toward chip-sized scalable quantum repeaters and quantum computers. This article reviews the recent experimental developments in optical control of single quantum dot spins for quantum information processing. We highlight demonstrations of a complete set of all-optical single-qubit operations on a single quantum dot spin: initialization, an arbitrary SU(2) gate, and measurement. We review the decoherence and dephasing mechanisms due to hyperfine interaction with the nuclear-spin bath, and show how the single-qubit operations can be combined to perform spin echo sequences that extend the qubit decoherence from a few nanoseconds to several microseconds, more than 5 orders of magnitude longer than the single-qubit gate time. Two-qubit coupling is discussed, both within a single chip by means of exchange coupling of nearby spins and optically induced geometric phases, as well as over longer-distances. Long-distance spin-spin entanglement can be generated if each spin can emit a photon that is entangled

  4. Ultrafast optical control of individual quantum dot spin qubits

    NASA Astrophysics Data System (ADS)

    De Greve, Kristiaan; Press, David; McMahon, Peter L.; Yamamoto, Yoshihisa

    2013-09-01

    Single spins in semiconductor quantum dots form a promising platform for solid-state quantum information processing. The spin-up and spin-down states of a single electron or hole, trapped inside a quantum dot, can represent a single qubit with a reasonably long decoherence time. The spin qubit can be optically coupled to excited (charged exciton) states that are also trapped in the quantum dot, which provides a mechanism to quickly initialize, manipulate and measure the spin state with optical pulses, and to interface between a stationary matter qubit and a ‘flying’ photonic qubit for quantum communication and distributed quantum information processing. The interaction of the spin qubit with light may be enhanced by placing the quantum dot inside a monolithic microcavity. An entire system, consisting of a two-dimensional array of quantum dots and a planar microcavity, may plausibly be constructed by modern semiconductor nano-fabrication technology and could offer a path toward chip-sized scalable quantum repeaters and quantum computers. This article reviews the recent experimental developments in optical control of single quantum dot spins for quantum information processing. We highlight demonstrations of a complete set of all-optical single-qubit operations on a single quantum dot spin: initialization, an arbitrary SU(2) gate, and measurement. We review the decoherence and dephasing mechanisms due to hyperfine interaction with the nuclear-spin bath, and show how the single-qubit operations can be combined to perform spin echo sequences that extend the qubit decoherence from a few nanoseconds to several microseconds, more than 5 orders of magnitude longer than the single-qubit gate time. Two-qubit coupling is discussed, both within a single chip by means of exchange coupling of nearby spins and optically induced geometric phases, as well as over longer-distances. Long-distance spin-spin entanglement can be generated if each spin can emit a photon that is

  5. Directional Amplification with a Josephson Circuit

    NASA Astrophysics Data System (ADS)

    Abdo, Baleegh; Sliwa, Katrina; Frunzio, Luigi; Devoret, Michel

    2013-07-01

    Nonreciprocal devices perform crucial functions in many low-noise quantum measurements, usually by exploiting magnetic effects. In the proof-of-principle device presented here, on the other hand, two on-chip coupled Josephson parametric converters (JPCs) achieve directionality by exploiting the nonreciprocal phase response of the JPC in the transmission-gain mode. The nonreciprocity of the device is controlled in situ by varying the amplitude and phase difference of two independent microwave pump tones feeding the system. At the desired working point and for a signal frequency of 8.453 GHz, the device achieves a forward power gain of 15 dB within a dynamical bandwidth of 9 MHz, a reverse gain of -6dB, and suppression of the reflected signal by 8 dB. We also find that the amplifier adds a noise equivalent to less than 1.5 photons at the signal frequency (referred back to the input). It can process up to 3 photons at the signal frequency per inverse dynamical bandwidth. With a directional amplifier operating along the principles of this device, qubit and readout preamplifier could be integrated on the same chip.

  6. SLUG Microwave Amplifier as a Nonreciprocal Gain Element for Scalable Qubit Readout

    NASA Astrophysics Data System (ADS)

    Thorbeck, Ted; Leonard, Edward; Zhu, Shaojiang; McDermott, Robert

    Josephson parametric amplifiers for superconducting qubits require several stages of cryogenic isolation to protect the qubit from strong microwave pump tones and downstream noise. But isolators and circulators are large, expensive and magnetic, so they are an obstacle to scaling up a superconducting quantum computer. In contrast, the SLUG (Superconducting Low-inductance Undulatory Galvanometer) is a high gain, broadband, low noise microwave amplifier that provides built-in reverse isolation. Here, we describe the dependence of the SLUG reverse isolation on signal frequency and device operating point. We show that the reverse isolation of the SLUG can be as large as or larger than that of a bulk commercial isolator. Finally, we discuss the use of the SLUG to read out a transmon qubit without isolators or circulators.

  7. Noise and Directionality in a SLUG Microwave Amplifier for Superconducting Qubit Readout

    NASA Astrophysics Data System (ADS)

    Thorbeck, Ted; Zhu, Shaojiang; Leonard, Edward; McDermott, Robert

    2015-03-01

    Josephson parametric amplifiers have been widely used for low-noise dispersive readout of superconducting qubits. However, multiple stages of cryogenic isolation are required to protect the qubit from the strong microwave pump tone and from the high temperature noise of downstream gain stages. We want to remove circulators and isolators from the measurement chain because they are bulky, expensive, and magnetic. The SLUG (superconducting low-inductance undulatory galvanometer) is a microwave amplifier that achieves broad bandwidth, low added noise, and high gain. In this talk we discuss measurements of the SLUG added noise (less than photon system added noise). We describe theoretical and experimental investigations of the SLUG reverse isolation. Finally, we discuss backaction of the SLUG on the measured qubit, and we present strategies for the suppression of SLUG backaction.

  8. Using Spontaneous Emission of a Qubit as a Resource for Feedback Control.

    PubMed

    Campagne-Ibarcq, P; Jezouin, S; Cottet, N; Six, P; Bretheau, L; Mallet, F; Sarlette, A; Rouchon, P; Huard, B

    2016-08-01

    Persistent control of a transmon qubit is performed by a feedback protocol based on continuous heterodyne measurement of its fluorescence. By driving the qubit and cavity with microwave signals whose amplitudes depend linearly on the instantaneous values of the quadratures of the measured fluorescence field, we show that it is possible to stabilize permanently the qubit in any targeted state. Using a Josephson mixer as a phase-preserving amplifier, it was possible to reach a total measurement efficiency η=35%, leading to a maximum of 59% of excitation and 44% of coherence for the stabilized states. The experiment demonstrates multiple-input multiple-output analog Markovian feedback in the quantum regime. PMID:27541448

  9. Universal two-qubit interactions, measurement, and cooling for quantum simulation and computing

    NASA Astrophysics Data System (ADS)

    Kapit, Eliot

    2015-07-01

    By coupling pairs of superconducting qubits through a small Josephson junction with a time-dependent flux bias, we show that arbitrary interactions involving any combination of Pauli matrices can be generated with a small number of drive tones applied through the flux bias of the coupling junction. We then demonstrate that similar (though not fully universal) results can be achieved in capacitively coupled qubits by exploiting the higher energy states of the devices through multiphoton drive signals applied to the qubits' flux degrees of freedom. By using this mechanism to couple a qubit to a detuned resonator, the qubit's rotating-frame state can be nondestructively measured along any direction on the Bloch sphere. Finally, we describe how the frequency-converting nature of the couplings can be used to engineer a mechanism analogous to dynamic nuclear polarization in NMR systems, capable of cooling an array of qubits well below the ambient temperature, and outline how higher-order interactions, such as local three-body terms, can be engineered through the same couplings. Our results demonstrate that a programmable quantum simulator for large classes of interacting spin models could be engineered with the same physical hardware.

  10. Systematically Generated Two-Qubit Braids for Fibonacci Anyons

    NASA Astrophysics Data System (ADS)

    Zeuch, Daniel; Carnahan, Caitlin; Bonesteel, N. E.

    We show how two-qubit Fibonacci anyon braids can be generated using a simple iterative procedure which, in contrast to previous methods, does not require brute force search. Our construction is closely related to that of, but with the new feature that it can be used for three-anyon qubits as well as four-anyon qubits. The iterative procedure we use, which was introduced by Reichardt, generates sequences of three-anyon weaves that asymptotically conserve the total charge of two of the three anyons, without control over the corresponding phase factors. The resulting two-qubit gates are independent of these factors and their length grows as log 1/ ɛ, where ɛ is the error, which is asymptotically better than the Solovay-Kitaev method.

  11. Confocal Annular Josephson Tunnel Junctions

    NASA Astrophysics Data System (ADS)

    Monaco, Roberto

    2016-04-01

    The physics of Josephson tunnel junctions drastically depends on their geometrical configurations and here we show that also tiny geometrical details play a determinant role. More specifically, we develop the theory of short and long annular Josephson tunnel junctions delimited by two confocal ellipses. The behavior of a circular annular Josephson tunnel junction is then seen to be simply a special case of the above result. For junctions having a normalized perimeter less than one, the threshold curves in the presence of an in-plane magnetic field of arbitrary orientations are derived and computed even in the case with trapped Josephson vortices. For longer junctions, a numerical analysis is carried out after the derivation of the appropriate motion equation for the Josephson phase. We found that the system is modeled by a modified and perturbed sine-Gordon equation with a space-dependent effective Josephson penetration length inversely proportional to the local junction width. Both the fluxon statics and dynamics are deeply affected by the non-uniform annulus width. Static zero-field multiple-fluxon solutions exist even in the presence of a large bias current. The tangential velocity of a traveling fluxon is not determined by the balance between the driving and drag forces due to the dissipative losses. Furthermore, the fluxon motion is characterized by a strong radial inward acceleration which causes electromagnetic radiation concentrated at the ellipse equatorial points.

  12. Confocal Annular Josephson Tunnel Junctions

    NASA Astrophysics Data System (ADS)

    Monaco, Roberto

    2016-09-01

    The physics of Josephson tunnel junctions drastically depends on their geometrical configurations and here we show that also tiny geometrical details play a determinant role. More specifically, we develop the theory of short and long annular Josephson tunnel junctions delimited by two confocal ellipses. The behavior of a circular annular Josephson tunnel junction is then seen to be simply a special case of the above result. For junctions having a normalized perimeter less than one, the threshold curves in the presence of an in-plane magnetic field of arbitrary orientations are derived and computed even in the case with trapped Josephson vortices. For longer junctions, a numerical analysis is carried out after the derivation of the appropriate motion equation for the Josephson phase. We found that the system is modeled by a modified and perturbed sine-Gordon equation with a space-dependent effective Josephson penetration length inversely proportional to the local junction width. Both the fluxon statics and dynamics are deeply affected by the non-uniform annulus width. Static zero-field multiple-fluxon solutions exist even in the presence of a large bias current. The tangential velocity of a traveling fluxon is not determined by the balance between the driving and drag forces due to the dissipative losses. Furthermore, the fluxon motion is characterized by a strong radial inward acceleration which causes electromagnetic radiation concentrated at the ellipse equatorial points.

  13. Superconducting qubits on the way to a quantum processor

    NASA Astrophysics Data System (ADS)

    Wilhelm, Frank

    2007-03-01

    Experimental research on supeconducting qubits has seen an enormous progress in recent years. About 10 years after its first theoretical proposals, experiments have demonstrated the necessary building blocks for the exploration of quantum information along several avenues: Single qubit-rotations, long coherence times, high-fidelity nondemolition readout, two-qubit interactions and gates, coupling to delocalized qubit modes. With this progress, analogies to other qubit candidates such as magnetic resonance systems, atomic, and optical systems are evident, but we also see the specific strengths of superconducting qubits play out - in situ tunable strong qubit-qubit coupling, strong coupling between qubits and the quantized electromagnetic field, strong intrinsic nonlinearity, and the possibility to fabricate large circuits. Most of these achievements will be discussed later in the session. I will give an introduction to superconducting qubits in the perspective of quantum information processing [1] accessible to outsiders in the field. I will put the different elements of the session in the perspective of an actual scalable architecture which allows for fault-tolerant quantum computation [1,2]. In order to make further progress in direction, the fidelities of quantum operations need to be improved. I will discuss the crucial topic of understanding and further supressing noise from material defects in these systems, which can influence both the phase and bit-flip error rate [3,4]. I will show, how optimal control theory can help to find fast and high-fidelity shaped pulses for superconducting qubits, even though they, other than spin 1/2 systems, have relatively close leakage levels outside the qubit manyfold [5,6]. This technique also allows to optimize pulses in the presence of telegraph noise [6]. Finally, I will describe how the strong nonlinearity of Josephson circuit can be used for the generation of single microwave photons [7] and lead to a nonlinear

  14. Wireless Josephson Junction Arrays

    NASA Astrophysics Data System (ADS)

    Adams, Laura

    2015-03-01

    We report low temperature, microwave transmission measurements on a wireless two- dimensional network of Josephson junction arrays composed of superconductor-insulator -superconductor tunnel junctions. Unlike their biased counterparts, by removing all electrical contacts to the arrays and superfluous microwave components and interconnects in the transmission line, we observe new collective behavior in the transmission spectra. In particular we will show emergent behavior that systematically responds to changes in microwave power at fixed temperature. Likewise we will show the dynamic and collective response of the arrays while tuning the temperature at fixed microwave power. We discuss these spectra in terms of the Berezinskii-Kosterlitz-Thouless phase transition and Shapiro steps. We gratefully acknowledge the support Prof. Steven Anlage at the University of Maryland and Prof. Allen Goldman at the University of Minnesota. Physics and School of Engineering and Applied Sciences.

  15. Filter function formalism beyond pure dephasing and non-Markovian noise in singlet-triplet qubits

    NASA Astrophysics Data System (ADS)

    Barnes, Edwin; Rudner, Mark S.; Martins, Frederico; Malinowski, Filip K.; Marcus, Charles M.; Kuemmeth, Ferdinand

    2016-03-01

    The filter function formalism quantitatively describes the dephasing of a qubit by a bath that causes Gaussian fluctuations in the qubit energies with an arbitrary noise power spectrum. Here, we extend this formalism to account for more general types of noise that couple to the qubit through terms that do not commute with the qubit's bare Hamiltonian. Our approach applies to any power spectrum that generates slow noise fluctuations in the qubit's evolution. We demonstrate our formalism in the case of singlet-triplet qubits subject to both quasistatic nuclear noise and 1 /ωα charge noise and find good agreement with recent experimental findings. This comparison shows the efficacy of our approach in describing real systems and additionally highlights the challenges with distinguishing different types of noise in free induction decay experiments.

  16. Simple Electronic Analog of a Josephson Junction.

    ERIC Educational Resources Information Center

    Henry, R. W.; And Others

    1981-01-01

    Demonstrates that an electronic Josephson junction analog constructed from three integrated circuits plus an external reference oscillator can exhibit many of the circuit phenomena of a real Josephson junction. Includes computer and other applications of the analog. (Author/SK)

  17. The a.c. Josephson effect without superconductivity

    PubMed Central

    Gaury, Benoit; Weston, Joseph; Waintal, Xavier

    2015-01-01

    Superconductivity derives its most salient features from the coherence of the associated macroscopic wave function. The related physical phenomena have now moved from exotic subjects to fundamental building blocks for quantum circuits such as qubits or single photonic modes. Here we predict that the a.c. Josephson effect—which transforms a d.c. voltage Vb into an oscillating signal cos (2eVbt/ħ)—has a mesoscopic counterpart in normal conductors. We show that when a d.c. voltage Vb is applied to an electronic interferometer, there exists a universal transient regime where the current oscillates at frequency eVb/h. This effect is not limited by a superconducting gap and could, in principle, be used to produce tunable a.c. signals in the elusive 0.1–10-THz ‘terahertz gap’. PMID:25765929

  18. Multidimensional Josephson vortices in spin-orbit-coupled Bose-Einstein condensates: Snake instability and decay through vortex dipoles

    NASA Astrophysics Data System (ADS)

    Gallemí, A.; Guilleumas, M.; Mayol, R.; Mateo, A. Muñoz

    2016-03-01

    We analyze the dynamics of Josephson vortex states in two-component Bose-Einstein condensates with Rashba-Dresselhaus spin-orbit coupling by using the Gross-Pitaevskii equation. In one dimension, both in homogeneous and harmonically trapped systems, we report on stationary states containing doubly charged, static Josephson vortices. In multidimensional systems, we find stable Josephson vortices in a regime of parameters typical of current experiments with 87Rb atoms. In addition, we discuss the instability regime of Josephson vortices in disk-shaped condensates, where the snake instability operates and vortex dipoles emerge. We study the rich dynamics that they exhibit in different regimes of the spin-orbit-coupled condensate depending on the orientation of the Josephson vortices.

  19. Intrinsic Josephson effect and single Cooper pair tunneling

    NASA Astrophysics Data System (ADS)

    Yamashita, Tsutomu; Kim, Sang-Jae; Latyshev, Yuri; Nakajima, Kensuke

    2000-06-01

    We proposed a new, small and fast switching gate based on the intrinsic Josephson effect of single crystals of a cuprate superconductor. The switching time is of subpicosecond order, and the operating frequency is up to several terahertz. We used the focused-ion-beam (FIB) method for the fabrication of small Bi 2Sr 2CaCu 2O 8 (Bi-2212) stacked intrinsic Josephson junctions (IJJ) with in-plane size down to the submicron level without the degradation of their Tc. We observed clear Fraunhofer patterns in Ic- B curves and flux-flow velocity of up to 10 6 m/s for the stack junctions with the size of several micrometer scale. For the submicron junction, the low-temperature behavior is governed by the Coulomb-charging effects. This is the first observation of the Coulomb-charging effects in layered high- Tc materials.

  20. The Josephson Effect and e/h

    ERIC Educational Resources Information Center

    Clarke, John

    1970-01-01

    Discusses the theory of the Josephson Effect, the derivation of the Josephson voltage-frequency relation, and methods of measuring the fundamental constatn ratio e/h. Various types of Josephson junctions are described. The impact of the measurement of e/h upin the fundamental constants and quantum electro-dynamics is briefly discussed.…

  1. Critical Current Oscillations of Josephson Junctions with Ferromagnetic Layers

    NASA Astrophysics Data System (ADS)

    Glick, Joseph A.; Khasawneh, Mazin A.; Niedzielski, Bethany M.; Loloee, Reza; Pratt, W. P., Jr.; Birge, Norman O.

    Josephson junctions containing ferromagnetic layers are of considerable interest for the development of practical cryogenic memory and superconducting qubits. Such junctions exhibit a phase shift of π for certain ranges of ferromagnetic layer thickness. We present studies of Nb based micron-scale Josephson junctions using ferromagnetic layers of Ni, Ni81Fe19, or Ni65Co20Fe15. By applying an external magnetic field, the critical current of the junctions containing Ni81Fe19 and Ni65Co20Fe15 is found to follow a characteristic Fraunhofer pattern, and displays the clear switching behavior expected of single-domain magnets. However, the junctions containing Ni exhibit more complex behaviors. The maximum value of the critical current, extracted from the Fraunhofer patterns, oscillates as a function of the ferromagnetic layer thickness, indicating transitions in the phase difference across the junction between values of zero and π. We compare the data to previous work and to models of the 0- π transitions based on existing clean and dirty limit theories. This work was supported by IARPA via ARO Contract W911NF-14-C-0115.

  2. Theory for collective macroscopic tunneling in high- Tc intrinsic Josephson junctions

    NASA Astrophysics Data System (ADS)

    Machida, M.; Koyama, T.

    2007-10-01

    On the basis of the theory for the capacitive coupling in intrinsic Josephson junctions (IJJ's), we theoretically study the macroscopic quantum tunneling in the switching dynamics into the voltage states in IJJ. The effective action obtained by using the path integral formalism reveals that the capacitive coupling splits each of the lowest and higher quantum levels, which are given inside Josephson potential barrier of the single junction derived by dropping off the coupling, into levels composed of the number of junction (N). This level splitting can cause multiple low-frequency Rabi-oscillations and enhance the switching probability compared to the conventional Caldeira-Leggett theory. Furthermore, a possibility as a naturally built-in multi-qubit is discussed.

  3. PREFACE: Nobel Symposium 141: Qubits for Future Quantum Information Nobel Symposium 141: Qubits for Future Quantum Information

    NASA Astrophysics Data System (ADS)

    Claeson, Tord; Delsing, Per; Wendin, Göran

    2009-12-01

    correction, have yet to be solved. It has been predicted that quantum computers will be able to perform certain complicated computations or simulations in minutes or hours instead of years as with present computers. So far there exist very few useful quantum algorithms; however there is hope that the development of these will be stimulated once there is a breakthrough in hardware. Remarkable progress has been made in quantum engineering and quantum measurements, but a large scale quantum computer is still far off. Quantum communication and cryptography are much closer to the market than a quantum computer. The development of quantum information has meant a large push in the field of quantum physics, that previously could only be studied in the microscopic world. Artificial atoms, realized by circuit technology and mimicking the properties of 'natural' atoms, are one example of the new possibilities opened up by quantum engineering. Several different types of qubits have been suggested. Some are based upon microscopic entities, like atoms and ions in traps, or nuclear spins in molecules. They can have long coherence times (i.e. a long period allowing many operations, of the order of 10 000, to be performed before the state needs to be refreshed) but they are difficult to integrate into large systems. Other qubits are based upon solid state components that facilitate integration and coupling between qubits, but they suffer from interactions with the environment and their coherent states have a limited lifetime. Advanced experiments have been performed with superconducting Josephson junctions and many breakthroughs have been reported in the last few years. They have an advantage in the inherent coherence of superconducting Cooper pairs over macroscopic distances. We chose to focus the Nobel Symposium on Qubits for Future Quantum Information on superconducting qubits to allow for depth in discussions, but at the same time to allow comparison with other types of qubits that may

  4. Deterministic doping and the exploration of spin qubits

    SciTech Connect

    Schenkel, T.; Weis, C. D.; Persaud, A.; Lo, C. C.; Chakarov, I.; Schneider, D. H.; Bokor, J.

    2015-01-09

    Deterministic doping by single ion implantation, the precise placement of individual dopant atoms into devices, is a path for the realization of quantum computer test structures where quantum bits (qubits) are based on electron and nuclear spins of donors or color centers. We present a donor - quantum dot type qubit architecture and discuss the use of medium and highly charged ions extracted from an Electron Beam Ion Trap/Source (EBIT/S) for deterministic doping. EBIT/S are attractive for the formation of qubit test structures due to the relatively low emittance of ion beams from an EBIT/S and due to the potential energy associated with the ions' charge state, which can aid single ion impact detection. Following ion implantation, dopant specific diffusion mechanisms during device processing affect the placement accuracy and coherence properties of donor spin qubits. For bismuth, range straggling is minimal but its relatively low solubility in silicon limits thermal budgets for the formation of qubit test structures.

  5. Josephson junction Q-spoiler

    DOEpatents

    Clarke, J.; Hilbert, C.; Hahn, E.L.; Sleator, T.

    1986-03-25

    An automatic Q-spoiler comprising at least one Josephson tunnel junction connected in an LC circuit for flow of resonant current therethrough. When in use in a system for detecting the magnetic resonance of a gyromagnetic particle system, a high energy pulse of high frequency energy irradiating the particle system will cause the critical current through the Josephson tunnel junctions to be exceeded, causing the tunnel junctions to act as resistors and thereby damp the ringing of the high-Q detection circuit after the pulse. When the current has damped to below the critical current, the Josephson tunnel junctions revert to their zero-resistance state, restoring the Q of the detection circuit and enabling the low energy magnetic resonance signals to be detected.

  6. Josephson junction Q-spoiler

    DOEpatents

    Clarke, John; Hilbert, Claude; Hahn, Erwin L.; Sleator, Tycho

    1988-01-01

    An automatic Q-spoiler comprising at least one Josephson tunnel junction connected in an LC circuit for flow of resonant current therethrough. When in use in a system for detecting the magnetic resonance of a gyromagnetic particle system, a high energy pulse of high frequency energy irradiating the particle system will cause the critical current through the Josephson tunnel junctions to be exceeded, causing the tunnel junctions to act as resistors and thereby damp the ringing of the high-Q detection circuit after the pulse. When the current has damped to below the critical current, the Josephson tunnel junctions revert to their zero-resistance state, restoring the Q of the detection circuit and enabling the low energy magnetic resonance signals to be detected.

  7. Dynamics of Josephson pancakes in layered superconductors

    SciTech Connect

    Mints, R.G.; Snapiro, I.B.

    1994-03-01

    We consider a pointlike vortex in a layered superconductor with linear defects in the superconducting layers. We treat these defects as Josephson junctions with high critical current density. We consider the electrodynamics of these junctions within the framework of nonlocal Josephson electrodynamics. We show that Josephson current through a linear defect in a superconducting layer results in a pointlike vortex with a superconducting core residing in this layer (Josephson pancake). We find the mobility of a Josephson pancake. We consider a small amplitude wave in a Josephson junction with nonlocal electrodynamics. We treat a bending wave for an infinite stack of Josephson pancakes. We find the dispersion relation for these waves. We show that their velocities tend to a certain finite limit when the wavelength tends to infinity.

  8. Entanglement generation and quantum information transfer between spatially-separated qubits in different cavities

    NASA Astrophysics Data System (ADS)

    Yang, Chui-Ping; Su, Qi-Ping; Nori, Franco

    2013-11-01

    The generation and control of quantum states of spatially-separated qubits distributed in different cavities constitute fundamental tasks in cavity quantum electrodynamics (QED). An interesting question in this context is how to prepare entanglement and realize quantum information transfer between qubits located at different cavities, which are important in large-scale quantum information processing. In this paper, we consider a physical system consisting of two cavities and three qubits. Two of the qubits are placed in two different cavities while the remaining one acts as a coupler, which is used to connect the two cavities. We propose an approach for generating quantum entanglement and implementing quantum information transfer between the two spatially-separated inter-cavity qubits. The quantum operations involved in this proposal are performed by a virtual photon process; thus the cavity decay is greatly suppressed during operations. In addition, to complete these tasks, only one coupler qubit and one operation step are needed. Moreover, there is no need to apply classical pulses, so that the engineering complexity is much reduced and the operation procedure is greatly simplified. Finally, our numerical results illustrate that high-fidelity implementation of this proposal using superconducting phase qubits and one-dimensional transmission line resonators is feasible for current circuit QED implementations. This proposal can also be applied to other types of superconducting qubits, including flux and charge qubits.

  9. Fabrication of submicron La2-xSrxCuO4 intrinsic Josephson junction stacks

    NASA Astrophysics Data System (ADS)

    Kubo, Yuimaru; Takahide, Yamaguchi; Tanaka, Takayoshi; Ueda, Shinya; Ishii, Satoshi; Tsuda, Shunsuke; Islam, ATM Nazmul; Tanaka, Isao; Takano, Yoshihiko

    2011-02-01

    Intrinsic Josephson junction (IJJ) stacks of cuprate superconductors have potential to be implemented as intrinsic phase qubits working at relatively high temperatures. We report success in fabricating submicron La2-xSrxCuO4 (LSCO) IJJ stacks carved out of single crystals. We also show a new fabrication method in which argon ion etching is performed after focused ion beam etching. As a result, we obtained an LSCO IJJ stack in which resistive multibranches appeared. It may be possible to control the number of stacked IJJs with an accuracy of a single IJJ by developing this method.

  10. Bottom-up superconducting and Josephson junction devices inside a group-IV semiconductor.

    PubMed

    Shim, Yun-Pil; Tahan, Charles

    2014-01-01

    Superconducting circuits are exceptionally flexible, enabling many different devices from sensors to quantum computers. Separately, epitaxial semiconductor devices such as spin qubits in silicon offer more limited device variation but extraordinary quantum properties for a solid-state system. It might be possible to merge the two approaches, making single-crystal superconducting devices out of a semiconductor by utilizing the latest atomistic fabrication techniques. Here we propose superconducting devices made from precision hole-doped regions within a silicon (or germanium) single crystal. We analyse the properties of this superconducting semiconductor and show that practical superconducting wires, Josephson tunnel junctions or weak links, superconducting quantum interference devices (SQUIDs) and qubits are feasible. This work motivates the pursuit of 'bottom-up' superconductivity for improved or fundamentally different technology and physics. PMID:24985349

  11. A Very Small Logical Qubit

    NASA Astrophysics Data System (ADS)

    Kapit, Eliot

    Superconducting qubits are among the most promising platforms for building a quantum computer. However, individual qubit coherence times are not far past the scalability threshold for quantum error correction, meaning that millions of physical devices would be required to construct a useful quantum computer. Consequently, further increases in coherence time are very desirable. In this letter, we blueprint a simple circuit consisting of two transmon qubits and two additional lossy qubits or resonators, which is passively protected against all single qubit quantum error channels through a combination of continuous driving and engineered dissipation. Photon losses are rapidly corrected through two-photon drive fields implemented with driven SQUID couplings, and dephasing from random potential fluctuations is heavily suppressed by the drive fields used to implement the multi-qubit Hamiltonian. Comparing our theoretical model to published noise estimates from recent experiments on flux and transmon qubits, we find that logical state coherence could be improved by a factor of forty or more compared to the individual qubit T1 and T2 using this technique.

  12. Characterizing Ensembles of Superconducting Qubits

    NASA Astrophysics Data System (ADS)

    Sears, Adam; Birenbaum, Jeff; Hover, David; Rosenberg, Danna; Weber, Steven; Yoder, Jonilyn L.; Kerman, Jamie; Gustavsson, Simon; Kamal, Archana; Yan, Fei; Oliver, William

    We investigate ensembles of up to 48 superconducting qubits embedded within a superconducting cavity. Such arrays of qubits have been proposed for the experimental study of Ising Hamiltonians, and efficient methods to characterize and calibrate these types of systems are still under development. Here we leverage high qubit coherence (> 70 μs) to characterize individual devices as well as qubit-qubit interactions, utilizing the common resonator mode for a joint readout. This research was funded by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA) under Air Force Contract No. FA8721-05-C-0002. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of ODNI, IARPA, or the US Government.

  13. Dynamically corrected gates for an exchange-only qubit

    NASA Astrophysics Data System (ADS)

    Hickman, G. T.; Wang, Xin; Kestner, J. P.; Das Sarma, S.

    2013-10-01

    We provide analytical composite pulse sequences that perform dynamical decoupling concurrently with arbitrary rotations for a qubit coded in the spin state of a triple quantum dot. The sequences are designed to respect realistic experimental constraints such as strictly nonnegative couplings. Logical errors and leakage errors are simultaneously corrected. A short pulse sequence is presented to compensate nuclear noise and a longer sequence is presented to simultaneously compensate nuclear and charge noise. The capability developed in this work provides a clear prescription for combatting the relevant sources of noise that currently hinder exchange-only qubit experiments.

  14. PHONONS IN INTRINSIC JOSEPHSON SYSTEMS

    SciTech Connect

    C. PREIS; K. SCHMALZL; ET AL

    2000-10-01

    Subgap structures in the I-V curves of layered superconductors are explained by the excitation of phonons by Josephson oscillations. In the presence of a magnetic field applied parallel to the layers additional structures due to fluxon motion appear. Their coupling with phonons is investigated theoretically and a shift of the phonon resonances in strong magnetic fields is predicted.

  15. Supercurrent in Graphene Josephson Transistors

    NASA Astrophysics Data System (ADS)

    Bao, Wenzhong; Miao, Feng; Liu, Gang; Lau, Chunning

    2008-03-01

    We investigate electrical transport in single or bi-layer graphene devices coupled to superconducting electrodes. In these two-dimensional Josephson junctions, we observed gate tunable supercurrent, multiple Andreev reflections and hysteretic current-voltage characteristics. Latest experimental progress on dependence of supercurrent on temperature, number of layers and source-drain separation will be discussed.

  16. Phase transition of dissipative Josephson arrays in a magnetic field

    SciTech Connect

    Kampf, A.; Schoen, G.

    1988-04-01

    The phase diagram of an array of Josephson junctions in a transverse magnetic field is investigated. The capacitive interactions of charges on the superconducting islands and the associated quantum-mechanical effects, as well as the dissipation due to the flow of normal Ohmic currents, are taken into account. The mean-field approximation of this system can be mapped onto the tight-binding Schroedinger equation for Bloch electrons in a magnetic field, which had been analyzed by Hofstadter. We show how the transition temperature depends on the dissipation and the charging energy.

  17. Josephson current and multiple Andreev reflections in graphene SNS junctions

    NASA Astrophysics Data System (ADS)

    Du, Xu; Skachko, Ivan; Andrei, Eva Y.

    2008-05-01

    The Josephson effect and superconducting proximity effect were observed in superconductor-graphene-superconductor (SGS) Josephson junctions with coherence lengths comparable to the distance between the superconducting leads. By comparing the measured gate dependence of the proximity induced subgap features (multiple Andreev reflections) and of the supercurrent to theoretical predictions, we find that the diffusive junction model yields close quantitative agreement with the results. By contrast, predictions of the ballistic SGS model are inconsistent with the data. We show that all SGS devices reported so far, our own as well as those of other groups, fall in the diffusive junction category. This is attributed to substrate induced potential fluctuations due to trapped charges and to the invasiveness of the metallic leads.

  18. Nonlocal supercurrent in mesoscopic multiterminal SNS Josephson junction in the low-temperature limit

    NASA Astrophysics Data System (ADS)

    Golikova, T. E.; Wolf, M. J.; Beckmann, D.; Batov, I. E.; Bobkova, I. V.; Bobkov, A. M.; Ryazanov, V. V.

    2014-03-01

    A nonlocal supercurrent was observed in mesoscopic planar SNS Josephson junctions with additional normal-metal electrodes, where nonequilibrium quasiparticles were injected from a normal-metal electrode into one of the superconducting banks of the Josephson junction in the absence of a net transport current through the junction. We claim that the observed effect is due to a supercurrent counterflow, appearing to compensate for the quasiparticle flow in the SNS weak link. We have measured the responses of SNS junctions for different distances between the quasiparticle injector and the SNS junction at temperatures far below the superconducting transition temperature. The charge-imbalance relaxation length was estimated by using a modified Kadin, Smith, and Skocpol scheme in the case of a planar geometry. The model developed allows us to describe the interplay of charge imbalance and Josephson effects in the nanoscale proximity system in detail.

  19. Rate of tunneling nonequilibrium quasiparticles in superconducting qubits

    NASA Astrophysics Data System (ADS)

    Ansari, Mohammad H.

    2015-04-01

    In superconducting qubits the lifetime of quantum states cannot be prolonged arbitrarily by decreasing temperature. At low temperature quasiparticles tunneling between the electromagnetic environment and superconducting islands takes the condensate state out of equilibrium due to charge imbalance. We obtain the tunneling rate from a phenomenological model of non-equilibrium, where nonequilibrium quasiparticle tunnelling stimulates a temperature-dependent chemical potential shift in the superconductor. As a result we obtain a non-monotonic behavior for relaxation rate as a function of temperature. Depending on the fabrication parameters for some qubits, the lowest tunneling rate of nonequilibrium quasiparticles can take place only near the onset temperature below which nonequilibrium quasiparticles dominate over equilibrium one. Our theory also indicates that such tunnelings can influence the probability of transitions in qubits through a coupling to the zero-point energy of phase fluctuations.

  20. A proposal for the realization of universal quantum gates via superconducting qubits inside a cavity

    SciTech Connect

    Obada, A.-S.F.; Hessian, H.A.; Mohamed, A.-B.A.; Homid, Ali H.

    2013-07-15

    A family of quantum logic gates is proposed via superconducting (SC) qubits coupled to a SC-cavity. The Hamiltonian for SC-charge qubits inside a single mode cavity is considered. Three- and two-qubit operations are generated by applying a classical magnetic field with the flux. Therefore, a number of quantum logic gates are realized. Numerical simulations and calculation of the fidelity are used to prove the success of these operations for these gates. -- Highlights: •A family of quantum logic gates is proposed via SC-qubits coupled to a cavity. •Three- and two-qubit operations are generated via a classical field with the flux. •Numerical simulations and calculation of the fidelity are used to prove the success of these operations for these gates.

  1. Interface between topological and superconducting qubits.

    PubMed

    Jiang, Liang; Kane, Charles L; Preskill, John

    2011-04-01

    We propose and analyze an interface between a topological qubit and a superconducting flux qubit. In our scheme, the interaction between Majorana fermions in a topological insulator is coherently controlled by a superconducting phase that depends on the quantum state of the flux qubit. A controlled-phase gate, achieved by pulsing this interaction on and off, can transfer quantum information between the topological qubit and the superconducting qubit. PMID:21517365

  2. Photonic phase transition in circuit quantum electrodynamics lattices coupled to superconducting phase qubits

    NASA Astrophysics Data System (ADS)

    Liu, YiMin; Jin, WuYin; You, JiaBin

    2014-11-01

    A hybrid quantum architecture was proposed to engineer a localization-delocalization phase transition of light in a two-dimension square lattices of superconducting coplanar waveguide resonators, which are interconnected by current-biased Josephson junction phase qubits. We find that the competition between the on-site repulsion and the nonlocal photonic hopping leads to the Mott insulator-superfluid transition. By using the mean-field approach and the quantum master equation, the phase boundary between these two different phases could be obtained when the dissipative effects of superconducting resonators and phase qubit are considered. The good tunability of the effective on-site repulsion and photon-hopping strengths enable quantum simulation on condensed matter physics and many-body models using such a superconducting resonator lattice system. The experimental feasibility is discussed using the currently available technology in the circuit QED.

  3. Quantum dynamics in the bosonic Josephson junction

    SciTech Connect

    Chuchem, Maya; Cohen, Doron; Smith-Mannschott, Katrina; Hiller, Moritz; Kottos, Tsampikos; Vardi, Amichay

    2010-11-15

    We employ a semiclassical picture to study dynamics in a bosonic Josephson junction with various initial conditions. Phase diffusion of coherent preparations in the Josephson regime is shown to depend on the initial relative phase between the two condensates. For initially incoherent condensates, we find a universal value for the buildup of coherence in the Josephson regime. In addition, we contrast two seemingly similar on-separatrix coherent preparations, finding striking differences in their convergence to classicality as the number of particles increases.

  4. Deterministic Quantum Teleportation of a Particular Six-qubit State Using Six-qubit Cluster State

    NASA Astrophysics Data System (ADS)

    Tan, Xiaoqing; Zhang, Xiaoqian; Song, Tingting

    2016-01-01

    A new application of the six-qubit cluster state for quantum teleportation of a particular six-qubit state is proposed. In this scheme, the sender performs three controlled-NOT operations to turn the particular six-qubit state into a three-qubit state. The receiver can reconstruct the particular six-qubit state by performing some appropriate unitary transformations on her qubits. The probability of success is 100 % and the fidelity is 1.

  5. Comment on"Teleportation Protocol of Three-Qubit State Using Four-Qubit Quantum Channels"

    NASA Astrophysics Data System (ADS)

    Wei, Zhao-Hui; Zha, Xin-Wei; Yu, Yan

    2016-06-01

    Recently, Choudhury (Int. J. Theor. Phys. 10, 1007 2016), proposed a teleportation protocol of three-qubit state using four-qubit quantum channels.According to their scheme the three-qubit entangled states could be teleported by use of three simultaneous quantum channels of four-qubit cluster states. In this paper,we emphasize that the same three-qubit entangled states can be teleported perfectly by using only one quantum channel of four-qubit cluster states.

  6. Quantum Teleportation of Three and Four-Qubit State Using Multi-qubit Cluster States

    NASA Astrophysics Data System (ADS)

    Li, Yuan-hua; Li, Xiao-lan; Nie, Li-ping; Sang, Ming-huang

    2016-03-01

    We provide various schemes for quantum teleportation by using the four and five qubit cluster states. Explicit protocols for the perfect quantum teleportation of three and four qubit states are illustrated. It is found that the four-qubit cluster state can be used for perfect quantum teleportation of a special form of three-qubit state and the five-qubit cluster state can be used for perfect quantum teleportation of a special form of four-qubit state.

  7. Global relaxation of superconducting qubits

    SciTech Connect

    Ojanen, T.; Niskanen, A. O.; Nakamura, Y.; Abdumalikov, A. A. Jr.

    2007-09-01

    We consider coupled quantum two-state systems (qubits) exposed to a global relaxation process. The global relaxation refers to the assumption that qubits are coupled to the same quantum bath with approximately equal strengths, appropriate for long-wavelength environmental fluctuations. We show that interactions do not spoil the picture of Dicke's subradiant and super-radiant states where quantum interference effects lead to striking deviations from the independent relaxation picture. Remarkably, the system possess a stable entangled state and a state decaying faster than single qubit excitations. We propose a scheme for how these effects can be experimentally accessed in superconducting flux qubits and, possibly, used in constructing long-lived entangled states.

  8. Coherent controlization using superconducting qubits

    PubMed Central

    Friis, Nicolai; Melnikov, Alexey A.; Kirchmair, Gerhard; Briegel, Hans J.

    2015-01-01

    Coherent controlization, i.e., coherent conditioning of arbitrary single- or multi-qubit operations on the state of one or more control qubits, is an important ingredient for the flexible implementation of many algorithms in quantum computation. This is of particular significance when certain subroutines are changing over time or when they are frequently modified, such as in decision-making algorithms for learning agents. We propose a scheme to realize coherent controlization for any number of superconducting qubits coupled to a microwave resonator. For two and three qubits, we present an explicit construction that is of high relevance for quantum learning agents. We demonstrate the feasibility of our proposal, taking into account loss, dephasing, and the cavity self-Kerr effect. PMID:26667893

  9. Novel approaches to high fidelity qubit state measurement in circuit quantum electrodynamics

    NASA Astrophysics Data System (ADS)

    Ginossar, Eran

    2011-03-01

    Qubit state measurement (`readout') in solid state systems is an open problem, which is currently the subject of intensive experimental and theoretical research. Achieving high fidelity in a single-shot measurement is an interesting quantum control problem, as well as an important component for the successful implementation of quantum information protocols. For superconducting qubits we can distinguish between linear dispersive and nonlinear methods, the latter relying on the bistability of a nonlinear resonator. In the context of circuit quantum electrodynamics, the transmon qubit is strongly coupled to a linear resonator and described by a generalized Jaynes-Cummings model (JCM) with external drive and dissipation. Recent novel approaches to achieve high-fidelity readout in the dispersive regime rely on the intrinsic nonlinearity of the JCM and its ultimate linearity in the high excitation regime. In the degenerate regime we rely on the photon blockade and precise transient dynamics of the system. This regime presents a theoretical challenge and the driven damped JCM model exhibits a dynamical phase transition. Another proposed approach extends the Josephson Bifurcation Amplifier and employs the dynamical effects of frequency chirping of the drive on the coupled qubit-resonator system. We will discuss the physics of these different regimes and describe the readout schemes which have been demonstrated by recent experiments and quantum simulations, as well as the role of quantum fluctuations and optimal control.

  10. Deterministic entanglement of two transmon qubits by parity measurement and digital feedback

    NASA Astrophysics Data System (ADS)

    Ristè, Diego; Dukalski, Marcin; Watson, Christopher; de Lange, Gijs; Tiggelman, Marijn; Blanter, Yaroslav; Lehnert, Konrad; Schouten, Raymond; Dicarlo, Leonardo

    2014-03-01

    While quantum measurement typically collapses quantum superpositions into a basis state, a special type of joint measurement, detecting the parity of qubit excitations, can create entanglement. Building on recent developments in quantum nondemolition measurement and feedback control in circuit QED, we realize a continuous-time parity meter for two 3D-transmon qubits using a dispersively coupled cavity and Josephson parametric amplification. Starting from a maximal superposition, we first generate entanglement with up to 88 % fidelity to the closest Bell state by postselecting on the odd-parity result. The infidelity is due to measurement-induced dephasing, arising from imperfect cavity resonance matching in the odd-parity subspace and finite transmission in the even. We then incorporate the parity meter into a digital qubit feedback loop to turn the generation of entanglement from probabilistic to deterministic, achieving 66 % fidelity to the targeted Bell state. This combination of parity measurement and conditional qubit control is at the basis of modern error correction protocols. Research funded by FOM, NWO, and the European projects SOLID and SCALEQIT.

  11. Tenfold increase in the Rabi decay time of the quantum dot hybrid qubit

    NASA Astrophysics Data System (ADS)

    Thorgrimsson, Brandur; Kim, Dohun; Simmons, C. B.; Ward, Daniel R.; Foote, Ryan H.; Savage, D. E.; Lagally, M. G.; Friesen, Mark; Coppersmith, S. N.; Eriksson, M. A.

    The quantum dot hybrid qubit is formed from three electrons in a double quantum dot. In previous work, we showed that the hybrid qubit has the speed of a charge qubit and the stability of a spin qubit. Here, we show that the hybrid qubit is also highly tunable, and can be tuned into regimes with desirable coherence properties. By changing the interdot tunnel rate by only 25%, from 5 GHz to 6.25 GHz, we are able to increase the Rabi decay time by a factor of ten, from 18 ns to 177 ns. We attribute this improvement to the refinement of an extended ``sweet spot'' in the energy dispersion of the hybrid qubit, where the qubit is less susceptible to charge noise, which is a dominant source of decoherence. This work was supported in part by ARO (W911NF-12-0607) and NSF (DMR-1206915 and PHY-1104660). Development and maintenance of the growth facilities used for fabricating samples is sup- ported by DOE (DE-FG02-03ER46028). This research utilized NSF-supported shared facilities at the University of Wisconsin-Madison.

  12. Local cloning of entangled qubits

    SciTech Connect

    Choudhary, Sujit K.; Kunkri, Samir; Rahaman, Ramij; Roy, Anirban

    2007-11-15

    We discuss the exact cloning of orthogonal but entangled qubits under local operations and classical communication. The amount of entanglement necessary in a blank copy is obtained for various cases. Surprisingly, this amount is more than 1 ebit for certain sets of two nonmaximal but equally entangled states of two qubits. To clone any three Bell states, at least log{sub 2} 3 ebit is necessary.

  13. Josephson junctions and dark energy

    NASA Astrophysics Data System (ADS)

    Jetzer, Philippe; Straumann, Norbert

    2006-08-01

    In a recent paper Beck and Mackey [C. Beck, M.C. Mackey, astro-ph/0603397] argue that the argument we gave in our paper [Ph. Jetzer, N. Straumann, Phys. Lett. B 606 (2005) 77, astro-ph/0411034] to disprove their claim that dark energy can be discovered in the Lab through noise measurements of Josephson junctions is incorrect. In particular, they emphasize that the measured noise spectrum in Josephson junctions is a consequence of the fluctuation dissipation theorem, while our argument was based on equilibrium statistical mechanics. In this note we show that the fluctuation dissipation relation does not depend upon any shift of vacuum (zero-point) energies, and therefore, as already concluded in our previous paper, dark energy has nothing to do with the proposed measurements.

  14. Characterization of qubit dephasing by Landau-Zener-Stückelberg-Majorana interferometry.

    PubMed

    Forster, F; Petersen, G; Manus, S; Hänggi, P; Schuh, D; Wegscheider, W; Kohler, S; Ludwig, S

    2014-03-21

    Controlling coherent interaction at avoided crossings and the dynamics there is at the heart of quantum information processing. A particularly intriguing dynamics is observed in the Landau-Zener regime, where periodic passages through the avoided crossing result in an interference pattern carrying information about qubit properties. In this Letter, we demonstrate a straightforward method, based on steady-state experiments, to obtain all relevant information about a qubit, including complex environmental influences. We use a two-electron charge qubit defined in a lateral double quantum dot as test system and demonstrate a long coherence time of T2 ≃ 200 ns, which is limited by electron-phonon interaction. PMID:24702402

  15. High-fidelity singlet-triplet S -T- qubits in inhomogeneous magnetic fields

    NASA Astrophysics Data System (ADS)

    Wong, Clement H.; Eriksson, M. A.; Coppersmith, S. N.; Friesen, Mark

    2015-07-01

    We propose an optimized set of quantum gates for a singlet-triplet qubit in a double quantum dot with two electrons utilizing the S -T- subspace. Qubit rotations are driven by the applied magnetic field and a field gradient provided by a micromagnet. We optimize the fidelity of this qubit as a function of the magnetic fields, taking advantage of "sweet spots" where the rotation frequencies are independent of the energy level detuning, providing protection against charge noise. We simulate gate operations and qubit rotations in the presence of quasistatic noise from charge and nuclear spins as well as leakage to nonqubit states. Our results show that, for silicon quantum dots, gate fidelities greater than 99 % should be realizable, for rotations about two nearly orthogonal axes.

  16. Josephson-Majorana cycle in topological single-electron hybrid transistors

    NASA Astrophysics Data System (ADS)

    Didier, Nicolas; Gibertini, Marco; Moghaddam, Ali G.; König, Jürgen; Fazio, Rosario

    2013-07-01

    Charge transport through a small topological superconducting island in contact with a normal and a superconducting electrode occurs through a cycle that involves coherent oscillations of Cooper pairs and tunneling in/out the normal electrode through a Majorana bound state, the Josephson-Majorana cycle. We illustrate this mechanism by studying the current-voltage characteristics of a superconductor-topological superconductor-normal metal single-electron transistor. At low bias and temperature the Josephson-Majorana cycle is the dominant mechanism for transport. We discuss a three-terminal configuration where the nonlocal character of the Majorana bound states is emergent.

  17. Ultimately short ballistic vertical graphene Josephson junctions

    PubMed Central

    Lee, Gil-Ho; Kim, Sol; Jhi, Seung-Hoon; Lee, Hu-Jong

    2015-01-01

    Much efforts have been made for the realization of hybrid Josephson junctions incorporating various materials for the fundamental studies of exotic physical phenomena as well as the applications to superconducting quantum devices. Nonetheless, the efforts have been hindered by the diffusive nature of the conducting channels and interfaces. To overcome the obstacles, we vertically sandwiched a cleaved graphene monoatomic layer as the normal-conducting spacer between superconducting electrodes. The atomically thin single-crystalline graphene layer serves as an ultimately short conducting channel, with highly transparent interfaces with superconductors. In particular, we show the strong Josephson coupling reaching the theoretical limit, the convex-shaped temperature dependence of the Josephson critical current and the exceptionally skewed phase dependence of the Josephson current; all demonstrate the bona fide short and ballistic Josephson nature. This vertical stacking scheme for extremely thin transparent spacers would open a new pathway for exploring the exotic coherence phenomena occurring on an atomic scale. PMID:25635386

  18. Quantum Teleportation of A Four-qubit State by Using Six-qubit Cluster State

    NASA Astrophysics Data System (ADS)

    Li, Yuan-hua; Sang, Ming-huang; Wang, Xian-ping; Nie, Yi-you

    2016-08-01

    We propose a scheme for perfect quantum teleportation of a special form of four-qubit state by using a six-qubit cluster state as quantum channel. In our scheme, the sender only needs six-qubit von-Neumann projective measurements, and the receiver can reconstruct the original four-qubit state by applying the appropriate unitary operation.

  19. Quantum Teleportation of A Four-qubit State by Using Six-qubit Cluster State

    NASA Astrophysics Data System (ADS)

    Li, Yuan-hua; Sang, Ming-huang; Wang, Xian-ping; Nie, Yi-you

    2016-03-01

    We propose a scheme for perfect quantum teleportation of a special form of four-qubit state by using a six-qubit cluster state as quantum channel. In our scheme, the sender only needs six-qubit von-Neumann projective measurements, and the receiver can reconstruct the original four-qubit state by applying the appropriate unitary operation.

  20. Quantum Teleportation of a Three-qubit State using a Five-qubit Cluster State

    NASA Astrophysics Data System (ADS)

    Liu, Zhong-min; Zhou, Lin

    2014-12-01

    Recently Muralidharan and Panigrahi (Phys. Rev. A 78, 062333 2008) had shown that using a five-qubit cluster state as quantum channel, it is possible to teleport an arbitrary single-qubit state and an arbitrary two-qubit state. In this paper, we investigate this channel for the teleportation of a special form of three-qubit state.

  1. Nanoscale broadband transmission lines for spin qubit control

    NASA Astrophysics Data System (ADS)

    Dehollain, J. P.; Pla, J. J.; Siew, E.; Tan, K. Y.; Dzurak, A. S.; Morello, A.

    2013-01-01

    The intense interest in spin-based quantum information processing has caused an increasing overlap between the two traditionally distinct disciplines of magnetic resonance and nanotechnology. In this work we discuss rigorous design guidelines to integrate microwave circuits with charge-sensitive nanostructures, and describe how to simulate such structures accurately and efficiently. We present a new design for an on-chip, broadband, nanoscale microwave line that optimizes the magnetic field used to drive a spin-based quantum bit (or qubit) while minimizing the disturbance to a nearby charge sensor. This new structure was successfully employed in a single-spin qubit experiment, and shows that the simulations accurately predict the magnetic field values even at frequencies as high as 30 GHz.

  2. Adiabatic Mach-Zehnder Interferometry on a Quantized Bose-Josephson Junction

    SciTech Connect

    Lee, Chaohong

    2006-10-13

    We propose a scheme to achieve Mach-Zehnder interferometry using a quantized Bose-Josephson junction with a negative charging energy. The quantum adiabatic evolution through a dynamical bifurcation is used to accomplish the beam splitting and recombination. The negative charging energy ensures the existence of a path-entangled state which enhances the phase measurement precision to the Heisenberg limit. A feasible detection procedure is also presented. The scheme should be realizable with current technology.

  3. Nonlocality of cluster states of qubits

    SciTech Connect

    Scarani, Valerio; Acin, Antonio; Schenck, Emmanuel; Aspelmeyer, Markus

    2005-04-01

    We investigate cluster states of qubits with respect to their nonlocal properties. We demonstrate that a Greenberger-Horne-Zeilinger (GHZ) argument holds for any cluster state: more precisely, it holds for any partial, thence mixed, state of a small number of connected qubits (five, in the case of one-dimensional lattices). In addition, we derive a Bell inequality that is maximally violated by the four-qubit cluster state and is not violated by the four-qubit GHZ state.

  4. Improved superconducting qubit readout by qubit-induced nonlinearities.

    PubMed

    Boissonneault, Maxime; Gambetta, J M; Blais, Alexandre

    2010-09-01

    In dispersive readout schemes, qubit-induced nonlinearity typically limits the measurement fidelity by reducing the signal-to-noise ratio (SNR) when the measurement power is increased. Contrary to seeing the nonlinearity as a problem, here we propose to use it to our advantage in a regime where it can increase the SNR. We show analytically that such a regime exists if the qubit has a many-level structure. We also show how this physics can account for the high-fidelity avalanchelike measurement recently reported by Reed et al. [arXiv:1004.4323v1]. PMID:20867500

  5. Double sweet-spot operation of the resonant exchange qubit in three-electron quantum dots

    NASA Astrophysics Data System (ADS)

    Burkard, Guido

    The resonant exchange (RX) qubit is a promising variant of the exchange-only spin qubit in a triple quantum dot which responds to a narrow-band resonant frequency. But the advantage of a permanently applied exchange splitting for spin control generally entails an increased susceptibility to charge noise. We have investigated the influence of electrical charge noise on a resonant exchange (RX) qubit by taking into account uncorrelated noise in each quantum dot, giving rise to two independent noisy bias parameters ɛ and Δ. Calculating the energy splitting of the two qubit states as a function of these two bias detuning parameters, we have identified ``sweet spots,'' where the qubit is least susceptible to noise. Our investigation shows that the sweet spots exist within the low-bias regime, in which the bias detuning parameters have the same magnitude as the hopping parameters between the dots. By calculating and comparing the charge dephasing rates at the various operating points of the RX qubit, we identify a new favorable operating regime for the RX qubit in the case of weak noise, based on these double sweet spots. In contrast, spin noise can be mitigated using exchange-based dynamical decoupling sequences that have been optimized using two different strategies, Uhrig dynamical decoupling (UDD) and optimized filter function dynamical decoupling (OFDD). Finally, we give a brief outlook towards the possibility of long-distance coupling between resonant exchange qubits mediated by a microwave cavity. Supported by DFG through SFB 767 and ARO through Grant No. W911NF-15-1-0149.

  6. Lower bound of concurrence for qubit systems

    NASA Astrophysics Data System (ADS)

    Zhu, Xue-Na; Fei, Shao-Ming

    2013-11-01

    We study the concurrence of four-qubit quantum states and provide analytical lower bounds of concurrence in terms of the monogamy inequality of concurrence for qubit systems. It is shown that these lower bounds are able to improve the existing bounds and detect entanglement better. The approach is generalized to arbitrary qubit systems.

  7. Cascade of parametric resonances in coupled Josephson junctions

    NASA Astrophysics Data System (ADS)

    Shukrinov, Yu. M.; Azemtsa-Donfack, H.; Rahmonov, I. R.; Botha, A. E.

    2016-06-01

    We found that the coupled system of Josephson junctions under external electromagnetic radiation demonstrates a cascade of parametric instabilities. These instabilities appear along the IV characteristics within bias current intervals corresponding to Shapiro step subharmonics and lead to charging in the superconducting layers. The amplitudes of the charge oscillations increase with increasing external radiation power. We demonstrate the existence of longitudinal plasma waves at the corresponding bias current values. An essential advantage of the parametric instabilities in the case of subharmonics is the lower amplitude of radiation that is needed for the creation of the longitudinal plasma wave. This fact gives a unique possibility to create and control longitudinal plasma waves in layered superconductors. We propose a novel experiment for studying parametric instabilities and the charging of superconducting layers based on the simultaneous variation of the bias current and radiation amplitude.

  8. Entanglement structures in qubit systems

    NASA Astrophysics Data System (ADS)

    Rangamani, Mukund; Rota, Massimiliano

    2015-09-01

    Using measures of entanglement such as negativity and tangles we provide a detailed analysis of entanglement structures in pure states of non-interacting qubits. The motivation for this exercise primarily comes from holographic considerations, where entanglement is inextricably linked with the emergence of geometry. We use the qubit systems as toy models to probe the internal structure, and introduce some useful measures involving entanglement negativity to quantify general features of entanglement. In particular, our analysis focuses on various constraints on the pattern of entanglement which are known to be satisfied by holographic sates, such as the saturation of Araki-Lieb inequality (in certain circumstances), and the monogamy of mutual information. We argue that even systems as simple as few non-interacting qubits can be useful laboratories to explore how the emergence of the bulk geometry may be related to quantum information principles.

  9. Heralded amplification of photonic qubits.

    PubMed

    Bruno, Natalia; Pini, Vittorio; Martin, Anthony; Verma, Varun B; Nam, Sae Woo; Mirin, Richard; Lita, Adriana; Marsili, Francesco; Korzh, Boris; Bussières, Félix; Sangouard, Nicolas; Zbinden, Hugo; Gisin, Nicolas; Thew, Rob

    2016-01-11

    We demonstrate postselection free heralded qubit amplification for Time-Bin qubits and single photon states in an all-fibre, telecom-wavelength, scheme that highlights the simplicity, stability and potential for fully integrated photonic solutions. Exploiting high-efficiency superconducting detectors, the gain, fidelity and the performance of the amplifier are studied as a function of loss. We also demonstrate the first heralded single photon amplifier with independent sources. This provides a significant advance towards demonstrating device-independent quantum key distribution as well as fundamental tests of quantum mechanics over extended distances. PMID:26832244

  10. Pinning-modulated non-collective Josephson-vortex motion in stacked Josephson junctions.

    SciTech Connect

    Jin, Y.-D.; Lee, G.-H.; Lee, H.-J.; Bae, M.-H.; Koshelev, A. E.; Pohang Univ. of Science and Technology; Univ. of Illinois

    2009-01-01

    Josephson vortices in naturally stacked Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} tunneling junctions display rich dynamic behavior that derives from the coexistence of three basic states: static Josephson vortex lattice, coherently moving lattice, and incoherent quasiparticle tunneling state. The rich structure of hysteretic branches observed in the current-voltage characteristics can be understood as combinatorial combinations of these three states which are realized in different junctions and evolve separately with magnetic field and bias current. In particular, the multiple Josephson vortex flow branches at low-bias currents arise from the individual depinning of Josephson vortex rows in each junction.

  11. New Phenomena in Josephson SINIS Junctions

    NASA Astrophysics Data System (ADS)

    Volkov, A. F.

    1995-06-01

    We analyze the dc and ac Josephson effects in SaINISb junctions in which an additional bias current flows in the N layer. The case of low temperatures and voltages \\(eV, T<<Δ\\) is considered in the dirty limit. We show that the critical Josephson current may change sign, and the considered SINIS junction may become a π junction if the voltage drop across the N/Sa interface exceeds a certain value \\(eVN>Δ/2\\). The ac Josephson effect may arise even if the current flows only through the N/Sa interface, whereas the current through the Sb/N interface is absent.

  12. Towards optimizing two-qubit operations in three-electron double quantum dots

    NASA Astrophysics Data System (ADS)

    Frees, Adam; Gamble, John King; Mehl, Sebastian; Friesen, Mark; Coppersmith, S. N.

    The successful implementation of single-qubit gates in the quantum dot hybrid qubit motivates our interest in developing a high fidelity two-qubit gate protocol. Recently, extensive work has been done to characterize the theoretical limitations and advantages in performing two-qubit operations at an operation point located in the charge transition region. Additionally, there is evidence to support that single-qubit gate fidelities improve while operating in the so-called ``far-detuned'' region, away from the charge transition. Here we explore the possibility of performing two-qubit gates in this region, considering the challenges and the benefits that may present themselves while implementing such an operational paradigm. This work was supported in part by ARO (W911NF-12-0607) (W911NF-12-R-0012), NSF (PHY-1104660), ONR (N00014-15-1-0029). The authors gratefully acknowledge support from the Sandia National Laboratories Truman Fellowship Program, which is funded by the Laboratory Directed Research and Development (LDRD) Program. Sandia is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.

  13. Long-range, low-noise gates for dopant and quantum dot spin qubits

    NASA Astrophysics Data System (ADS)

    Srinivasa, V.; Xu, H.; Medford, J.; Taylor, J. M.

    2014-03-01

    Coupling spins by exchange interactions provides a rapid, tunable method of entanglement generation. However, this necessarily occurs only at short distances, and often incurs susceptibility to charge noise. To address these challenges, we consider two approaches. First, we investigate the coupling of two qubits localized on spatially separated impurity atoms or quantum dots. We show that a third multi-electron, multi-level quantum dot can mediate an exchange interaction between the qubits that is tunable via gate voltage control of level splittings and tunneling amplitudes. This approach suggests an experimentally accessible method for coupling donor electron spins in silicon via a hybrid impurity-dot system. Second, we discuss the resonant exchange (RX) qubit, defined within a triple quantum dot in the three-electron regime. Electric field control of the dipole moment of the RX qubit at microwave frequencies enables single-qubit and two-qubit gates that are protected against low-frequency charge noise. Support from DARPA MTO and the NSF funded Physics Frontier Center at the JQI is gratefully acknowledged.

  14. Interface between Topological and Superconducting Qubits

    NASA Astrophysics Data System (ADS)

    Jiang, Liang; Kane, Charles; Preskill, John

    2011-03-01

    We propose and analyze an interface between a topological qubit and a superconducting flux qubit. In our scheme, the interaction between Majorana fermions in a topological insulator is coherently controlled by a superconducting phase that depends on the quantum state of the flux qubit. A controlled phase gate, achieved by pulsing this interaction on and off, can transfer quantum information between the topological qubit and the superconducting qubit. This work was supported by the Sherman Fairchild Foundation, by NSF grants DMR-0906175 and PHY-0803371, by DOE grant DE-FG03-92-ER40701, and by NSA/ARO grant W911NF-09-1-0442.

  15. Parametric Amplifier and Oscillator Based on Josephson Junction Circuitry

    NASA Astrophysics Data System (ADS)

    Yamamoto, T.; Koshino, K.; Nakamura, Y.

    While the demand for low-noise amplification is ubiquitous, applications where the quantum-limited noise performance is indispensable are not very common. Microwave parametric amplifiers with near quantum-limited noise performance were first demonstrated more than 20 years ago. However, there had been little effort until recently to improve the performance or the ease of use of these amplifiers, partly because of a lack of any urgent motivation. The emergence of the field of quantum information processing in superconducting systems has changed this situation dramatically. The need to reliably read out the state of a given qubit using a very weak microwave probe within a very short time has led to renewed interest in these quantum-limited microwave amplifiers, which are already widely used as tools in this field. Here, we describe the quantum mechanical theory for one particular parametric amplifier design, called the flux-driven Josephson parametric amplifier, which we developed in 2008. The theory predicts the performance of this parametric amplifier, including its gain, bandwidth, and noise temperature. We also present the phase detection capability of this amplifier when it is operated with a pump power that is above the threshold, i.e., as a parametric phase-locked oscillator or parametron.

  16. Optimizing bandwidth and dynamic range of lumped Josephson parametric amplifiers

    NASA Astrophysics Data System (ADS)

    Eddins, A.; Vijay, R.; Macklin, C.; Minev, Z.; Siddiqi, I.

    2013-03-01

    Superconducting parametric amplifiers have revolutionized the field of quantum measurement by providing high gain, ultra-low noise amplification. They have been used successfully for high-fidelity qubit state measurements, probing nano-mechanical resonators, quantum feedback, and for microwave quantum optics experiments. Though several designs exist, a simple and robust architecture is the Lumped Josephson Parametric Amplifier (LJPA). This device consists of a capacitively shunted SQUID directly coupled to a transmission line to form a low quality factor (Q) nonlinear resonator. We discuss amplifiers which can be tuned over the full 4-8 GHz band with 20-25 dB of gain and 10 - 50 MHz of signal bandwidth. However, similar to other parametric amplifiers employing a resonant circuit, the LJPA suffers from low dynamic range and has a -1 dB gain compression point of order -130 dBm. We explore new designs comprised of an array of SQUIDs to improve the dynamic range. We will present the results of numerical simulations and preliminary experiments. We will also briefly discuss improvements obtained from different biasing methods and packaging. This research was supported by the Army Research Office under a QCT grant.

  17. Polaritonic Rabi and Josephson Oscillations

    PubMed Central

    Rahmani, Amir; Laussy, Fabrice P.

    2016-01-01

    The dynamics of coupled condensates is a wide-encompassing problem with relevance to superconductors, BECs in traps, superfluids, etc. Here, we provide a unified picture of this fundamental problem that includes i) detuning of the free energies, ii) different self-interaction strengths and iii) finite lifetime of the modes. At such, this is particularly relevant for the dynamics of polaritons, both for their internal dynamics between their light and matter constituents, as well as for the more conventional dynamics of two spatially separated condensates. Polaritons are short-lived, interact only through their material fraction and are easily detuned. At such, they bring several variations to their atomic counterpart. We show that the combination of these parameters results in important twists to the phenomenology of the Josephson effect, such as the behaviour of the relative phase (running or oscillating) or the occurence of self-trapping. We undertake a comprehensive stability analysis of the fixed points on a normalized Bloch sphere, that allows us to provide a generalized criterion to identify the Rabi and Josephson regimes in presence of detuning and decay. PMID:27452872

  18. Polaritonic Rabi and Josephson Oscillations

    NASA Astrophysics Data System (ADS)

    Rahmani, Amir; Laussy, Fabrice P.

    2016-07-01

    The dynamics of coupled condensates is a wide-encompassing problem with relevance to superconductors, BECs in traps, superfluids, etc. Here, we provide a unified picture of this fundamental problem that includes i) detuning of the free energies, ii) different self-interaction strengths and iii) finite lifetime of the modes. At such, this is particularly relevant for the dynamics of polaritons, both for their internal dynamics between their light and matter constituents, as well as for the more conventional dynamics of two spatially separated condensates. Polaritons are short-lived, interact only through their material fraction and are easily detuned. At such, they bring several variations to their atomic counterpart. We show that the combination of these parameters results in important twists to the phenomenology of the Josephson effect, such as the behaviour of the relative phase (running or oscillating) or the occurence of self-trapping. We undertake a comprehensive stability analysis of the fixed points on a normalized Bloch sphere, that allows us to provide a generalized criterion to identify the Rabi and Josephson regimes in presence of detuning and decay.

  19. Testing of Josephson Spectrometer with Waveguide Coupling

    NASA Astrophysics Data System (ADS)

    Lyatti, M.; Gundareva, I.; Pavlovskii, V.; Poppe, U.; Divin, Y.

    2014-05-01

    One of the challenges in public security is the quick and reliable identification of threat liquids in bottles, when vapour analysis is not possible. Recently, we demonstrated that it is possible to rapidly identify liquids by EM measurements of their dielectric functions in the sub-THz range with a high-Tc Josephson spectrometer. Following this approach, we have developed a Josephson spectrometer with a new radiation coupling system, based on dielectric waveguides. In this paper, we present the results of spectroscopic measurements on liquid samples of various purities including 30% H2O2/H2O, performed using our Josephson spectrometer with waveguide coupling. Also, the signal and noise characteristics of a classical Josephson detector used in our liquid identifier were numerically simulated and the power dynamic range was estimated for a wide spread of junction parameters.

  20. Phonon Josephson junction with nanomechanical resonators

    NASA Astrophysics Data System (ADS)

    Barzanjeh, Shabir; Vitali, David

    2016-03-01

    We study coherent phonon oscillations and tunneling between two coupled nonlinear nanomechanical resonators. We show that the coupling between two nanomechanical resonators creates an effective phonon Josephson junction, which exhibits two different dynamical behaviors: Josephson oscillation (phonon-Rabi oscillation) and macroscopic self-trapping (phonon blockade). Self-trapping originates from mechanical nonlinearities, meaning that when the nonlinearity exceeds its critical value, the energy exchange between the two resonators is suppressed, and phonon Josephson oscillations between them are completely blocked. An effective classical Hamiltonian for the phonon Josephson junction is derived and its mean-field dynamics is studied in phase space. Finally, we study the phonon-phonon coherence quantified by the mean fringe visibility, and show that the interaction between the two resonators may lead to the loss of coherence in the phononic junction.

  1. Microwave integrated circuit for Josephson voltage standards

    NASA Technical Reports Server (NTRS)

    Holdeman, L. B.; Toots, J.; Chang, C. C. (Inventor)

    1980-01-01

    A microwave integrated circuit comprised of one or more Josephson junctions and short sections of microstrip or stripline transmission line is fabricated from thin layers of superconducting metal on a dielectric substrate. The short sections of transmission are combined to form the elements of the circuit and particularly, two microwave resonators. The Josephson junctions are located between the resonators and the impedance of the Josephson junctions forms part of the circuitry that couples the two resonators. The microwave integrated circuit has an application in Josephson voltage standards. In this application, the device is asymmetrically driven at a selected frequency (approximately equal to the resonance frequency of the resonators), and a d.c. bias is applied to the junction. By observing the current voltage characteristic of the junction, a precise voltage, proportional to the frequency of the microwave drive signal, is obtained.

  2. Tunable TiN or NbTiN resonators and couplers using nonlinear kinetic inductance for superconducting qubits

    NASA Astrophysics Data System (ADS)

    Vissers, Michael; Gao, Jiansong; Bockstiegel, Clint; Sandberg, Martin; Pappas, David

    2014-03-01

    Nitride superconductors such as TiN and NbTiN have a nonlinear kinetic inductance when driven at high current. Using this current-tunable reactance, we have designed superconducting devices that are tunable with a DC current without using Josephson junctions. We show that when the DC current is directly coupled to a lumped element resonator, the resonant frequency can be tuned by >4% without inducing loss. In other circuits, we can use a DC current to independently tune the coupling of a long microwave transmission line to a standard superconducting resonator from zero to maximum coupling. In addition to characterizing the non-linear current response of these materials, these tunable devices could be used as a tunable coupler in transmon qubits, by adjusting the strength of the cavity's Purcell effect to the qubit as needed. They also have potential to be used as tunable filters or parametric amplifiers in superconducting circuits.

  3. Design of a dc SQUID Phase Qubit with Controlled Coupling to the Microwave Signal

    NASA Astrophysics Data System (ADS)

    Budoyo, R. P.; Przybysz, A. J.; Cooper, B. K.; Kwon, H.; Kim, Z.; Cheng, B.; Dragt, A. J.; Anderson, J. R.; Lobb, C. J.; Wellstood, F. C.; Khalil, M.; Gladchenko, S.; Stoutimore, M.; Palmer, B. S.; Osborn, K. D.

    2011-03-01

    We have designed an Al/ Al Ox /Al dc SQUID phase qubit on a sapphire substrate with a qubit junction area of 0.3 μ m 2 to minimize loss associated with two-level systems in the junction oxide barrier. The qubit junction is shunted with a 1.5 pF interdigitated capacitor, and is isolated from the bias leads by an LC filter and an inductive isolation network using a larger Josephson junction. A previous device we built with similar parameters had its relaxation time T1 limited by coupling to the microwave line. To reduce this coupling, we adopted a transmission line design and verified the coupling strength using microwave simulations. The new design will also allow us to measure the coupling to the SQUID by throughput measurements. We will discuss our design, the microwave simulations, our estimates for the overall coherence time due to losses and noise from various sources, and our progress towards testing the device. Acknowledgement: DOD, JQI, and CNAM.

  4. Spectroscopy Measurements of Magnesium Diboride Josephson Junctions

    NASA Astrophysics Data System (ADS)

    Mlack, J. T.; Lambert, J. G.; Carabello, S. A.; Thrailkill, Z. E.; Galwaduge, P. T.; Ramos, R. C.

    2010-03-01

    MgB2 has the highest Tc of the conventional superconductors at 39K and exhibits two superconducting energy bands. This material is also inexpensive to produce and has been utilized in new designs for MRI, RF cavities, and Josephson junctions. We report results of recent spectroscopy and transport measurements of Josephson junctions made of MgB2 obtained from our collaborators. We investigate its transport characteristics at sub-kelvin temperatures as well as its responses to resonant microwave activation.

  5. Quantum Coherence in a Superfluid Josephson Junction

    SciTech Connect

    Narayana, Supradeep; Sato, Yuki

    2011-02-04

    We report a new kind of experiment in which we take an array of nanoscale apertures that form a superfluid {sup 4}He Josephson junction and apply quantum phase gradients directly along the array. We observe collective coherent behaviors from aperture elements, leading to quantum interference. Connections to superconducting and Bose-Einstein condensate Josephson junctions as well as phase coherence among the superfluid aperture array are discussed.

  6. Nonlinear coupling between a nitrogen-vacancy-center ensemble and a superconducting qubit.

    PubMed

    Chen, Qiong; Wen, Jun; Yang, W L; Feng, M; Du, Jiangfeng

    2015-01-26

    By exchange of virtual microwave photon induced by a transmission line resonator, the nonlinear interaction between a nitrogen-vacancy-center ensemble (NVE) and a superconducting charge qubit is achieved in circuit quantum electrodynamics, where the nonlinear coupling results from the second order of the coupling between the magnetic field of the transmission line resonator and the charge qubit. In our case, the nonlinear coupling can be much enhanced by a factor of the total spin number in the NVE. As an application, we present a potentially practical scheme to realize the squeezing of the NVE using the nonlinear coupling, which is within reach of the currently available technology. PMID:25835919

  7. Qubit transient dynamics at tunneling Fermi-edge singularity

    NASA Astrophysics Data System (ADS)

    Ponomarenko, V. V.; Larkin, I. A.

    2016-03-01

    We consider tunneling of spinless electrons from a single-channel emitter into an empty collector through an interacting resonant level of the quantum dot. When all Coulomb screening of sudden charge variations of the dot during the tunneling is realized by the emitter channel, the system is described with an exactly solvable model of a dissipative qubit. We derive the corresponding Bloch equation for its quantum evolution. We further use it to specify the qubit transient dynamics towards its stationary quantum state after a sudden change of the level position. We demonstrate that the time-dependent tunneling current characterizing this dynamics exhibits an oscillating behavior for a wide range of the model parameters.

  8. Two-center black holes, qubits, and elliptic curves

    NASA Astrophysics Data System (ADS)

    Lévay, Péter

    2011-07-01

    We relate the U-duality invariants characterizing two-center extremal black-hole solutions in the stu, st2, and t3 models of N=2, d=4 supergravity to the basic invariants used to characterize entanglement classes of four-qubit systems. For the elementary example of a D0D4-D2D6 composite in the t3 model we illustrate how these entanglement invariants are related to some of the physical properties of the two-center solution. Next we show that it is possible to associate elliptic curves to charge configurations of two-center composites. The hyperdeterminant of the hypercube, a four-qubit polynomial invariant of order 24 with 2 894 276 terms, is featuring the j invariant of the elliptic curve. We present some evidence that this quantity and its straightforward generalization should play an important role in the physics of two-center solutions.

  9. Josephson 32-bit shift register

    SciTech Connect

    Yuh, P.F.; Yao, C.T.; Bradley, P. )

    1991-03-01

    This paper reports on a 32-bit shift register designed by edge-triggered gates tested with {plus minus}25% bias margin and {plus minus}81% input margin for the full array. Simulations have shown {plus minus}55% bias margin at 3.3 GHz and working up to a maximum frequency of 30 GHz with a junction current density of 2000A/cm{sup 2} although the shift register has only been tested up to 500 MHz, limited by instrumentation. This edge-triggered gate consisting of a pair of conventional Josephson logic gates in series has the advantages of wide margins, short reset time, and insensitivity to global parameter-variations.

  10. 8 π -periodic Josephson effects in a quantum dot/ quantum spin-Hall josephson junction system

    NASA Astrophysics Data System (ADS)

    Hui, Hoi-Yin; Sau, Jay

    2015-03-01

    Josephson junctions made of conventional s-wave superconductors display 2 π periodicity. On the other hand, 4 π -periodic fractional Josephson effect is known to be a characteristic signature of topological superconductors and Majorana fermions [1]. Zhang and Kane have shown that Josephson junctions made of topological superconductors are 8 π -periodic if interaction is used to avoid dissipation [2]. Here we present a general argument for how time-reversal symmetry and Z2 non-trivial topology constrains the Josephson periodicity to be 8 π . We then illustrate this through a microscopic model of a quantum dot in a quantum spin-hall Josephson junction. Work supported by NSF-JQI-PFC, LPS-CMTC and Microsoft Q.

  11. Splitting Unknown Qubit State Using Five-Qubit Entangled State

    NASA Astrophysics Data System (ADS)

    Li, Dong-fen; Wang, Rui-jin; Zhang, Feng-li; Qin, Zhi-guang; Baagyere, Edward

    2016-04-01

    In this paper, we demonstrate how a five-qubit entangled state can be used to realize the deterministic Quantum State Sharing (QSTS) of a single and two-qubit state among three parties by performing von-Neumann measurement and bell-state measurements. In our scheme, any of the two agents has the ability to reconstruct the original state if he/she collaborates with the other agent,otherwise an individual agent does not have enough information to reconstruct the original state. The paper also outlines the various measurements and mathematical framework of the scheme. The security analysis of our scheme against two attacks scenarios prove that the scheme is secure against an eavesdropper attack and a malicious attacks.

  12. Quantum nonlocality via local contextuality with qubit-qubit entanglement

    NASA Astrophysics Data System (ADS)

    Saha, Debashis; Cabello, Adán; Choudhary, Sujit K.; Pawłowski, Marcin

    2016-04-01

    Quantum nonlocality can be revealed "via local contextuality" in qudit-qudit entangled systems with d >2 , that is, through the violation of inequalities containing Alice-Bob correlations that admit a local description, and Alice-Alice correlations (between the results of sequences of measurements on Alice's subsystem) that admit a local (but contextual) description. A fundamental question to understand the respective roles of entanglement and local contextuality is whether nonlocality via local contextuality exists when the parties have only qubit-qubit entanglement. Here we respond affirmatively to this question. This result further clarifies the connection between contextuality and nonlocality and opens the door for observing nonlocality via local contextuality in actual experiments.

  13. Coupling qubits in circuit-QED cavities connected by a bridge qubit

    NASA Astrophysics Data System (ADS)

    Kim, Mun Dae; Kim, Jaewan

    2016-01-01

    We analyze a coupling scheme for qubits in different cavities of circuit-QED architecture. In contrast to the usual scheme where the cavities are coupled by an interface capacitance we employ a bridge qubit connecting cavities to mediate two-qubit coupling. This active-coupling scheme makes it possible to switch on or off and adjust the strength of qubit-qubit coupling, which is essential for scalability of quantum circuits. By transforming the Hamiltonian we obtain an exact expression of two-qubit coupling in the rotating-wave approximation. For the general case of n qubits the Hamiltonian can produce the W state as an eigenstate of the system. We calculate the decay rate of the coupled qubit-resonator system to find that it is viable in real experiments.

  14. Enhanced Dynamic Range in N-SQUID Lumped Josephson Parametric Amplifiers

    NASA Astrophysics Data System (ADS)

    Eddins, A.; Levenson-Falk, E. M.; Toyli, D. M.; Vijay, R.; Minev, Z.; Siddiqi, I.

    2014-03-01

    Simultaneously providing high gain and nearly quantum-limited noise performance, superconducting parametric amplifiers (paramps) have been used successfully for high fidelity qubit readout, quantum feedback, and microwave quantum optics experiments. The Lumped Josephson Parametric Amplifier (LJPA) consists of a capacitively shunted SQUID coupled to a transmission line to form a nonlinear resonator. Like other paramps employing a resonant circuit, the LJPA's dynamic range-a potentially key ingredient for multiplexing-is limited. Simple theory predicts that the dynamic range can be increased without any reduction in bandwidth or gain by distributing the resonator nonlinearity over a series array of SQUIDs. We fabricated such array devices with up to 5 SQUIDs and observed a clear increase in the critical power for bifurcation about which parametric gain occurs. We discuss in detail amplifier performance as a function of the number of SQUIDs in the array. This research was supported by the Army Research Office under a QCT grant.

  15. Strain-tunable Josephson current in graphene-superconductor junction

    NASA Astrophysics Data System (ADS)

    Wang, Y.; Liu, Y.; Wang, B.

    2013-10-01

    Strain effects on Josephson current in a graphene-superconductor junction are explored theoretically. It is demonstrated that the supercurrent is an oscillatory function of zigzag direction strain with a strain-dependent oscillating frequency. Interestingly, it is found that the Josephson current under armchair direction strain can be turned on/off with a cutoff strain. In view of the on/off properties of the Josephson current, we propose the strained graphene Josephson junction to be utilized as a supercurrent switch.

  16. Dynamics of two coupled semiconductor spin qubits in a noisy environment

    NASA Astrophysics Data System (ADS)

    Das Sarma, S.; Throckmorton, Robert E.; Wu, Yang-Le

    2016-07-01

    We theoretically consider the temporal dynamics of two coupled spin qubits (e.g., semiconductor quantum dots) driven by the interqubit spin-spin coupling. The presence of environmental noise (e.g., charge traps, nuclear spins, random magnetic impurities) is accounted for by including random magnetic field and random interqubit coupling terms in the Hamiltonian. Both Heisenberg coupling and Ising coupling between the spin qubits are considered, corresponding respectively to exchange and capacitive gates as appropriate for single spin and singlet-triplet semiconductor qubit systems, respectively. Both exchange (Heisenberg) and capacitive (Ising) coupling situations can be solved numerically exactly even in the presence of noise, leading to the key findings that (i) the steady-state return probability to the initial state remains close to unity in the presence of strong noise for many, but not all, starting spin configurations, and (ii) the return probability as a function of time is oscillatory with a characteristic noise-controlled decay toward the steady-state value. We also provide results for the magnetization dynamics of the coupled two-qubit system. Our predicted dynamics can be directly tested in the already existing semiconductor spin qubit setups providing insight into their coherent interaction dynamics. Retention of the initial state spin memory even in the presence of strong environmental noise has important implications for quantum computation using spin qubits.

  17. Circuit QED with hole-spin qubits in Ge/Si nanowire quantum dots

    NASA Astrophysics Data System (ADS)

    Kloeffel, Christoph; Trif, Mircea; Stano, Peter; Loss, Daniel

    2013-12-01

    We propose a setup for universal and electrically controlled quantum information processing with hole spins in Ge/Si core/shell nanowire quantum dots (NW QDs). Single-qubit gates can be driven through electric-dipole-induced spin resonance, with spin-flip times shorter than 100 ps. Long-distance qubit-qubit coupling can be mediated by the cavity electric field of a superconducting transmission line resonator, where we show that operation times below 20 ns seem feasible for the entangling iSWAP gate. The absence of Dresselhaus spin-orbit interaction (SOI) and the presence of an unusually strong Rashba-type SOI enable precise control over the transverse qubit coupling via an externally applied, perpendicular electric field. The latter serves as an on-off switch for quantum gates and also provides control over the g factor, so single- and two-qubit gates can be operated independently. Remarkably, we find that idle qubits are insensitive to charge noise and phonons, and we discuss strategies for enhancing noise-limited gate fidelities.

  18. Tunable-cavity QED with phase qubits

    NASA Astrophysics Data System (ADS)

    Whittaker, Jed D.; da Silva, Fabio; Allman, Michael Shane; Lecocq, Florent; Cicak, Katarina; Sirois, Adam; Teufel, John; Aumentado, Jose; Simmonds, Raymond W.

    2014-03-01

    We describe a tunable-cavity QED architecture with an rf SQUID phase qubit inductively coupled to a single-mode, resonant cavity with a tunable frequency that allows for both tunneling and dispersive measurements. Dispersive measurement is well characterized by a three-level model, strongly dependent on qubit anharmonicity, qubit-cavity coupling and detuning. The tunable cavity frequency provides dynamic control over the coupling strength and qubit-cavity detuning helping to minimize Purcell losses and cavity-induced dephasing during qubit operation. The maximum decay time T1 = 1 . 5 μs is limited by dielectric losses from a design geometry similar to planar transmon qubits. This work supported by NIST and NSA grant EAO140639.

  19. Universal quantum computation with unlabelled qubits

    NASA Astrophysics Data System (ADS)

    Severini, Simone

    2006-06-01

    We show that an nth root of the Walsh-Hadamard transform (obtained from the Hadamard gate and a cyclic permutation of the qubits), together with two diagonal matrices, namely a local qubit-flip (for a fixed but arbitrary qubit) and a non-local phase-flip (for a fixed but arbitrary coefficient), can do universal quantum computation on n qubits. A quantum computation, making use of n qubits and based on these operations, is then a word of variable length, but whose letters are always taken from an alphabet of cardinality three. Therefore, in contrast with other universal sets, no choice of qubit lines is needed for the application of the operations described here. A quantum algorithm based on this set can be interpreted as a discrete diffusion of a quantum particle on a de Bruijn graph, corrected on-the-fly by auxiliary modifications of the phases associated with the arcs.

  20. Purification of Logic-Qubit Entanglement

    NASA Astrophysics Data System (ADS)

    Zhou, Lan; Sheng, Yu-Bo

    2016-07-01

    Recently, the logic-qubit entanglement shows its potential application in future quantum communication and quantum network. However, the entanglement will suffer from the noise and decoherence. In this paper, we will investigate the first entanglement purification protocol for logic-qubit entanglement. We show that both the bit-flip error and phase-flip error in logic-qubit entanglement can be well purified. Moreover, the bit-flip error in physical-qubit entanglement can be completely corrected. The phase-flip in physical-qubit entanglement error equals to the bit-flip error in logic-qubit entanglement, which can also be purified. This entanglement purification protocol may provide some potential applications in future quantum communication and quantum network.

  1. Purification of Logic-Qubit Entanglement.

    PubMed

    Zhou, Lan; Sheng, Yu-Bo

    2016-01-01

    Recently, the logic-qubit entanglement shows its potential application in future quantum communication and quantum network. However, the entanglement will suffer from the noise and decoherence. In this paper, we will investigate the first entanglement purification protocol for logic-qubit entanglement. We show that both the bit-flip error and phase-flip error in logic-qubit entanglement can be well purified. Moreover, the bit-flip error in physical-qubit entanglement can be completely corrected. The phase-flip in physical-qubit entanglement error equals to the bit-flip error in logic-qubit entanglement, which can also be purified. This entanglement purification protocol may provide some potential applications in future quantum communication and quantum network. PMID:27377165

  2. Purification of Logic-Qubit Entanglement

    PubMed Central

    Zhou, Lan; Sheng, Yu-Bo

    2016-01-01

    Recently, the logic-qubit entanglement shows its potential application in future quantum communication and quantum network. However, the entanglement will suffer from the noise and decoherence. In this paper, we will investigate the first entanglement purification protocol for logic-qubit entanglement. We show that both the bit-flip error and phase-flip error in logic-qubit entanglement can be well purified. Moreover, the bit-flip error in physical-qubit entanglement can be completely corrected. The phase-flip in physical-qubit entanglement error equals to the bit-flip error in logic-qubit entanglement, which can also be purified. This entanglement purification protocol may provide some potential applications in future quantum communication and quantum network. PMID:27377165

  3. Precise Heater Controller with rf-Biased Josephson Junctions

    NASA Technical Reports Server (NTRS)

    Green, Colin J.; Sergatskov, Dmitri A.; Duncan, R. V.

    2003-01-01

    Paramagnetic susceptibility thermometers used in fundamental physics experiments are capable of measuring temperature changes with a precision of a part in 2 x 10(exp 10). However, heater controllers are only able to control open-loop power dissipation to about a part in 10(exp 5). We used an array of rf-biased Josephson junctions to precisely control the electrical power dissipation in a heater resistor mounted on a thermally isolated cryogenic platform. Theoretically, this method is capable of controlling the electrical power dissipation to better than a part in 10(exp 12). However, this level has not yet been demonstrated experimentally. The experiment consists of a liquid helium cell that also functions as a high-resolution PdMn thermometer, with a heater resistor mounted on it. The cell is thermally connected to a temperature-controlled cooling stage via a weak thermal link. The heater resistor is electrically connected to the array of Josephson junctions using superconducting wire. An rf-biased array of capacitively shunted Josephson junctions drives the voltage across the heater. The quantized voltage across the resistor is Vn = nf(h/2e), where h is Planck's constant, f is the array biasing frequency, e is the charge of an electron, and n is the integer quantum state of the Josephson array. This results in an electrical power dissipation on the cell of Pn = (Vn)(sup 2/R), where R is the heater resistance. The change of the quantum state of the array changes the power dissipated in the heater, which in turn, results in the change of the cell temperature. This temperature change is compared to the expected values based on the known thermal standoff resistance of the cell from the cooling stage. We will present our initial experimental results and discuss future improvements. This work has been funded by the Fundamental Physics Discipline of the Microgravity Science Office of NASA, and supported by a no-cost equipment loan from Sandia National Laboratories.

  4. Single-qubit optical quantum fingerprinting.

    PubMed

    Horn, Rolf T; Babichev, S A; Marzlin, Karl-Peter; Lvovsky, A I; Sanders, Barry C

    2005-10-01

    We analyze and demonstrate the feasibility and superiority of linear optical single-qubit fingerprinting over its classical counterpart. For one-qubit fingerprinting of two-bit messages, we prepare "tetrahedral" qubit states experimentally and show that they meet the requirements for quantum fingerprinting to exceed the classical capability. We prove that shared entanglement permits 100% reliable quantum fingerprinting, which will outperform classical fingerprinting even with arbitrary amounts of shared randomness. PMID:16241707

  5. Ballistic-like supercurrent in suspended graphene Josephson weak links

    NASA Astrophysics Data System (ADS)

    Mizuno, Naomi; Nielsen, Bent; Du, Xu

    2013-11-01

    The interplay of the massless Dirac fermions in graphene and the Cooper pair states in a superconductor has the potential to give rise to exotic physical phenomena and useful device applications. But to date, the junctions formed between graphene and superconductors on conventional substrates have been highly disordered. Charge scattering and potential fluctuations caused by such disorder are believed to have prevented the emergence or observation of new physics. Here we propose to address this problem by forming suspended graphene-superconductor junctions. We demonstrate the fabrication of high-quality suspended monolayer graphene-NbN Josephson junctions with device mobility in excess of 150,000 cm2 per Vs, minimum carrier density below 1010 cm-2, and the flow of a supercurrent at critical temperatures greater than 2 K. The characteristics of our Josephson junctions are consistent with ballistic transport, with a linear dependence on the Fermi energy that reflects of linear dispersion of massless Dirac fermions.

  6. Ballistic-like supercurrent in suspended graphene Josephson weak links.

    PubMed

    Mizuno, Naomi; Nielsen, Bent; Du, Xu

    2013-01-01

    The interplay of the massless Dirac fermions in graphene and the Cooper pair states in a superconductor has the potential to give rise to exotic physical phenomena and useful device applications. But to date, the junctions formed between graphene and superconductors on conventional substrates have been highly disordered. Charge scattering and potential fluctuations caused by such disorder are believed to have prevented the emergence or observation of new physics. Here we propose to address this problem by forming suspended graphene-superconductor junctions. We demonstrate the fabrication of high-quality suspended monolayer graphene-NbN Josephson junctions with device mobility in excess of 150,000 cm(2) per Vs, minimum carrier density below 10(10) cm(-2), and the flow of a supercurrent at critical temperatures greater than 2 K. The characteristics of our Josephson junctions are consistent with ballistic transport, with a linear dependence on the Fermi energy that reflects of linear dispersion of massless Dirac fermions. PMID:24225412

  7. Quantacell: powerful charging of quantum batteries

    NASA Astrophysics Data System (ADS)

    Binder, Felix C.; Vinjanampathy, Sai; Modi, Kavan; Goold, John

    2015-07-01

    We study the problem of charging a quantum battery in finite time. We demonstrate an analytical optimal protocol for the case of a single qubit. Extending this analysis to an array of N qubits, we demonstrate that an N-fold advantage in power per qubit can be achieved when global operations are permitted. The exemplary analytic argument for this quantum advantage in the charging power is backed up by numerical analysis using optimal control techniques. It is demonstrated that the quantum advantage for power holds when, with cyclic operation in mind, initial and final states are required to be separable.

  8. Dressed qubits in nuclear spin baths

    SciTech Connect

    Wu Lianao

    2010-04-15

    We present a method to encode a dressed qubit into the product state of an electron spin localized in a quantum dot and its surrounding nuclear spins via a dressing transformation. In this scheme, the hyperfine coupling and a portion of a nuclear dipole-dipole interaction become logic gates, while they are the sources of decoherence in electron-spin qubit proposals. We discuss errors and corrections for the dressed qubits. Interestingly, the effective Hamiltonian of nuclear spins is equivalent to a pairing Hamiltonian, which provides the microscopic mechanism to protect dressed qubits against decoherence.

  9. Teleportation capability, distillability, and nonlocality on three-qubit states

    SciTech Connect

    Lee, Soojoon; Joo, Jaewoo; Kim, Jaewan

    2007-07-15

    In this paper, we consider teleportation capability, distillability, and nonlocality on three-qubit states. In order to investigate some relations among them, we first find the explicit formulas of the quantities about the maximal teleportation fidelity on three-qubit states. We show that if any three-qubit state is useful for three-qubit teleportation then the three-qubit state is distillable into a Greenberger-Horne-Zeilinger state, and that if any three-qubit state violates a specific form of Mermin inequality then the three-qubit state is useful for three-qubit teleportation.

  10. Observation of 0–π transition in SIsFS Josephson junctions

    SciTech Connect

    Ruppelt, N. Vavra, O.; Kohlstedt, H.; Sickinger, H.; Menditto, R.; Goldobin, E.; Koelle, D.; Kleiner, R.

    2015-01-12

    The 0–π transition in Superconductor-Insulator-superconductor-Ferromagnet-Superconductor (SIsFS) Josephson junctions (JJs) was investigated experimentally. As predicted by theory, an s-layer inserted into a ferromagnetic SIFS junction can enhance the critical current density up to the value of an SIS tunnel junction. We fabricated Nb′ | AlO{sub x} | Nb | Ni{sub 60}Cu{sub 40} | Nb JJs with wedge-like s (Nb) and F (Ni{sub 60}Cu{sub 40}) layers and studied the Josephson effect as a function of the s- and F-layer thickness, d{sub s} and d{sub F}, respectively. For d{sub s} = 11 nm, π-JJs with SIFS-type j{sub c}(d{sub F}) and critical current densities up to j{sub c}{sup π}=60 A/cm{sup 2} were obtained at 4.2 K. Thicker d{sub s} led to a drastic increase of the critical current decay length, accompanied by the unexpected disappearance of the 0–π transition dip in the j{sub c}(d{sub F}) dependence. Our results are relevant for superconducting memories, rapid single flux quantum logic circuits, and solid state qubits.

  11. Gate Tuning of Different Phase-Particle Escape Regimes in Graphene-Based Josephson Junctions

    NASA Astrophysics Data System (ADS)

    Lee, Gil-Ho; Jeong, Dongchan; Choi, Jae-Hyun; Doh, Yong-Joo; Lee, Hu-Jong

    2012-02-01

    Graphene-based Josephson junctions (GJJs) provide a unique system to investigate superconducting proximity effect with in-situ tunable Josephson coupling strength. While the phase-coherent behaviors of a GJJ under a magnetic field and microwave irradiation have been observed previouslyootnotetextH. B. Heersche et al., Nature 446, 56 (2007); D. Jeong et al. Phys. Rev. B 83, 094503 (2011)., we investigated the stochastic switching behavior of the supercurrent in this system. Here, we present the observation of the three different escaping regimes for a phase particle from a washboard potential of the GJJ; macroscopic quantum tunneling (MQT), thermal activation (TA), and phase diffusion (PD).ootnotetextG.-H. Lee et al., Phys. Rev. Lett. 107, 146605 (2011). The crossover temperature (T^*MQT) between the classical to quantum regime can be controlled by the gate voltage, implying that discrete energy levels of a phase particle are also gate-tunable. Moreover, direct observation of energy level quantization (ELQ) by microwave spectroscopy shows the consistent gate dependence of T^*MQT. A new class of hybrid quantum devices such as a gate-tunable phase qubit is potentially realized by utilizing the MQT and ELQ behavior of the GJJs.

  12. Deterministic LOCC transformation of three-qubit pure states and entanglement transfer

    SciTech Connect

    Tajima, Hiroyasu

    2013-02-15

    A necessary and sufficient condition of the possibility of a deterministic local operations and classical communication (LOCC) transformation of three-qubit pure states is given. The condition shows that the three-qubit pure states are a partially ordered set parametrized by five well-known entanglement parameters and a novel parameter; the five are the concurrences C{sub AB}, C{sub AC}, C{sub BC}, the tangle {tau}{sub ABC} and the fifth parameter J{sub 5} of Acin et al. (2000) Ref. [19], while the other new one is the entanglement charge Q{sub e}. The order of the partially ordered set is defined by the possibility of a deterministic LOCC transformation from a state to another state. In this sense, the present condition is an extension of Nielsen's work (Nielsen (1999) [14]) to three-qubit pure states. We also clarify the rules of transfer and dissipation of the entanglement which is caused by deterministic LOCC transformations. Moreover, the minimum number of times of measurements to reproduce an arbitrary deterministic LOCC transformation between three-qubit pure states is given. - Highlights: Black-Right-Pointing-Pointer We obtained a necessary and sufficient condition for deterministic LOCC of 3 qubits. Black-Right-Pointing-Pointer We clarified rules of entanglement flow caused by measurements. Black-Right-Pointing-Pointer We found a new parameter which is interpreted as 'Charge of Entanglement'. Black-Right-Pointing-Pointer We gave a set of entanglements which determines whether two states are LU-eq. or not. Black-Right-Pointing-Pointer Our approach to deterministic LOCC of 3 qubits may be applicable to N qubits.

  13. PREFACE: Nobel Symposium 141: Qubits for Future Quantum Information Nobel Symposium 141: Qubits for Future Quantum Information

    NASA Astrophysics Data System (ADS)

    Claeson, Tord; Delsing, Per; Wendin, Göran

    2009-12-01

    correction, have yet to be solved. It has been predicted that quantum computers will be able to perform certain complicated computations or simulations in minutes or hours instead of years as with present computers. So far there exist very few useful quantum algorithms; however there is hope that the development of these will be stimulated once there is a breakthrough in hardware. Remarkable progress has been made in quantum engineering and quantum measurements, but a large scale quantum computer is still far off. Quantum communication and cryptography are much closer to the market than a quantum computer. The development of quantum information has meant a large push in the field of quantum physics, that previously could only be studied in the microscopic world. Artificial atoms, realized by circuit technology and mimicking the properties of 'natural' atoms, are one example of the new possibilities opened up by quantum engineering. Several different types of qubits have been suggested. Some are based upon microscopic entities, like atoms and ions in traps, or nuclear spins in molecules. They can have long coherence times (i.e. a long period allowing many operations, of the order of 10 000, to be performed before the state needs to be refreshed) but they are difficult to integrate into large systems. Other qubits are based upon solid state components that facilitate integration and coupling between qubits, but they suffer from interactions with the environment and their coherent states have a limited lifetime. Advanced experiments have been performed with superconducting Josephson junctions and many breakthroughs have been reported in the last few years. They have an advantage in the inherent coherence of superconducting Cooper pairs over macroscopic distances. We chose to focus the Nobel Symposium on Qubits for Future Quantum Information on superconducting qubits to allow for depth in discussions, but at the same time to allow comparison with other types of qubits that may

  14. Long-range spin current driven by superconducting phase difference in a josephson junction with double layer ferromagnets.

    PubMed

    Hikino, S; Yunoki, S

    2013-06-01

    We theoretically study spin current through ferromagnet (F) in a Josephson junction composed of s-wave superconductors and two layers of ferromagnets. Using quasiclassical theory, we show that the long-range spin current can be driven by the superconducting phase difference without a voltage drop. The origin of this spin current is due to spin-triplet Cooper pairs (STCs) formed by electrons of equal spin, which are induced by the proximity effect inside the F. We find that the spin current carried by the STCs exhibits long-range propagation in the F even where the Josephson charge current is practically zero. We also show that this spin current persists over a remarkably longer distance than the ordinary spin current carried by spin polarized conduction electrons in the F. Our results thus indicate the promising potential of Josephson junctions based on multilayer ferromagnets for spintronics applications with long-range propagating spin current. PMID:25167525

  15. On the role of the four-qubit state in two-qubit gate teleportation

    NASA Astrophysics Data System (ADS)

    Sousa, P. R. M.; Mendes, F. V.; Ramos, R. V.

    2016-05-01

    The full analysis of quantum protocols requires the knowledge of the role of quantum states, bases of measurement and quantum gates involved. In what concerns the famous two-qubit quantum gate teleportation protocol, the role of the basis of measurement was considered in a recent work by Mendes and Ramos. In this work, we analyze the role of the four-qubit state used as resource. We show that the quantum two-qubit gate teleportation divides the set of pure four-qubit states in two classes. For one class, deterministic and probabilistic teleportation can be achieved, while for the other class, probabilistic remote two-qubit gate preparation is achieved.

  16. Weak measurement and quantum steering of spin qubits in silicon

    NASA Astrophysics Data System (ADS)

    Morello, Andrea; Muhonen, Juha; Simmons, Stephanie; Freer, Solomon; Dehollain, Juan; McCallum, Jeffrey; Jamieson, David; Itoh, Kohei; Dzurak, Andrew

    Single-shot, projective measurements have been demonstrated with very high fidelities on both the electron and the nuclear spin of single implanted phosphorus (31P) donors in silicon. Here we present a series of experiments where the measurement strength is continousuly reduced, giving access to the regime of weak measurement of single spins.For the electron qubit, the measurement strength is set by the measurement time compared to the spin-dependent tunneling time between the 31P donor and a charge reservoir. For the nuclear qubit, the measurement strength is set by the rotation angle of an ESR pulse.We have demonstrated quantum steering of the spin states, with curious and useful applications. We can improve the fidelity of electron qubit initialization by steering it towards the ground state, thus bypassing thermal effects on the initialization process. We can also accurately measure the electron-reservoir tunnel coupling, without the electron ever tunneling away from the 31P atom. Finally, these techniques allow the study of weak values and Leggett-Garg inequalities. Present address: AMOLF, Amsterdam, The Netherlands.

  17. Implementing N-quantum phase gate via circuit QED with qubit-qubit interaction

    NASA Astrophysics Data System (ADS)

    Said, T.; Chouikh, A.; Essammouni, K.; Bennai, M.

    2016-02-01

    We propose a method for realizing a quantum phase gate of one qubit simultaneously controlling N target qubits based on the qubit-qubit interaction. We show how to implement the proposed gate with one transmon qubit simultaneously controlling N transmon qubits in a circuit QED driven by a strong microwave field. In our scheme, the operation time of this phase gate is independent of the number N of qubits. On the other hand, this gate can be realized in a time of nanosecond-scale much smaller than the decoherence time and dephasing time both being the time of microsecond-scale. Numerical simulation of the occupation probabilities of the second excited lever shows that the scheme could be achieved efficiently within current technology.

  18. Scanning Josephson spectroscopy on the atomic scale

    NASA Astrophysics Data System (ADS)

    Randeria, Mallika T.; Feldman, Benjamin E.; Drozdov, Ilya K.; Yazdani, Ali

    2016-04-01

    The Josephson effect provides a direct method to probe the strength of the pairing interaction in superconductors. By measuring the phase fluctuating Josephson current between a superconducting tip of a scanning tunneling microscope and a BCS superconductor with isolated magnetic adatoms on its surface, we demonstrate that the spatial variation of the pairing order parameter can be characterized on the atomic scale. This system provides an example where the local pairing potential suppression is not directly reflected in the spectra measured via quasiparticle tunneling. Spectroscopy with such superconducting tips also shows signatures of previously unexplored Andreev processes through individual impurity-bound Shiba states. The atomic resolution achieved here establishes scanning Josephson spectroscopy as a promising technique for the study of novel superconducting phases.

  19. A Josephson radiation comb generator

    PubMed Central

    Solinas, P.; Gasparinetti, S.; Golubev, D.; Giazotto, F.

    2015-01-01

    We propose the implementation of a Josephson Radiation Comb Generator (JRCG) based on a dc superconducting quantum interference device (SQUID) driven by an external magnetic field. When the magnetic flux crosses a diffraction node of the critical current interference pattern, the superconducting phase undergoes a jump of π and a voltage pulse is generated at the extremes of the SQUID. Under periodic drive this allows one to generate a sequence of sharp, evenly spaced voltage pulses. In the frequency domain, this corresponds to a comb-like structure similar to the one exploited in optics and metrology. With this device it is possible to generate up to several hundreds of harmonics of the driving frequency. For example, a chain of 50 identical high-critical-temperature SQUIDs driven at 1 GHz can deliver up to a 0.5 nW at 200 GHz. The availability of a fully solid-state radiation comb generator such as the JRCG, easily integrable on chip, may pave the way to a number of technological applications, from metrology to sub-millimeter wave generation. PMID:26193628

  20. A Josephson radiation comb generator.

    PubMed

    Solinas, P; Gasparinetti, S; Golubev, D; Giazotto, F

    2015-01-01

    We propose the implementation of a Josephson Radiation Comb Generator (JRCG) based on a dc superconducting quantum interference device (SQUID) driven by an external magnetic field. When the magnetic flux crosses a diffraction node of the critical current interference pattern, the superconducting phase undergoes a jump of π and a voltage pulse is generated at the extremes of the SQUID. Under periodic drive this allows one to generate a sequence of sharp, evenly spaced voltage pulses. In the frequency domain, this corresponds to a comb-like structure similar to the one exploited in optics and metrology. With this device it is possible to generate up to several hundreds of harmonics of the driving frequency. For example, a chain of 50 identical high-critical-temperature SQUIDs driven at 1 GHz can deliver up to a 0.5 nW at 200 GHz. The availability of a fully solid-state radiation comb generator such as the JRCG, easily integrable on chip, may pave the way to a number of technological applications, from metrology to sub-millimeter wave generation. PMID:26193628

  1. Quantum coherent dynamics in a DC SQUID phase qubit using an LC filter

    NASA Astrophysics Data System (ADS)

    Kwon, Hyeokshin

    2011-12-01

    A dc SQUID phase qubit consists of two Josephson junctions in a loop. One junction acts as a qubit with two lowest energy levels forming the ∣0> and ∣1> status. The second junction and the loop inductance act to isolate the qubit junction from noise. In this thesis, I report on the improvement of the relaxation time and the coherence time in a dc SQUID phase qubit that used an LC filter. I also report the measurement of anomalous switching curves. In order to improve the relaxation and coherence times, I used two isolation networks, an LC isolation network and an inductive isolation network, to decouple the device from the current bias lines. This produced a very large total effective resistance of the input leads that increases the relaxation time of the qubit. In addition, I connected a low-loss SiNx shunting capacitor across the qubit junction to reduce dielectric losses. I measured two dc SQUID phase qubits. Device DS6 had a 4 (mum) 2 Al/AlOx/Al qubit junction with a critical current of 0.5 muA and a 1 pF shunting capacitor. It used an LC filter made from a 10 nH inductor and a 145 pF capacitor. The capacitors contained N-H rich SiN x which produced a loss tangent of about 7x10-4. Device DS8 had a 2 (mum)2 Al/AlOx/Al qubit junction with a critical current of 77 nA and an LC filter similar to the first one. The shunting capacitor contained Si-H rich SiNx. Using a pulse readout technique, I measured the characteristics of the qubits, including the transition spectrum, Rabi oscillations, relaxation, Ramsey fringes and state tomography. The best relaxation time T 1 for device DS6 was 32 ns and 280 ns for device DS8. The best Rabi decay time T' for DS6 was 42 ns while for device DS8 it was 120 ns. From these and other data I obtained estimates for the best coherence time T2 in device DS6 of 61 ns and 76 ns in device DS8. In DS8, I observed anomalous switching curves; i.e. switching curves which were qualitatively different from conventional switching curves. In

  2. Fabrication of submicron La{sub 2-x}Sr{sub x}CuO{sub 4} intrinsic Josephson junction stacks

    SciTech Connect

    Kubo, Yuimaru; Takano, Yoshihiko; Takahide, Yamaguchi; Ueda, Shinya; Ishii, Satoshi; Tsuda, Shunsuke; Tanaka, Takayoshi; Islam, ATM Nazmul; Tanaka, Isao

    2011-02-01

    Intrinsic Josephson junction (IJJ) stacks of cuprate superconductors have potential to be implemented as intrinsic phase qubits working at relatively high temperatures. We report success in fabricating submicron La{sub 2-x}Sr{sub x}CuO{sub 4} (LSCO) IJJ stacks carved out of single crystals. We also show a new fabrication method in which argon ion etching is performed after focused ion beam etching. As a result, we obtained an LSCO IJJ stack in which resistive multibranches appeared. It may be possible to control the number of stacked IJJs with an accuracy of a single IJJ by developing this method.

  3. Robust interface between flying and topological qubits

    PubMed Central

    Xue, Zheng-Yuan; Gong, Ming; Liu, Jia; Hu, Yong; Zhu, Shi-Liang; Wang, Z. D.

    2015-01-01

    Hybrid architectures, consisting of conventional and topological qubits, have recently attracted much attention due to their capability in consolidating robustness of topological qubits and universality of conventional qubits. However, these two kinds of qubits are normally constructed in significantly different energy scales, and thus the energy mismatch is a major obstacle for their coupling, which can support the exchange of quantum information between them. Here we propose a microwave photonic quantum bus for a strong direct coupling between the topological and conventional qubits, where the energy mismatch is compensated by an external driving field. In the framework of tight-binding simulation and perturbation approach, we show that the energy splitting of Majorana fermions in a finite length nanowire, which we use to define topological qubits, is still robust against local perturbations due to the topology of the system. Therefore, the present scheme realizes a rather robust interface between the flying and topological qubits. Finally, we demonstrate that this quantum bus can also be used to generate multipartitie entangled states with the topological qubits. PMID:26216201

  4. Josephson junctions with alternating critical current density

    SciTech Connect

    Mints, R.G.; Kogan, V.G.

    1997-04-01

    The magnetic-field dependence of the critical current I{sub c}(H) is considered for a short Josephson junction with the critical current density j{sub c} alternating along the tunnel contact. Two model cases, periodic and randomly alternating j{sub c}, are treated in detail. Recent experimental data on I{sub c}(H) for grain-boundary Josephson junctions in YBa{sub 2}Cu{sub 3}O{sub x} are discussed. {copyright} {ital 1997} {ital The American Physical Society}

  5. Flux Cloning in Josephson Transmission Lines

    SciTech Connect

    Gulevich, D.R.; Kusmartsev, F.V.

    2006-07-07

    We describe a novel effect related to the controlled birth of a single Josephson vortex. In this phenomenon, the vortex is created in a Josephson transmission line at a T-shaped junction. The 'baby' vortex arises at the moment when a 'mother' vortex propagating in the adjacent transmission line passes the T-shaped junction. In order to give birth to a new vortex, the mother vortex must have enough kinetic energy. Its motion can also be supported by an externally applied driving current. We determine the critical velocity and the critical driving current for the creation of the baby vortices and briefly discuss the potential applications of the found effect.

  6. Symmetry protected Josephson supercurrents in three-dimensional topological insulators.

    PubMed

    Cho, Sungjae; Dellabetta, Brian; Yang, Alina; Schneeloch, John; Xu, Zhijun; Valla, Tonica; Gu, Genda; Gilbert, Matthew J; Mason, Nadya

    2013-01-01

    Coupling the surface state of a topological insulator to an s-wave superconductor is predicted to produce the long-sought Majorana quasiparticle excitations. However, superconductivity has not been measured in surface states when the bulk charge carriers are fully depleted, that is, in the true topological regime relevant for investigating Majorana modes. Here we report measurements of d.c. Josephson effects in topological insulator-superconductor junctions as the chemical potential is moved through the true topological regime characterized by the presence of only surface currents. We compare our results with three-dimensional quantum transport simulations, and determine the effects of bulk/surface mixing, disorder and magnetic field; in particular, we show that the supercurrent is largely carried by surface states, due to the inherent topology of the bands, and that it is robust against disorder. Our results thus clarify key open issues regarding the nature of supercurrents in topological insulators. PMID:23575693

  7. Multi-terminal Josephson junctions as topological matter

    NASA Astrophysics Data System (ADS)

    Riwar, Roman-Pascal; Houzet, Manuel; Meyer, Julia S.; Nazarov, Yuli V.

    2016-04-01

    Topological materials and their unusual transport properties are now at the focus of modern experimental and theoretical research. Their topological properties arise from the bandstructure determined by the atomic composition of a material and as such are difficult to tune and naturally restricted to <=3 dimensions. Here we demonstrate that n-terminal Josephson junctions with conventional superconductors may provide novel realizations of topology in n-1 dimensions, which have similarities, but also marked differences with existing 2D or 3D topological materials. For n>=4, the Andreev subgap spectrum of the junction can accommodate Weyl singularities in the space of the n-1 independent superconducting phases, which play the role of bandstructure quasimomenta. The presence of these Weyl singularities enables topological transitions that are manifested experimentally as changes of the quantized transconductance between two voltage-biased leads, the quantization unit being 4e2/h, where e is the electric charge and h is the Planck constant.

  8. Multi-terminal Josephson junctions as topological matter

    PubMed Central

    Riwar, Roman-Pascal; Houzet, Manuel; Meyer, Julia S.; Nazarov, Yuli V.

    2016-01-01

    Topological materials and their unusual transport properties are now at the focus of modern experimental and theoretical research. Their topological properties arise from the bandstructure determined by the atomic composition of a material and as such are difficult to tune and naturally restricted to ≤3 dimensions. Here we demonstrate that n-terminal Josephson junctions with conventional superconductors may provide novel realizations of topology in n−1 dimensions, which have similarities, but also marked differences with existing 2D or 3D topological materials. For n≥4, the Andreev subgap spectrum of the junction can accommodate Weyl singularities in the space of the n−1 independent superconducting phases, which play the role of bandstructure quasimomenta. The presence of these Weyl singularities enables topological transitions that are manifested experimentally as changes of the quantized transconductance between two voltage-biased leads, the quantization unit being 4e2/h, where e is the electric charge and h is the Planck constant. PMID:27040917

  9. Dynamics of a Josephson array in a resonant cavity

    NASA Astrophysics Data System (ADS)

    Almaas, E.; Stroud, D.

    2002-04-01

    We derive dynamical equations for a Josephson array coupled to a resonant cavity by applying the Heisenberg equations of motion to a model Hamiltonian described by us earlier [Phys. Rev. B 63, 144522 (2001); 64, 179902(E) (2001)]. By means of a canonical transformation, we also show that, in the absence of an applied current and dissipation, our model reduces to one described by Shnirman et al [Phys. Rev. Lett. 79, 2371 (1997)] for coupled qubits, and that it corresponds to a capacitive coupling between the array and the cavity mode. From extensive numerical solutions of the model in one dimension, we find that the array locks into a coherent, periodic state above a critical number of active junctions; that the current-voltage characteristics of the array have self-induced resonant steps (SIRS's) that when Na active junctions are synchronized on a SIRS, the energy emitted into the resonant cavity is quadratic in Na; and that when a fixed number of junctions is biased on a SIRS, the energy is linear in the input power. All these results are in agreement with recent experiments. By choosing the initial conditions carefully, we can drive the array into any of a variety of different integer SIRS's. We tentatively identify terms in the equations of motion which give rise to both the SIRS's and the coherence threshold. We also find higher-order integer SIRS's and fractional SIRS's in some simulations. We conclude that a resonant cavity can produce threshold behavior and SIRS's even in a one-dimensional array with appropriate experimental parameters, and that the experimental data, including the coherent emission, can be understood from classical equations of motion.

  10. Can a strain yield a qubit?

    NASA Astrophysics Data System (ADS)

    Benjamin, Colin

    2015-03-01

    A Josepshon qubit is designed via the application of a tensile strain to a topological insulator surface, sandwiched between two s-wave superconductors. The strain applied leads to a shift in Dirac point without changing the conducting states existing on the surface of a topological insulator. This strain applied can be tuned to form a π-junction in such a structure. Combining two such junctions in a ring architecture leads to the ground state of the ring being in a doubly degenerate state- ``0'' and ``1'' states of the qubit. A qubit designed this way is easily controlled via the tunable strain. We report on the conditions necessary to design such a qubit. Finally the operating time of a single qubit phase gate is derived. This work was supported by funds from Dept. of Science and Technology (Nanomission), Govt. of India, Grant No. SR/NM/NS-1101/2011.

  11. Mixed qubits cannot be universally broadcast

    SciTech Connect

    Chen Lin; Chen Yixin

    2007-06-15

    We show that no universal quantum cloning machine exists that can broadcast an arbitrary mixed qubit with a constant fidelity. Based on this result, we investigate the dependent quantum cloner in the sense that some parameter of the input qubit {rho}{sub s}({theta},{omega},{lambda}) is regarded as constant in the fidelity. For the case of constant {omega}, we establish the 1{yields}2 optimal symmetric dependent cloner with a fidelity 1/2. It is also shown that the 1{yields}M optimal quantum cloning machine for pure qubits is also optimal for mixed qubits, when {lambda} is the unique parameter in the fidelity. For general N{yields}M broadcasting of mixed qubits, the situation is very different.

  12. Experimental quantum coding against qubit loss error

    PubMed Central

    Lu, Chao-Yang; Gao, Wei-Bo; Zhang, Jin; Zhou, Xiao-Qi; Yang, Tao; Pan, Jian-Wei

    2008-01-01

    The fundamental unit for quantum computing is the qubit, an isolated, controllable two-level system. However, for many proposed quantum computer architectures, especially photonic systems, the qubits can be lost or can leak out of the desired two-level systems, posing a significant obstacle for practical quantum computation. Here, we experimentally demonstrate, both in the quantum circuit model and in the one-way quantum computer model, the smallest nontrivial quantum codes to tackle this problem. In the experiment, we encode single-qubit input states into highly entangled multiparticle code words, and we test their ability to protect encoded quantum information from detected 1-qubit loss error. Our results prove in-principle the feasibility of overcoming the qubit loss error by quantum codes. PMID:18682562

  13. Low-temperature 1 /f noise in microwave dielectric constant of amorphous dielectrics in Josephson qubits

    NASA Astrophysics Data System (ADS)

    Burin, Alexander L.; Matityahu, Shlomi; Schechter, Moshe

    2015-11-01

    The analytical solution for the low-temperature 1 /f noise in the microwave dielectric constant of amorphous films at frequency ν0˜5 GHz due to tunneling two-level systems (TLSs) is derived within the standard tunneling model including the weak dipolar or elastic TLS-TLS interactions. The 1 /f frequency dependence is caused by TLS spectral diffusion characterized by the width growing logarithmically with time. Temperature and field dependencies are predicted for the noise spectral density in typical glasses with universal TLSs. The satisfactory interpretation of the recent experiment by J. Burnett et al. [Nat. Commun. 5, 4119 (2014), 10.1038/ncomms5119] in Pt-capped Nb superconducting resonators is attained by assuming a smaller density of TLSs compared to ordinary glasses, which is consistent with the very high internal quality factor in those samples.

  14. Theory, modeling and simulation of superconducting qubits

    SciTech Connect

    Berman, Gennady P; Kamenev, Dmitry I; Chumak, Alexander; Kinion, Carin; Tsifrinovich, Vladimir

    2011-01-13

    We analyze the dynamics of a qubit-resonator system coupled with a thermal bath and external electromagnetic fields. Using the evolution equations for the set of Heisenberg operators that describe the whole system, we derive an expression for the resonator field, that includes the resonator-drive, the resonator-bath, and resonator-qubit interactions. The renormalization of the resonator frequency, caused by the qubit-resonator interaction, is accounted for. Using the solutions for the resonator field, we derive the equation that describes the qubit dynamics. The dependence of the qubit evolution during the measurement time on the fidelity of a single-shot measurement is studied. The relation between the fidelity and measurement time is shown explicitly. We proposed a novel adiabatic method for the phase qubit measurement. The method utilizes a low-frequency, quasi-classical resonator inductively coupled to the qubit. The resonator modulates the qubit energy, and the back reaction of the qubit causes a shift in the phase of the resonator. The resonator phase shift can be used to determine the qubit state. We have simulated this measurement taking into the account the energy levels outside the phase qubit manifold. We have shown that, for qubit frequencies in the range of 8-12GHZ, a resonator frequency of 500 MHz and a measurement time of 100 ns, the phase difference between the two qubit states is greater than 0.2 rad. This phase difference exceeds the measurement uncertainty, and can be detected using a classical phase-meter. A fidelity of 0.9999 can be achieved for a relaxation time of 0.5 ms. We also model and simulate a microstrip-SQUID amplifier of frequency about 500 MHz, which could be used to amplify the resonator oscillations in the phase qubit adiabatic measurement. The voltage gain and the amplifier noise temperature are calculated. We simulate the preparation of a generalized Bell state and compute the relaxation times required for achieving high

  15. Unraveling of a detailed-balance-preserved quantum master equation and continuous feedback control of a measured qubit

    NASA Astrophysics Data System (ADS)

    Luo, JunYan; Jin, Jinshuang; Wang, Shi-Kuan; Hu, Jing; Huang, Yixiao; He, Xiao-Ling

    2016-03-01

    We present a generic unraveling scheme for a detailed-balance-preserved quantum master equation applicable for stochastic point processes in mesoscopic transport. It enables us to investigate continuous measurement of a qubit on the level of single quantum trajectories, where essential correlations between the inherent dynamics of the qubit and detector current fluctuations are revealed. Based on this unraveling scheme, feedback control of the charge qubit is implemented to achieve a desired pure state in the presence of the detailed-balance condition. With sufficient feedback strength, coherent oscillations of the measured qubit can be maintained for arbitrary qubit-detector coupling. Competition between the loss and restoration of coherence entailed, respectively, by measurement back action and feedback control is reflected in the noise power spectrum of the detector's output. It is demonstrated unambiguously that the signal-to-noise ratio is significantly enhanced with increasing feedback strength and could even exceed the well-known Korotkov-Averin bound in quantum measurement. The proposed unraveling and feedback scheme offers a transparent and straightforward approach to effectively sustaining ideal coherent oscillations of a charge qubit in the field of quantum computation.

  16. Quantum effects and the dissipation by quasiparticle tunneling in arrays of Josephson junctions

    SciTech Connect

    Kampf, A.; Schoen, G.

    1987-09-01

    We investigate the influence of dissipative quasiparticle tunneling currents on quantum effects and phase transitions in d-dimensional arrays of Josephson junctions. We show how the dissipative phase transition, which is known from single junctions at zero temperature, is modified due to the multidimensional coupling. The transition depends on the strength of the dissipation but also on the ratio of Josephson coupling energy to the capacitive charging energy e/sup 2//2C. It separates an ordered (superconducting) regime from a disordered (resistive) regime where fluctuations prevent phase coherence. In arrays with small capacitance junctions and weak dissipation, the disordered phase persists down to zero temperature. Finite temperatures modify the phase diagram significantly. A reentrant transition between a resistive and a superconducting state is found for weak dissipation. We also make contact with the familiar phase transitions of d-dimensional XY models and show how the charging energy and dissipation in Josephson-junction arrays influence these transitions. The results are of relevance for granular superconductors.

  17. Spin-qubit inspired architectures for superconducting quantum computing

    NASA Astrophysics Data System (ADS)

    Shim, Yun-Pil; Tahan, Charles

    2015-03-01

    In recent years, the superconducting qubit community has achieved single and two-qubit benchmarked gate fidelities approaching 99.9%, fast readout with novel superconducting amplifiers, distributed entanglement, and other milestones on the road to fault-tolerant quantum information processing. Obviously, this is a field that could use some help from the semiconductor qubit community! Here we present theoretical work on superconducting qubit systems inspired by our experience with semiconductor qubits. We discuss initialization, single- and two-qubit gate operations, and measurement schemes for an encoded qubit in a two-dimensional architecture. Our results motivate new ways of designing or operating superconducting quantum information processors.

  18. Topological quantum memory interfacing atomic and superconducting qubits

    NASA Astrophysics Data System (ADS)

    Xue, ZhengYuan; Yin, ZhangQi; Chen, Yan; Wang, ZiDan; Zhu, ShiLiang

    2016-06-01

    We propose a scheme to manipulate a topological spin qubit which is realized with cold atoms in a one-dimensional optical lattice. In particular, by introducing a quantum opto-electro-mechanical interface, we are able to first transfer a superconducting qubit state to an atomic qubit state and then to store it into the topological spin qubit. In this way, an efficient topological quantum memory could be constructed for the superconducting qubit. Therefore, we can consolidate the advantages of both the noise resistance of the topological qubits and the scalability of the superconducting qubits in this hybrid architecture.

  19. Supercurrent in van der Waals Josephson junction

    PubMed Central

    Yabuki, Naoto; Moriya, Rai; Arai, Miho; Sata, Yohta; Morikawa, Sei; Masubuchi, Satoru; Machida, Tomoki

    2016-01-01

    Supercurrent flow between two superconductors with different order parameters, a phenomenon known as the Josephson effect, can be achieved by inserting a non-superconducting material between two superconductors to decouple their wavefunctions. These Josephson junctions have been employed in fields ranging from digital to quantum electronics, yet their functionality is limited by the interface quality and use of non-superconducting material. Here we show that by exfoliating a layered dichalcogenide (NbSe2) superconductor, the van der Waals (vdW) contact between the cleaved surfaces can instead be used to construct a Josephson junction. This is made possible by recent advances in vdW heterostructure technology, with an atomically flat vdW interface free of oxidation and inter-diffusion achieved by eliminating all heat treatment during junction preparation. Here we demonstrate that this artificially created vdW interface provides sufficient decoupling of the wavefunctions of the two NbSe2 crystals, with the vdW Josephson junction exhibiting a high supercurrent transparency. PMID:26830754

  20. Axion mass estimates from resonant Josephson junctions

    NASA Astrophysics Data System (ADS)

    Beck, Christian

    2015-03-01

    Recently it has been proposed that dark matter axions from the galactic halo can produce a small Shapiro step-like signal in Josephson junctions whose Josephson frequency resonates with the axion mass (Beck, 2013). Here we show that the axion field equations in a voltage-driven Josephson junction environment allow for a nontrivial solution where the axion-induced electrical current manifests itself as an oscillating supercurrent. The linear change of phase associated with this nontrivial solution implies the formal existence of a large magnetic field in a tiny surface area of the weak link region of the junction which makes incoming axions decay into microwave photons. We derive a condition for the design of Josephson junction experiments so that they can act as optimum axion detectors. Four independent recent experiments are discussed in this context. The observed Shapiro step anomalies of all four experiments consistently point towards an axion mass of (110±2) μeV. This mass value is compatible with the recent BICEP2 results and implies that Peccei-Quinn symmetry breaking was taking place after inflation.

  1. Supercurrent in van der Waals Josephson junction

    NASA Astrophysics Data System (ADS)

    Yabuki, Naoto; Moriya, Rai; Arai, Miho; Sata, Yohta; Morikawa, Sei; Masubuchi, Satoru; Machida, Tomoki

    2016-02-01

    Supercurrent flow between two superconductors with different order parameters, a phenomenon known as the Josephson effect, can be achieved by inserting a non-superconducting material between two superconductors to decouple their wavefunctions. These Josephson junctions have been employed in fields ranging from digital to quantum electronics, yet their functionality is limited by the interface quality and use of non-superconducting material. Here we show that by exfoliating a layered dichalcogenide (NbSe2) superconductor, the van der Waals (vdW) contact between the cleaved surfaces can instead be used to construct a Josephson junction. This is made possible by recent advances in vdW heterostructure technology, with an atomically flat vdW interface free of oxidation and inter-diffusion achieved by eliminating all heat treatment during junction preparation. Here we demonstrate that this artificially created vdW interface provides sufficient decoupling of the wavefunctions of the two NbSe2 crystals, with the vdW Josephson junction exhibiting a high supercurrent transparency.

  2. A compact transportable Josephson voltage standard

    SciTech Connect

    Hamilton, C.A.; Burroughs, C.J.; Kupferman, S.L.

    1996-06-01

    The development of a compact, portable 10 V Josephson calibration system is described. Its accuracy is the same as typical laboratory systems and its weight and volume are reduced by more than a factor of three. The new system will replace travelling voltage standards used within several NASA and DOE standards laboratories.

  3. Supercurrent in van der Waals Josephson junction.

    PubMed

    Yabuki, Naoto; Moriya, Rai; Arai, Miho; Sata, Yohta; Morikawa, Sei; Masubuchi, Satoru; Machida, Tomoki

    2016-01-01

    Supercurrent flow between two superconductors with different order parameters, a phenomenon known as the Josephson effect, can be achieved by inserting a non-superconducting material between two superconductors to decouple their wavefunctions. These Josephson junctions have been employed in fields ranging from digital to quantum electronics, yet their functionality is limited by the interface quality and use of non-superconducting material. Here we show that by exfoliating a layered dichalcogenide (NbSe2) superconductor, the van der Waals (vdW) contact between the cleaved surfaces can instead be used to construct a Josephson junction. This is made possible by recent advances in vdW heterostructure technology, with an atomically flat vdW interface free of oxidation and inter-diffusion achieved by eliminating all heat treatment during junction preparation. Here we demonstrate that this artificially created vdW interface provides sufficient decoupling of the wavefunctions of the two NbSe2 crystals, with the vdW Josephson junction exhibiting a high supercurrent transparency. PMID:26830754

  4. Electric Field Effect in Intrinsic Josephson Junctions

    NASA Astrophysics Data System (ADS)

    Koyama, T.

    The electric field effect in intrinsic Josephson junction stacks (IJJ's) is investigated on the basis of the capacitively-coupled IJJ model. We clarify the current-voltage characteristics of the IJJ's in the presence of an external electric field. It is predicted that the IJJ's show a dynamical transition to the voltage state as the external electric field is increased.

  5. TOPICAL REVIEW: Intrinsic Josephson junctions: recent developments

    NASA Astrophysics Data System (ADS)

    Yurgens, A. A.

    2000-08-01

    Some recent developments in the fabrication of intrinsic Josephson junctions (IJJ) and their application for studying high-temperature superconductors are discussed. The major advantages of IJJ and unsolved problems are outlined. The feasibility of three-terminal devices based on the stacked IJJ is briefly evaluated.

  6. Visualization of phase-coherent electron interference in a ballistic graphene Josephson junction

    NASA Astrophysics Data System (ADS)

    Allen, Monica; Shtanko, Oles; Fulga, Ion Cosma; Wang, Joel; Nurgaliev, Daniyar; Watanabe, Kenji; Taniguchi, Takashi; Akhmerov, Anton; Jarillo-Herrero, Pablo; Levitov, Leonid; Yacoby, Amir

    Graphene provides an appealing platform to explore electronic analogs of optics-like effects due to the nonclassical nature of ballistic charge transport. By coupling superconductors to a ballistic graphene sheet, we explore a new regime of superconducting transport in which phase-coherent interference of electron waves is a dominant feature. We employ Fraunhofer interferometry to achieve spatial imaging of cavity modes in a graphene Fabry-Perot resonator, embedded between two superconductors to form a Josephson junction. By visualizing current flow using Fourier methods, our measurements provide evidence of separate interference conditions for bulk and edge currents and elucidate the microscopic nature of interference at the crystal boundaries. We also observe modulation of the multiple Andreev reflection amplitude on and off resonance, a direct measure of cavity transparency. These results constitute a strong departure from conventional Josephson behavior and motivate further exploration of new effects at the intersection of superconductivity and electron-optics.

  7. Josephson-Majorana cycle in topological single-electron hybrid transistors

    NASA Astrophysics Data System (ADS)

    Didier, Nicolas; Gibertini, Marco; Moghaddam, Ali G.; Koenig, Juergen; Fazio, Rosario

    2013-03-01

    Charge transport through a small topological superconducting island in contact with a normal and a superconducting electrode occurs through a cycle which involves coherent oscillations of Cooper pairs and tunneling in/out the normal electrode through a Majorana bound state, the Josephson-Majorana cycle. We illustrate this mechanism by studying the current-voltage characteristics of a superconductor - topological superconductor - normal metal single-electron transistor. At low bias and temperature the Josephson-Majorana cycle is the dominant mechanism for transport. We discuss a three-terminal configuration that constitutes a direct probe of the non-local character of the Majorana bound states. Non-local cotunneling dominates over the local contributions and the current noise is maximally correlated independently of the length of the wire. Preprint: arXiv:1202.6357 The work is supported by CIFAR, by EU through projects QNEMS, IP-SOLID, GEOMDISS, NANOCTM and by DFG.

  8. Proposed spin-qubit controlled-not gate robust against noisy coupling

    SciTech Connect

    Kestner, J. P.; Das Sarma, S.

    2011-07-15

    We propose an implementation of the two-qubit gate in a quantum dot spin qubit system which is immune to charge noise problems. Our proposed implementation, if it could be realized in a physical system, would have the advantage of being robust against uncertainties and fluctuations in the tunnel coupling and barrier gate voltage pulse area. The key idea is to introduce an auxiliary dot and use an analog to the stimulated Raman adiabatic passage pulse sequence in three-level atomic systems, often referred to in the context of electron transport in quantum dot systems as Coherent Tunneling by Adiabatic Passage. Spin-dependent tunneling opens the possibility of performing two-qubit gate operations by this method.

  9. Spin-valley qubit in nanostructures of monolayer semiconductors: Optical control and hyperfine interaction

    NASA Astrophysics Data System (ADS)

    Wu, Yue; Tong, Qingjun; Liu, Gui-Bin; Yu, Hongyi; Yao, Wang

    2016-01-01

    We investigate the optical control possibilities of spin-valley qubit carried by single electrons localized in nanostructures of monolayer TMDs, including small quantum dots formed by lateral heterojunction and charged impurities. The quantum controls are discussed when the confinement induces valley hybridization and when the valley hybridization is absent. We show that the bulk valley and spin optical selection rules can be inherited in different forms in the two scenarios, both of which allow the definition of spin-valley qubit with desired optical controllability. We also investigate nuclear spin-induced decoherence and quantum control of electron-nuclear spin entanglement via intervalley terms of the hyperfine interaction. Optically controlled two-qubit operations in a single quantum dot are discussed.

  10. Two-axis control of a singlet–triplet qubit with an integrated micromagnet

    PubMed Central

    Wu, Xian; Ward, D. R.; Prance, J. R.; Kim, Dohun; Gamble, John King; Mohr, R. T.; Shi, Zhan; Savage, D. E.; Lagally, M. G.; Friesen, Mark; Coppersmith, S. N.; Eriksson, M. A.

    2014-01-01

    The qubit is the fundamental building block of a quantum computer. We fabricate a qubit in a silicon double-quantum dot with an integrated micromagnet in which the qubit basis states are the singlet state and the spin-zero triplet state of two electrons. Because of the micromagnet, the magnetic field difference ΔB between the two sides of the double dot is large enough to enable the achievement of coherent rotation of the qubit’s Bloch vector around two different axes of the Bloch sphere. By measuring the decay of the quantum oscillations, the inhomogeneous spin coherence time T2* is determined. By measuring T2* at many different values of the exchange coupling J and at two different values of ΔB, we provide evidence that the micromagnet does not limit decoherence, with the dominant limits on T2* arising from charge noise and from coupling to nuclear spins. PMID:25092298

  11. Leakage of The Quantum Dot Hybrid Qubit in The Strong Driving Regime

    NASA Astrophysics Data System (ADS)

    Yang, Yuan-Chi; Friesen, Mark; Coppersmith, S. N.

    Recent experimental demonstrations of high-fidelity single-qubit gates suggest that the quantum dot hybrid qubit is a promising candidate for large-scale quantum computing. The qubit is comprised of three electrons in a double quantum dot, and can be protected from charge noise by operating in an extended sweet-spot regime. Gate operations are based on exchange interactions mediated by an excited state. However, strong resonant driving causes unwanted leakage into the excited state. Here, we theoretically analyze leakage caused by strong driving, and explore methods for increasing gate fidelities. This work was supported in part by ARO (W911NF-12-0607), NSF (PHY-1104660), ONR (N00014-15-1-0029), and the University of Wisconsin-Madison.

  12. Quantum Information Splitting of Arbitrary Three-Qubit State by Using Seven-Qubit Entangled State

    NASA Astrophysics Data System (ADS)

    Li, Dong-fen; Wang, Rui-jin; Zhang, Feng-li; Deng, Fu-hu

    2015-06-01

    In this paper, we propose a scheme of quantum information splitting arbitrary three-qubit state by using seven-qubit entangled as quantum channel. The sender Alice first performs Bell-state measurements (BSMs) on her qubits pairs respectively and tells her measurement outcome to authorizers Bob to reconstruct the original state, then Charlie should carries out single-qubit measurement (SQM) on his qubits. According to the results from Alice and Charlie, Bob can reconstruct the original state by applying an appropriate unitary operation. After analyzing, the method achieved the desired effect of quantum information splitting (QIS). We also realize the QIS of arbitrary three-qubit state in cavity quantum electrodynamics (QED).

  13. Quantum Splitting a Two-qubit State with a Genuinely Entangled Five-qubit State

    NASA Astrophysics Data System (ADS)

    Sang, Ming-Huang; Dai, Hai-Lang

    2014-08-01

    A new application of the genuinely entangled five-qubit state is investigated for quantum information splitting of a particular type of two-qubit state. In this scheme, a genuinely entangled five-qubit state is shared by Alice (a sender), Charlie (a controller) and Bob (a receiver), and Alice only needs to perform two Bell-state measurements and Charlie performs a single-qubit measurement, Bob can reconstruct the two-qubit state by performing some appropriately unitary transformations on his qubits after he knows the measured results of both Alice and Charlie. This quantum information splitting scheme is deterministic, i.e. the probability of success is 100 %. The presented protocol is showed to be secure against certain eavesdropping attacks.

  14. Ultrafast gates for single atomic qubits.

    PubMed

    Campbell, W C; Mizrahi, J; Quraishi, Q; Senko, C; Hayes, D; Hucul, D; Matsukevich, D N; Maunz, P; Monroe, C

    2010-08-27

    We demonstrate single-qubit operations on a trapped atom hyperfine qubit using a single ultrafast pulse from a mode-locked laser. We shape the pulse from the laser and perform a π rotation of the qubit in less than 50 ps with a population transfer exceeding 99% and negligible effects from spontaneous emission or ac Stark shifts. The gate time is significantly shorter than the period of atomic motion in the trap (Ω(Rabi)/ν(trap)>10(4)), demonstrating that this interaction takes place deep within the strong excitation regime. PMID:20868145

  15. Finite-Size Bath in Qubit Thermodynamics

    NASA Astrophysics Data System (ADS)

    Pekola, J. P.; Suomela, S.; Galperin, Y. M.

    2016-09-01

    We discuss a qubit weakly coupled to a finite-size heat bath (calorimeter) from the point of view of quantum thermodynamics. The energy deposited to this environment together with the state of the qubit provides a basis to analyze the heat and work statistics of this closed combined system. We present results on two representative models, where the bath is composed of two-level systems or harmonic oscillators, respectively. Finally, we derive results for an open quantum system composed of the above qubit plus finite-size bath, but now the latter is coupled to a practically infinite bath of the same nature of oscillators or two-level systems.

  16. Multiple Multi-Qubit Quantum States Sharing

    NASA Astrophysics Data System (ADS)

    Qin, Hua-Wang; Dai, Yue-Wei

    2016-04-01

    A multiple multi-qubit quantum states sharing scheme is proposed, in which the dealer can share multiple multi-qubit quantum states among the participants through only one distribution and one recovery. The dealer encodes the secret quantum states into a special entangled state, and then distributes the particles of the entangled state to the participants. The participants perform the single-particle measurements on their particles, and can cooperate to recover the multiple multi-qubit quantum states. Compared to the existing schemes, our scheme is more efficient and more flexible in practice.

  17. Finite-Size Bath in Qubit Thermodynamics

    NASA Astrophysics Data System (ADS)

    Pekola, J. P.; Suomela, S.; Galperin, Y. M.

    2016-04-01

    We discuss a qubit weakly coupled to a finite-size heat bath (calorimeter) from the point of view of quantum thermodynamics. The energy deposited to this environment together with the state of the qubit provides a basis to analyze the heat and work statistics of this closed combined system. We present results on two representative models, where the bath is composed of two-level systems or harmonic oscillators, respectively. Finally, we derive results for an open quantum system composed of the above qubit plus finite-size bath, but now the latter is coupled to a practically infinite bath of the same nature of oscillators or two-level systems.

  18. Geometry of three-qubit entanglement

    SciTech Connect

    Levay, Peter

    2005-01-01

    A geometrical description of three-qubit entanglement is given. A part of the transformations corresponding to stochastic local operations and classical communication on the qubits is regarded as a gauge degree of freedom. Entangled states can be represented by the points of the Klein quadric Q, a space known from twistor theory. It is shown that three-qubit invariants are vanishing on special subspaces of Q with interesting geometric properties. An invariant characterizing the Greenberger-Horne-Zeilinger class is proposed. A geometric interpretation of the canonical decomposition and the inequality for distributed entanglement is also given.

  19. Maxwell's demon based on a single qubit

    NASA Astrophysics Data System (ADS)

    Pekola, J. P.; Golubev, D. S.; Averin, D. V.

    2016-01-01

    We propose and analyze Maxwell's demon based on a single qubit with avoided level crossing. Its operation cycle consists of adiabatic drive to the point of minimum energy separation, measurement of the qubit state, and conditional feedback. We show that the heat extracted from the bath at temperature T can ideally approach the Landauer limit of kBT ln2 per cycle even in the quantum regime. Practical demon efficiency is limited by the interplay of Landau-Zener transitions and coupling to the bath. We suggest that an experimental demonstration of the demon is fully feasible using one of the standard superconducting qubits.

  20. Strong monogamy inequalities for four qubits

    NASA Astrophysics Data System (ADS)

    Regula, Bartosz; Osterloh, Andreas; Adesso, Gerardo

    2016-05-01

    We investigate possible generalizations of the Coffman-Kundu-Wootters monogamy inequality to four qubits, accounting for multipartite entanglement in addition to the bipartite terms. We show that the most natural extension of the inequality does not hold in general, and we describe the violations of this inequality in detail. We investigate alternative ways to extend the monogamy inequality to express a constraint on entanglement sharing valid for all four-qubit states, and perform an extensive numerical analysis of randomly generated four-qubit states to explore the properties of such extensions.

  1. Stabilization of entanglement between remote transmon qubits

    NASA Astrophysics Data System (ADS)

    Motzoi, Felix; Sarovar, Mohan; Whaley, Birgitta

    2014-03-01

    Entanglement between remote qubits can be a valuable resource for scalable quantum computation and other quantum technologies. Here, we discuss non-unitary methods for generating and stabilizing such entanglement between remote superconducting qubits. While joint measurement of the qubits using a sequential probe allows for post-selected entanglement, adding feedback during the measurement conditioned on the outcome allows for deterministic entanglement. This can be supplemented or substituted for with reservoir engineering techniques, which allow for non-zero concurrence in the steady state even in the presence of dephasing. Both the dispersive and near-resonant regimes of circuit QED are analysed.

  2. Multimode mediated qubit-qubit coupling and dark-state symmetries in circuit quantum electrodynamics

    SciTech Connect

    Filipp, S.; Goeppl, M.; Fink, J. M.; Baur, M.; Bianchetti, R.; Steffen, L.; Wallraff, A.

    2011-06-15

    Microwave cavities with high quality factors enable coherent coupling of distant quantum systems. Virtual photons lead to a transverse interaction between qubits when they are nonresonant with the cavity but resonant with each other. We experimentally investigate the inverse scaling of the interqubit coupling with the detuning from a cavity mode and its proportionality to the qubit-cavity interaction strength. We demonstrate that the enhanced coupling at higher frequencies is mediated by multiple higher-harmonic cavity modes. Moreover, we observe dark states of the coupled qubit-qubit system and analyze their relation to the symmetry of the applied driving field at different frequencies.

  3. Quantum error correction beyond qubits

    NASA Astrophysics Data System (ADS)

    Aoki, Takao; Takahashi, Go; Kajiya, Tadashi; Yoshikawa, Jun-Ichi; Braunstein, Samuel L.; van Loock, Peter; Furusawa, Akira

    2009-08-01

    Quantum computation and communication rely on the ability to manipulate quantum states robustly and with high fidelity. To protect fragile quantum-superposition states from corruption through so-called decoherence noise, some form of error correction is needed. Therefore, the discovery of quantum error correction (QEC) was a key step to turn the field of quantum information from an academic curiosity into a developing technology. Here, we present an experimental implementation of a QEC code for quantum information encoded in continuous variables, based on entanglement among nine optical beams. This nine-wave-packet adaptation of Shor's original nine-qubit scheme enables, at least in principle, full quantum error correction against an arbitrary single-beam error.

  4. Gate Set Tomography on two qubits

    NASA Astrophysics Data System (ADS)

    Nielsen, Erik; Blume-Kohout, Robin; Gamble, John; Rudinger, Kenneth

    Gate set tomography (GST) is a method for characterizing quantum gates that does not require pre-calibrated operations, and has been used to both certify and improve the operation of single qubits. We analyze the performance of GST applied to a simulated two-qubit system, and show that Heisenberg scaling is achieved in this case. We present a GST analysis of preliminary two-qubit experimental data, and draw comparisons with the simulated data case. Finally, we will discuss recent theoretical developments that have improved the efficiency of GST estimation procedures, and which are particularly beneficial when characterizing two qubit systems. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.

  5. Quantum gates by qubit frequency modulation in circuit QED

    NASA Astrophysics Data System (ADS)

    Beaudoin, Felix; da Silva, Marcus P.; Johnson, Blake R.; Ohki, Thomas A.; Dutton, Zachary; Blais, Alexandre

    2012-02-01

    Several types of two-qubit gates have been realized experimentally in circuit QED. These are based, for example, on tuning the pair of qubits in resonance with each other [Majer, Nature 449, 443-447 (2007)] or on a microwave pulse on one qubit at the transition frequency of a second qubit [Chow, Phys. Rev. Lett. 107, 080502 (2011)]. Another realization is based on a sequence of blue-sideband transitions generated by microwave pulses [Leek, Phys. Rev. B 79, 180511(R) (2009)]. Here, we propose a different approach relying on oscillations of the qubit frequency using a flux-bias line. We explain how frequency modulation leads to tunable qubit-resonator and qubit-qubit interactions. We also show how this form of quantum control leads to faster (first-order) sideband transitions and consider applications to two-qubit gates.

  6. Electrical control of quantum dot spin qubits

    NASA Astrophysics Data System (ADS)

    Laird, Edward Alexander

    This thesis presents experiments exploring the interactions of electron spins with electric fields in devices of up to four quantum dots. These experiments are particularly motivated by the prospect of using electric fields to control spin qubits. A novel hyperfine effect on a single spin in a quantum dot is presented in Chapter 2. Fluctuations of the nuclear polarization allow single-spin resonance to be driven by an oscillating electric field. Spin resonance spectroscopy revealed a nuclear polarization built up inside the quantum dot device by driving the resonance. The evolution of two coupled spins is controlled by the combination of hyperfine interaction, which tends to cause spin dephasing, and exchange, which tends to prevent it. In Chapter 3, dephasing is studied in a device with tunable exchange, probing the crossover between exchange-dominated and hyperfine-dominated regimes. In agreement with theoretical predictions, oscillations of the spin conversion probability and saturation of dephasing are observed. Chapter 4 deals with a three-dot device, suggested as a potential qubit controlled entirely by exchange. Preparation and readout of the qubit state are demonstrated, together with one out of two coherent exchange operations needed for arbitrary manipulations. A new readout technique allowing rapid device measurement is described. In Chapter 5, an attempt to make a two-qubit gate using a four-dot device is presented. Although spin qubit operation has not yet been possible, the electrostatic interaction between pairs of dots was measured to be sufficient in principle for coherent qubit coupling.

  7. Single-shot readout of spin qubits in Si/SiGe quantum dots

    NASA Astrophysics Data System (ADS)

    Simmons, Christie

    2012-02-01

    Si/SiGe quantum dots are an attractive option for spin qubit development, because of the long coherence times for electron spins in silicon, arising from weak hyperfine interaction and low spin orbit coupling. I will present measurements of gate-defined single and double quantum dots formed in Si/SiGe semiconductor heterostuctures. Control of the gate voltages on these dots enables tuning of the tunnel coupling to the leads and to other dots. Careful tuning of these tunnel rates, in combination with fast, pulsed-gate manipulation and spin-to-charge conversion, allow spin state measurement using an integrated quantum point contact as a local charge detector. Single spin qubit readout relies on the Zeeman energy splitting from an external magnetic field for spin-to-charge conversion. Two-electron singlet-triplet qubits, on the other hand, can be measured by using Pauli spin blockade of tunneling between the dots to readout the qubit even at zero magnetic field. I will present real-time, single-shot readout measurements of both individual spin [1] and singlet-triplet qubits [2] in gated Si/SiGe quantum dots. Work performed in collaboration with J. R. Prance, Zhan Shi, B. J. Van Bael, Teck Seng Koh, D. E. Savage, M. G. Lagally, R. Joynt, L. R. Schreiber, L. M. K. Vandersypen, M. Friesen, S. N. Coppersmith, and M. A. Eriksson. [4pt] [1] C. B. Simmons et al. Physical Review Letters 106, 156804 (2011). [0pt] [2] J. R. Prance, et al., e-print: http://lanl.arxiv.org/abs/1110.6431

  8. Fabrication of Ultrasmall High-Quality Bi2Sr2CaCu2O8+δ Intrinsic Josephson Junctions

    NASA Astrophysics Data System (ADS)

    Matsumoto, Tetsuro; Kashiwaya, Hiromi; Shibata, Hajime; Eisaki, Hiroshi; Yoshida, Yoshiyuki; Kashiwaya, Satoshi

    2008-10-01

    The fabrication of ultrasmall high-quality intrinsic Josephson junctions (IJJs) using a cuprate superconductor is critical for the realization of a qubit. We investigated the mechanism of damage induced by a Ga+ beam in a Bi2Sr2CaCu2O8+δ IJJ during focused ion beam (FIB) processing. On the basis of the results, we developed a process that allowed the successful fabrication of an ultrasmall and high-quality IJJ. The damage induced by the FIB is reduced by restricting the direction of the Ga+ beam so that the junction area is not directly irradiated by the ion beam. The fabricated ultrasmall IJJ device has a junction area of 0.3 µm2 and shows excellent current-voltage characteristics.

  9. Local dissipation effects in two-dimensional quantum Josephson junction arrays with a magnetic field

    SciTech Connect

    Polak, T.P.; Kopec, T.K.

    2005-07-01

    We study the quantum phase transitions in two-dimensional arrays of Josephson-couples junctions with short range Josephson couplings (given by the Josephson energy E{sub J}) and the charging energy E{sub C}. We map the problem onto the solvable quantum generalization of the spherical model that improves over the mean-field theory method. The arrays are placed on the top of a two-dimensional electron gas separated by an insulator. We include effects of the local dissipation in the presence of an external magnetic flux f={phi}/{phi}{sub 0} in square lattice for several rational fluxes f=0,(1/2),(1/3),(1/4), and (1/6). We also have examined the T=0 superconducting-insulator phase boundary as a function of a dissipation {alpha}{sub 0} for two different geometry of the lattice: square and triangular. We have found a critical value of the dissipation parameter independent on geometry of the lattice and presence magnetic field.

  10. Superconducting-to-Normal State Switching Experiments using Graphene-based Josephson Junctions

    NASA Astrophysics Data System (ADS)

    Lambert, Joseph; Carabello, Steven; Ramos, Roberto

    2011-03-01

    We report results of ongoing superconductor-to-normal state switching experiments using graphene-based Josephson junctions. These devices consist of a single-layer graphene flake contacted by two superconducting parallel leads separated by a few hundred nanometers. Through the proximity effect, the superconducting state is induced in the graphene region below the leads and the Josephson supercurrent is mediated through the normal graphene region by multiple Andreev reflections. The Josephson effect has been firmly demonstrated in these devices, where supercurrents in the hysteretic current-voltage characteristic, Shapiro steps, the Fraunhofer-like diffraction pattern in the critical current versus external magnetic field, and the current-phase relationship have been observed. We report on work in progress, in measuring I-V characteristics, thermal activation and microwave resonant activation in graphene-based junctions, at various temperatures below 1 Kelvin. We modulate the density of charge carriers using a back-gate voltage, which tunes the critical current. This provides another knob for studying these state switching properties.

  11. Holographic Josephson junction from massive gravity

    NASA Astrophysics Data System (ADS)

    Hu, Ya-Peng; Li, Huai-Fan; Zeng, Hua-Bi; Zhang, Hai-Qing

    2016-05-01

    We study the holographic superconductor-normal metal-superconductor (SNS) Josephson junction in de Rham-Gabadadze-Tolley massive gravity. If the boundary theory is independent of spatial directions, i.e., if the chemical potential is homogeneous in spatial directions, we find that the graviton mass parameter will make it more difficult for the normal metal-superconductor phase transition to take place. In the holographic model of the Josephson junction, it is found that the maximal tunneling current will decrease according to the graviton mass parameter. Besides, the coherence length of the junction decreases as well with respect to the graviton mass parameter. If one interprets the graviton mass parameter as the effect of momentum dissipation in the boundary field theory, this indicates that the stronger the momentum dissipation is, the smaller the coherence length is.

  12. Josephson Circuits as Vector Quantum Spins

    NASA Astrophysics Data System (ADS)

    Samach, Gabriel; Kerman, Andrew J.

    While superconducting circuits based on Josephson junction technology can be engineered to represent spins in the quantum transverse-field Ising model, no circuit architecture to date has succeeded in emulating the vector quantum spin models of interest for next-generation quantum annealers and quantum simulators. Here, we present novel Josephson circuits which may provide these capabilities. We discuss our rigorous quantum-mechanical simulations of these circuits, as well as the larger architectures they may enable. This research was funded by the Office of the Director of National Intelligence (ODNI) and the Intelligence Advanced Research Projects Activity (IARPA) under Air Force Contract No. FA8721-05-C-0002. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of ODNI, IARPA, or the US Government.

  13. Work fluctuations in bosonic Josephson junctions

    NASA Astrophysics Data System (ADS)

    Lena, R. G.; Palma, G. M.; De Chiara, G.

    2016-05-01

    We calculate the first two moments and full probability distribution of the work performed on a system of bosonic particles in a two-mode Bose-Hubbard Hamiltonian when the self-interaction term is varied instantaneously or with a finite-time ramp. In the instantaneous case, we show how the irreversible work scales differently depending on whether the system is driven to the Josephson or Fock regime of the bosonic Josephson junction. In the finite-time case, we use optimal control techniques to substantially decrease the irreversible work to negligible values. Our analysis can be implemented in present-day experiments with ultracold atoms and we show how to relate the work statistics to that of the population imbalance of the two modes.

  14. Josephson π state induced by valley polarization

    NASA Astrophysics Data System (ADS)

    Wang, Jun; Yang, Y. H.; Chan, K. S.

    2014-02-01

    We theoretically explore possible π-state Josephson junctions made from graphene-like two-dimensional materials (G) with the honeycomb lattice structure. It is shown that the valley polarization in the G sheet could lead to a 0-π state transition of the Josephson junction because of the valley-singlet Cooper pairs acquiring a nonzero momentum. When the valley-mixing scattering exists in the interfaces of the junction due to lattice mismatch, an odd-frequency valley-triplet supercurrent flows in the system even though the G sheet is fully valley polarized, and the supercurrent is characterized by a rapid atomic-scale oscillation with a periodicity of three lattice constants.

  15. Topology of the three-qubit space of entanglement types

    SciTech Connect

    Walck, Scott N.; Glasbrenner, James K.; Lochman, Matthew H.; Hilbert, Shawn A.

    2005-11-15

    The three-qubit space of entanglement types is the orbit space of the local unitary action on the space of three-qubit pure states and, hence, describes the types of entanglement that a system of three qubits can achieve. We show that this orbit space is homeomorphic to a certain subspace of R{sup 6}, which we describe completely. We give a topologically based classification of three-qubit entanglement types, and we argue that the nontrivial topology of the three-qubit space of entanglement types forbids the existence of standard states with the convenient properties of two-qubit standard states.

  16. Enhancing the coherence of 3D qubits suitable for multi-qubit experiments

    NASA Astrophysics Data System (ADS)

    Abdo, Baleegh; McClure, Douglas; Paik, Hanhee; Sandberg, Martin; Gambetta, Jay; Dial, Oliver

    2015-03-01

    Superconducting qubits coupled to 3D cavities have several advantages over qubits coupled to planar cavities on the same chip, e.g., 1) they can be individually designed, tested, and integrated, 2) they exhibit better microwave hygiene than their 2D counterpart, and 3) they possess higher coherence times, in part because they have reduced participation ratios for surface dielectric layers that can be lossy. However, in order to implement the surface code using a multi-qubit system, 3D qubits and their corresponding microwave cavities have to meet several competing requirements in addition to preserving long coherence, such as large coupling of the 3D qubit to two adjacent microwave resonators, sufficient coupling to other qubits, and large microwave isolation between different cavities. Finding a qubit design that balances all of these requirements has proved challenging so far. In this work, we apply a combination of simulations and experiments to investigate a variety of loss mechanisms that are particularly relevant for multi-qubit systems. Based on this learning, we identify changes in the design and materials of these systems that can lead to enhancement of their coherence times. We acknowledge support from IARPA under Contract W911NF-10-1-0324.

  17. The validity of the RWA and gate operation speedup by violating RWA in resonant-driven qubit systems

    NASA Astrophysics Data System (ADS)

    Song, Yang

    The rotating wave approximation (RWA) is ubiquitously used in understanding (quasi)resonant driven systems and designing pulses for state evolution. Following the practice in atomic and NMR physics, a wide range of semiconducting qubit systems are driven resonantly to manipulate the qubit, including single-spin/resonant exchange (RX)/various singlet-triplet(ST)/spin-charge hybrid qubits. The purpose of this talk is twofold: (I) Examine the validity of RWA in different qubit systems and analyze the error in terms of quantum computation; (II) Present faster gate operations by going into RWA-invalid regime for resonant-driven qubits (esp. for ST and RX types). We measure the RWA-induced infidelity and discuss it in view of the fault-tolerant error correction threshold and operation speeds. Applying the analytical extension (two orders higher than RWA) greatly reduces the infidelity, in the regime where the RWA is attempted to be used. Moreover, we show that the resonant-driven system is not limited by the Rabi-like weak coupling limit and the associated slow gate speed, much smaller than the level splitting (e.g., the small Zeeman energy gradient in ST qubits). We demonstrate the universal one qubit gates for driving strength up to a few level splitting, achieving fast control with only simple sinusoidal pulses. We also solve for the `shifted sinusoidal' pulses needed for ST qubits where the exchange coupling cannot change signs. In collaboration with Xin Wang, Jason Kestner and Sankar Das Sarma, and supported by LPS-MPO-CMTC and IARPA-MQCO.

  18. Josephson Junctions Help Measure Resonance And Dispersion

    NASA Technical Reports Server (NTRS)

    Javadi, Hamid H. S.; Mcgrath, William R.; Bumble, Bruce; Leduc, Henry G.

    1994-01-01

    Electrical characteristics of superconducting microstrip transmission lines measured at millimeter and submillimeter wavelengths. Submicron Josephson (super-conductor/insulator/superconductor) junctions used as both voltage-controlled oscillators and detectors to measure frequencies (in range of hundreds of gigahertz) of high-order resonant electromagnetic modes of superconducting microstrip transmission-line resonators. This oscillator/detector approach similar to vacuum-tube grid dip meters and transistor dip meters used to probe resonances at much lower frequencies.

  19. A fast Josephson SFQ shift register

    SciTech Connect

    Kuo, F.; Whiteley, S.R.; Faris, S.M.

    1989-03-01

    A novel Josephson shift register has been designed and fabricated. The main data latch is DC biased; the data storage is accomplished through the transfer of a single flux quantum in or out of a SQUID superconducting loop. A two-phase sinusoidal clock with offset is used to operate the circuit. Simulations verify that the operating frequency can go beyond 70 GHz, with operating margins exceeding 20%. Preliminary measurements indicate that the circuit operates as intended.

  20. Josephson coupling mediated by quantum diffusion

    NASA Astrophysics Data System (ADS)

    Frydman, A.; Ovadyahu, Z.

    1995-07-01

    We present results on the transport properties of Pb/I/Pb junctions, where I is either a-Ge or a-InO film. At helium temperatures, such structures sustain non-dissipative currents and exhibit systematic sub-gap I-V modulation. The data are consistent with the existence of a Josephson-coupling mechanism involving quantum mechanical coherent diffusion within the Anderson-insulating barrier.

  1. Macroscopic quantum tunneling in Josephson tunnel junctions and Coulomb blockade in single small tunnel junctions

    SciTech Connect

    Cleland, A.N.

    1991-04-01

    Experiments investigating the process of macroscopic quantum tunneling in a moderately-damped, resistively shunted, Josephson junction are described, followed by a discussion of experiments performed on very small capacitance normal-metal tunnel junctions. The experiments on the resistively-shunted Josephson junction were designed to investigate a quantum process, that of the tunneling of the Josephson phase variable under a potential barrier, in a system in which dissipation plays a major role in the dynamics of motion. All the parameters of the junction were measured using the classical phenomena of thermal activation and resonant activation. Theoretical predictions are compared with the experimental results, showing good agreement with no adjustable parameters; the tunneling rate in the moderately damped (Q {approx} 1) junction is seen to be reduced by a factor of 300 from that predicted for an undamped junction. The phase is seen to be a good quantum-mechanical variable. The experiments on small capacitance tunnel junctions extend the measurements on the larger-area Josephson junctions from the region in which the phase variable has a fairly well-defined value, i.e. its wavefunction has a narrow width, to the region where its value is almost completely unknown. The charge on the junction becomes well-defined and is predicted to quantize the current through the junction, giving rise to the Coulomb blockade at low bias. I present the first clear observation of the Coulomb blockade in single junctions. The electrical environment of the tunnel junction, however, strongly affects the behavior of the junction: higher resistance leads are observed to greatly sharpen the Coulomb blockade over that seen with lower resistance leads. I present theoretical descriptions of how the environment influences the junctions; comparisons with the experimental results are in reasonable agreement.

  2. Radiation comb generation with extended Josephson junctions

    SciTech Connect

    Solinas, P.; Bosisio, R.; Giazotto, F.

    2015-09-21

    We propose the implementation of a Josephson radiation comb generator based on an extended Josephson junction subject to a time dependent magnetic field. The junction critical current shows known diffraction patterns and determines the position of the critical nodes when it vanishes. When the magnetic flux passes through one of such critical nodes, the superconducting phase must undergo a π-jump to minimize the Josephson energy. Correspondingly, a voltage pulse is generated at the extremes of the junction. Under periodic driving, this allows us to produce a comb-like voltage pulses sequence. In the frequency domain, it is possible to generate up to hundreds of harmonics of the fundamental driving frequency, thus mimicking the frequency comb used in optics and metrology. We discuss several implementations through a rectangular, cylindrical, and annular junction geometries, allowing us to generate different radiation spectra and to produce an output power up to 10 pW at 50 GHz for a driving frequency of 100 MHz.

  3. Josephson junction in a thin film

    SciTech Connect

    Kogan, V. G.; Dobrovitski, V. V.; Clem, J. R.; Mawatari, Yasunori; Mints, R. G.

    2001-04-01

    The phase difference {phi}(y) for a vortex at a line Josephson junction in a thin film attenuates at large distances as a power law, unlike the case of a bulk junction where it approaches exponentially the constant values at infinities. The field of a Josephson vortex is a superposition of fields of standard Pearl vortices distributed along the junction with the line density {phi}'(y)/2{pi}. We study the integral equation for {phi}(y) and show that the phase is sensitive to the ratio l/{Lambda}, where l={lambda}{sub J}{sup 2}/{lambda}{sub L}, {Lambda}=2{lambda}{sub L}{sup 2}/d, {lambda}{sub L}, and {lambda}{sub J} are the London and Josephson penetration depths, and d is the film thickness. For l<<{Lambda}, the vortex ''core'' of the size l is nearly temperature independent, while the phase ''tail'' scales as l{Lambda}/y{sup 2}={lambda}{sub J}2{lambda}{sub L}/d/y{sup 2}; i.e., it diverges as T{yields}T{sub c}. For l>>{Lambda}, both the core and the tail have nearly the same characteristic length l{Lambda}.

  4. Edge currents in frustrated Josephson junction ladders

    NASA Astrophysics Data System (ADS)

    Marques, A. M.; Santos, F. D. R.; Dias, R. G.

    2016-09-01

    We present a numerical study of quasi-1D frustrated Josephson junction ladders with diagonal couplings and open boundary conditions, in the large capacitance limit. We derive a correspondence between the energy of this Josephson junction ladder and the expectation value of the Hamiltonian of an analogous tight-binding model, and show how the overall superconducting state of the chain is equivalent to the minimum energy state of the tight-binding model in the subspace of one-particle states with uniform density. To satisfy the constraint of uniform density, the superconducting state of the ladder is written as a linear combination of the allowed k-states of the tight-binding model with open boundaries. Above a critical value of the parameter t (ratio between the intra-rung and inter-rung Josephson couplings) the ladder spontaneously develops currents at the edges, which spread to the bulk as t is increased until complete coverage is reached. Above a certain value of t, which varies with ladder size (t = 1 for an infinite-sized ladder), the edge currents are destroyed. The value t = 1 corresponds, in the tight-binding model, to the opening of a gap between two bands. We argue that the disappearance of the edge currents with this gap opening is not coincidental, and that this points to a topological origin for these edge current states.

  5. Very large thermophase in ferromagnetic Josephson junctions.

    PubMed

    Giazotto, F; Heikkilä, T T; Bergeret, F S

    2015-02-13

    The concept of thermophase refers to the appearance of a phase gradient inside a superconductor originating from the presence of an applied temperature bias across it. The resulting supercurrent flow may, in suitable conditions, fully counterbalance the temperature-bias-induced quasiparticle current therefore preventing the formation of any voltage drop, i.e., a thermovoltage, across the superconductor. Yet, the appearance of a thermophase is expected to occur in Josephson-coupled superconductors as well. Here, we theoretically investigate the thermoelectric response of a thermally biased Josephson junction based on a ferromagnetic insulator. In particular, we predict the occurrence of a very large thermophase that can reach π/2 across the contact for suitable temperatures and structure parameters; i.e., the quasiparticle thermal current can reach the critical current. Such a thermophase can be several orders of magnitude larger than that predicted to occur in conventional Josephson tunnel junctions. In order to assess experimentally the predicted very large thermophase, we propose a realistic setup realizable with state-of-the-art nanofabrication techniques and well-established materials, based on a superconducting quantum interference device. This effect could be of strong relevance in several low-temperature applications, for example, for revealing tiny temperature differences generated by coupling the electromagnetic radiation to one of the superconductors forming the junction. PMID:25723238

  6. On Josephson effects in insulating spin systems

    SciTech Connect

    Schilling, Andreas Grundmann, Henrik

    2012-09-15

    We discuss an experiment in which two magnetic insulators that both show a field-induced magnetic ordering transition are weakly coupled to one another and are placed into an external magnetic field. If the respective magnetic states can be interpreted as phase-coherent Bose-Einstein condensates of magnetic bosonic quasiparticles, one expects the occurrence of Josephson effects. For two identical systems, the resulting d.c. Josephson effect formally represents a constant quasiparticle Josephson current across the weak link, which turns out to be unobservable in an experiment. For magnetic insulators with different critical fields, a spontaneous alternating quasiparticle current develops with a leading oscillation frequency {omega}{sub a.c.} that is determined by the difference between the critical fields. As a result of the coupling, additional sidebands appear in the energy spectrum of the coupled device that would be absent without phase coherence. We discuss the primary conditions for such an effect to take place and conclude that its detection can be feasible for a proper choice of compounds with suitable and realistic material parameters.

  7. Overdamped Josephson junctions for digital applications

    NASA Astrophysics Data System (ADS)

    Febvre, P.; De Leo, N.; Fretto, M.; Sosso, A.; Belogolovskii, M.; Collot, R.; Lacquaniti, V.

    2013-01-01

    An interesting feature of Superconductor-Normal metal-Superconductor Josephson junctions for digital applications is due to their non-hysteretic current-voltage characteristics in a broad temperature range below Tc. This allows to design Single-Flux-Quantum (SFQ) cells without the need of external shunts. Two advantages can be drawn from this property: first the SFQ cells can be more compact which leads to a more integrated solution towards nano-devices and more complex circuits; second the absence of electrical parasitic elements associated with the wiring of resistors external to the Josephson junctions increases the performance of SFQ circuits, in particular regarding the ultimate speed of operation. For this purpose Superconductor-Normal metal-Insulator-Superconductor Nb/Al-AlOx/Nb Josephson junctions have been recently developed at INRiM with aluminum layer thicknesses between 30 and 100 nm. They exhibit non-hysteretic current-voltage characteristics with IcRn values higher than 0.5 mV in a broad temperature range and optimal Stewart McCumber parameters at 4.2 K for RSFQ applications. The main features of obtained SNIS junctions regarding digital applications are presented.

  8. Intrinsic Josephson Junctions with Intermediate Damping

    NASA Astrophysics Data System (ADS)

    Warburton, Paul A.; Saleem, Sajid; Fenton, Jon C.; Speller, Susie; Grovenor, Chris R. M.

    2011-03-01

    In cuprate superconductors, adjacent cuprate double-planes are intrinsically Josephson-coupled. For bias currents perpendicular to the planes, the current-voltage characteristics correspond to those of an array of underdamped Josephson junctions. We will discuss our experiments on sub-micron Tl-2212 intrinsic Josephson junctions (IJJs). The dynamics of the IJJs at the plasma frequency are moderately damped (Q ~ 8). This results in a number of counter-intuitive observations, including both a suppression of the effect of thermal fluctuations and a shift of the skewness of the switching current distributions from negative to positive as the temperature is increased. Simulations confirm that these phenomena result from repeated phase slips as the IJJ switches from the zero-voltage to the running state. We further show that increased dissipation counter-intuitively increases the maximum supercurrent in the intermediate damping regime (PRL vol. 103, art. no. 217002). We discuss the role of environmental dissipation on the dynamics and describe experiments with on-chip lumped-element passive components in order control the environment seen by the IJJs. Work supported by EPSRC.

  9. Probing qubit by qubit: Properties of the POVM and the information/disturbance tradeoff

    NASA Astrophysics Data System (ADS)

    Sparaciari, Carlo; Paris, Matteo G. A.

    2014-04-01

    We address the class of positive operator-valued measures (POVMs) for qubit systems that are obtained by coupling the signal qubit with a probe qubit and then performing a projective measurement on the sole probe system. These POVMs, which represent the simplest class of qubit POVMs, depends on 3 + 3 + 2 = 8 free parameters describing the initial preparation of the probe qubit, the Cartan representative of the unitary coupling, and the projective measurement at the output, respectively. We analyze in some detail the properties of the POVM matrix elements, and investigate their values for given ranges of the free parameters. We also analyze in detail the tradeoff between information and disturbance for different ranges of the free parameters, showing, among other things, that (i) typical values of the tradeoff are close to optimality and (ii) even using a maximally mixed probe one may achieve optimal tradeoff.

  10. A two-qubit logic gate in silicon.

    PubMed

    Veldhorst, M; Yang, C H; Hwang, J C C; Huang, W; Dehollain, J P; Muhonen, J T; Simmons, S; Laucht, A; Hudson, F E; Itoh, K M; Morello, A; Dzurak, A S

    2015-10-15

    Quantum computation requires qubits that can be coupled in a scalable manner, together with universal and high-fidelity one- and two-qubit logic gates. Many physical realizations of qubits exist, including single photons, trapped ions, superconducting circuits, single defects or atoms in diamond and silicon, and semiconductor quantum dots, with single-qubit fidelities that exceed the stringent thresholds required for fault-tolerant quantum computing. Despite this, high-fidelity two-qubit gates in the solid state that can be manufactured using standard lithographic techniques have so far been limited to superconducting qubits, owing to the difficulties of coupling qubits and dephasing in semiconductor systems. Here we present a two-qubit logic gate, which uses single spins in isotopically enriched silicon and is realized by performing single- and two-qubit operations in a quantum dot system using the exchange interaction, as envisaged in the Loss-DiVincenzo proposal. We realize CNOT gates via controlled-phase operations combined with single-qubit operations. Direct gate-voltage control provides single-qubit addressability, together with a switchable exchange interaction that is used in the two-qubit controlled-phase gate. By independently reading out both qubits, we measure clear anticorrelations in the two-spin probabilities of the CNOT gate. PMID:26436453

  11. Feynman's and Ohta's Models of a Josephson Junction

    ERIC Educational Resources Information Center

    De Luca, R.

    2012-01-01

    The Josephson equations are derived by means of the weakly coupled two-level quantum system model given by Feynman. Adopting a simplified version of Ohta's model, starting from Feynman's model, the strict voltage-frequency Josephson relation is derived. The contribution of Ohta's approach to the comprehension of the additional term given by the…

  12. The Josephson Effect: 50 Years of Science and Technology

    ERIC Educational Resources Information Center

    Warburton, Paul A.

    2011-01-01

    The Josephson effect, the 50th anniversary of which will be celebrated in 2012, remains one of the most spectacular manifestations of quantum mechanics in all of experimental science. It was first predicted in 1962 and then experimentally verified in 1963. At its most fundamental level the Josephson effect is nothing more than the electronic…

  13. Target attractor tracking of relative phase in Bosonic Josephson junction

    NASA Astrophysics Data System (ADS)

    Borisenok, Sergey

    2016-06-01

    The relative phase of Bosonic Josephson junction in the Josephson regime of Bose-Hubbard model is tracked via the target attractor (`synergetic') feedback algorithm with the inter-well coupling parameter presented as a control function. The efficiency of our approach is demonstrated numerically for Gaussian and harmonic types of target phases.

  14. Noise filtering of composite pulses for singlet-triplet qubits.

    PubMed

    Yang, Xu-Chen; Wang, Xin

    2016-01-01

    Semiconductor quantum dot spin qubits are promising candidates for quantum computing. In these systems, the dynamically corrected gates offer considerable reduction of gate errors and are therefore of great interest both theoretically and experimentally. They are, however, designed under the static-noise model and may be considered as low-frequency filters. In this work, we perform a comprehensive theoretical study of the response of a type of dynamically corrected gates, namely the supcode for singlet-triplet qubits, to realistic 1/f noises with frequency spectra 1/ω(α). Through randomized benchmarking, we have found that supcode offers improvement of the gate fidelity for α  1 and the improvement becomes exponentially more pronounced with the increase of the noise exponent in the range 1  α ≤ 3 studied. On the other hand, for small α, supcode will not offer any improvement. The δJ-supcode, specifically designed for systems where the nuclear noise is absent, is found to offer additional error reduction than the full supcode for charge noises. The computed filter transfer functions of the supcode gates are also presented. PMID:27383129

  15. Noise filtering of composite pulses for singlet-triplet qubits

    PubMed Central

    Yang, Xu-Chen; Wang, Xin

    2016-01-01

    Semiconductor quantum dot spin qubits are promising candidates for quantum computing. In these systems, the dynamically corrected gates offer considerable reduction of gate errors and are therefore of great interest both theoretically and experimentally. They are, however, designed under the static-noise model and may be considered as low-frequency filters. In this work, we perform a comprehensive theoretical study of the response of a type of dynamically corrected gates, namely the supcode for singlet-triplet qubits, to realistic 1/f noises with frequency spectra 1/ωα. Through randomized benchmarking, we have found that supcode offers improvement of the gate fidelity for α  1 and the improvement becomes exponentially more pronounced with the increase of the noise exponent in the range 1  α ≤ 3 studied. On the other hand, for small α, supcode will not offer any improvement. The δJ-supcode, specifically designed for systems where the nuclear noise is absent, is found to offer additional error reduction than the full supcode for charge noises. The computed filter transfer functions of the supcode gates are also presented. PMID:27383129

  16. Noise filtering of composite pulses for singlet-triplet qubits

    NASA Astrophysics Data System (ADS)

    Wang, Xin; Yang, Xu-Chen

    Dynamically corrected gates are useful measures to combat decoherence in spin qubit systems. They are, however, mostly designed assuming the static-noise model and may thus be considered low-frequency noise filters. In this talk we carefully examine the applicability of a particular type of dynamically corrected gates, namely the supcode designed for singlet-triplet qubits, under realistic 1 /fα noises. Through randomized benchmarking, we have found that supcode offers improvement of the gate fidelity for α > 1 and the improvement becomes exponentially more pronounced with the increase of the noise exponent α up to 3. On the other hand, for small α supcode will not offer any improvement. We also present the computed filter transfer functions for the supcode gates for nuclear and charge noise respectively and have found that they are consistent with the finding from the benchmarking. The work is supported by Grants from City University of Hong Kong (Projects No. 9610335 and No. 7200456).

  17. Sensitivity to electronics error in coupled double quantum dot qubits

    NASA Astrophysics Data System (ADS)

    Nielsen, Erik; Muller, Richard; Carroll, Malcolm

    2011-03-01

    Reducing the effects of electronics control error in double quantum dot (DQD) quantum bits (qubit) is a central challenge to the creation of a solid-state quantum computing architecture. We investigate a system of capacitively coupled DQDs which implement a variant of the controlled phase gate when using each DQD as a singlet-triplet qubit. We identify regimes in which the gate action is more robust to sources of noise such as error around the applied bias point due to electronics or charge noise. Energy spectra are found using a configuration interaction (CI) method that accurately captures the (2,0) configuration of the DQD system, which is important for operating in these potentially low-noise regimes. This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin company, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

  18. Singlet-triplet donor-quantum-dot qubit in silicon

    NASA Astrophysics Data System (ADS)

    Harvey-Collard, Patrick; Ten Eyck, Gregory A.; Wendt, Joel R.; Pluym, Tammy; Lilly, Michael P.; Carroll, Malcolm S.; Pioro-Ladrière, Michel

    2015-03-01

    Electron spins bound to phosphorus (P) donors in silicon (Si) are promising qubits due to their high fidelities, but donor-donor coupling is challenging. We propose an alternative two-electron singlet-triplet quantum-dot (QD) and donor (D) hybrid qubit. A QD is formed at a MOS 28-Si interface and is tunnel-coupled to implanted P. The proposed two-axis system is defined by the exchange and contact hyperfine interactions. We demonstrate that a few electron QD can be formed and tuned to interact with a donor. We investigate the spin filling of the QD-D system through charge-sensed (CS) magnetospectroscopy and identify spin-up loading consistent with a singlet-triplet splitting of ~100 μeV near a QD-D anti-crossing. We also demonstrate an enhanced CS readout contrast and time window due to the restricted relaxation path of the D through the QD. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. DOE's National Nuclear Security Administration under contract DE-AC04-94AL85000.

  19. Scalable in-situ qubit calibration during repetitive error detection

    NASA Astrophysics Data System (ADS)

    Kelly, J.; Barends, R.; Fowler, A.; Mutus, J.; Campbell, B.; Chen, Y.; Chen, Z.; Chiaro, B.; Dunsworth, A.; Jeffrey, E.; Lucero, E.; Megrant, A.; Neeley, M.; Neill, C.; O'Malley, P. J. J.; Roushan, P.; Sank, D.; Quintana, C.; Vainsencher, A.; Wenner, J.; White, T.; Martinis, J. M.

    A quantum computer protects a quantum state from the environment through the careful manipulations of thousands or millions of physical qubits. However, operating such quantities of qubits at the necessary level of precision is an open challenge, as optimal control parameters can vary between qubits and drift in time. We present a method to optimize physical qubit parameters while error detection is running using a nine qubit system performing the bit-flip repetition code. We demonstrate how gate optimization can be parallelized in a large-scale qubit array and show that the presented method can be used to simultaneously compensate for independent or correlated qubit parameter drifts. Our method is O(1) scalable to systems of arbitrary size, providing a path towards controlling the large numbers of qubits needed for a fault-tolerant quantum computer.

  20. Electrodynamics and intrinsic Josephson effects in multi-gap superconductors

    NASA Astrophysics Data System (ADS)

    Koyama, Tomio; Ota, Yukihiro; Machida, Masahiko

    2010-11-01

    We develop a theory for the Josephson effects in 2-gap intrinsic Josephson junction stacks (IJJ's). The coupled dynamical equations for the phase differences are derived from the low-energy effective Lagrangian. The equations can describe the longitudinal Josephson plasma and the Josephson-Leggett (JL) mode propagating in the direction perpendicular to the junctions. Numerical results for the I - V characteristics are presented. The I - V characteristics shows multiple-branch structure similar to that in Bi-2212 IJJ's. When the Josephson frequency is approached to the JL mode frequency in non-uniform voltage branches, the JL mode is resonantly excited. At the resonant voltage a step-like structure appears in the I - V curves in low-voltage branches.

  1. Spin Qubits with Semiconductor Quantum Dots

    NASA Astrophysics Data System (ADS)

    Tarucha, Seigo; Yamamoto, Michihisa; Oiwa, Akira; Choi, Byung-Soo; Tokura, Yasuhiro

    This section describes recent progresses on the research of spin qubits realized in semiconductor quantum dot (QD) systems. After we argue the scheme of initialization and detection of individual spin states, we discuss the key idea of the universal gates constituted with QDs proposed by D. Loss and D. P. DiVincenzo. In order to achieve universal quantum gate operations, we need single qubit coherent manipulations and two qubit controlled-NOT or control-Z gates. For the first type of gate, instead of the standard rf magnetic field driven electron spin resonance (ESR), we proposed and implemented electric dipole induced spin resonance (EDSR), which has various advantages over ESR, including low dissipation, individual access to the spins and integrability. We describes recent progress in the fast Rabi oscillations. The second type of gate can be realized by the exchange coupling between nearby QDs. We also discuss the experiments combining single- and two-qubit operations. Finally, we argue the progress of the coupling of the spins in QDs with the "flying qubits", namely, photons of visible or microwave and itinerant electrons in the wave guides.

  2. Classification of two-qubit states

    NASA Astrophysics Data System (ADS)

    Caban, Paweł; Rembieliński, Jakub; Smoliński, Kordian A.; Walczak, Zbigniew

    2015-12-01

    Verstraete, Dehaene and DeMoor showed that each of the two-qubit states can be generated from one of two canonical families of two-qubit states by means of transformations preserving the tensor structure of the state space. Precisely, each of such states can be generated from a three-parameter family of Bell-diagonal states or from three-parameter rank-deficient states. In this paper, we show that this classification of two-qubit states can be refined. In particular, we show that the latter canonical family of states can be reduced to three fixed states and a two-parameter family of two-qubit states. For this family of states, we provide a simple parametrization that guarantees positive semidefiniteness of the states and enables easier calculation of the Wootters concurrence and quantum discord. Moreover, we present a new general parametrization of all two-qubit states generated from the canonical families of states using sets of (pseudo)orthogonal four-vectors (frames). An advantage of the presented approach lies in the fact that the standard conditions for positive semidefiniteness of states are equivalent to (pseudo)orthogonality conditions for four-vectors serving as parameters (and appropriate conditions for parameters of the corresponding canonical family of states).

  3. Graphene-based qubits in quantum communications

    NASA Astrophysics Data System (ADS)

    Wu, G. Y.; Lue, N.-Y.

    2012-07-01

    We explore the potential application of graphene-based qubits in photonic quantum communications. In particular, the valley pair qubit in double quantum dots of gapped graphene is investigated as a quantum memory in the implementation of quantum repeaters. For the application envisioned here, our work extends the recent study of the qubit [Wu , arXiv:1104.0443; Phys. Rev. BPRBMDO1098-012110.1103/PhysRevB.84.195463 84, 195463 (2011)] to the case where the qubit is placed in an in-plane magnetic field configuration. It develops, for the configuration, a method of qubit manipulation, based on a unique ac electric field-induced, valley-orbit interaction-derived mechanism in gapped graphene. It also studies the optical response of graphene quantum dots in the configuration, in terms of valley excitation with respect to photonic polarization, and illustrates faithful photon ↔ valley quantum state transfers. This work suggests the interesting prospect of an all-graphene approach for the solid state components of a quantum network, e.g., quantum computers and quantum memories in communications.

  4. Interferometric distillation and determination of unknown two-qubit entanglement

    SciTech Connect

    Lee, S.-S. B.; Sim, H.-S.

    2009-05-15

    We propose a scheme for both distilling and quantifying entanglement, applicable to individual copies of an arbitrary unknown two-qubit state. It is realized in a usual two-qubit interferometry with local filtering. Proper filtering operation for the maximal distillation of the state is achieved by erasing single-qubit interference, and then the concurrence of the state is determined directly from the visibilities of two-qubit interference. We compare the scheme with full state tomography.

  5. Toroidal qubits: naturally-decoupled quiet artificial atoms

    PubMed Central

    Zagoskin, Alexandre M.; Chipouline, Arkadi; Il’ichev, Evgeni; Johansson, J. Robert; Nori, Franco

    2015-01-01

    The requirements of quantum computations impose high demands on the level of qubit protection from perturbations; in particular, from those produced by the environment. Here we propose a superconducting flux qubit design that is naturally protected from ambient noise. This decoupling is due to the qubit interacting with the electromagnetic field only through its toroidal moment, which provides an unusual qubit-field interaction, which is suppressed at low frequencies. PMID:26607667

  6. Asymmetric Bidirectional Controlled Teleportation via Seven-qubit Cluster State

    NASA Astrophysics Data System (ADS)

    Yang, Yu-Quan; Zha, Xin-Wei; Yu, Yan

    2016-05-01

    We propose a new protocol of asymmetric bidirectional controlled teleportation by using a seven-qubit cluster state as the quantum channel. That is to say Alice wants to transmit an arbitrary single-qubit state to Bob and Bob wants to transmit an arbitrary two qubit state to Alice via the control of the supervisor Charlie. One only need perform the Bell-state measurements and single-qubit measurement.

  7. Search for Second-Order Josephson tunneling in SFS Josephson junctions

    NASA Astrophysics Data System (ADS)

    Frolov, S. M.; Oboznov, V. A.

    2005-03-01

    SFS (Superconductor-Ferromagnet-Superconductor) Josephson junctions can exhibit transitions between ordinary Josephson (0-junction) tunneling and pi-junction behavior as a function of barrier thickness or temperature. Close to the 0-π crossover at which the first-order Josephson component vanishes, it has been predicted that second-order Josephson tunneling, characterized by a sin(2φ) component in the supercurrent, can dominate. If present, this component can be detected directly by measurements of the current-phase relation and can induce period doubling in the critical current diffraction patterns and generate half-integer Shapiro steps. However, such effects can also arise near the 0-π transition from a distribution of 0-junction and π-junction regions due to a non-uniform ferromagnetic barrier. We compare the results of measurements on junctions with uniform and non-uniform ferromagnetic barriers to determine whether observed second harmonics arise from a microscopic sin(2φ) component or from junction non-uniformity.

  8. Four-Qubit Entanglement Classification from String Theory

    SciTech Connect

    Borsten, L.; Dahanayake, D.; Duff, M. J.; Rubens, W.; Marrani, A.

    2010-09-03

    We invoke the black-hole-qubit correspondence to derive the classification of four-qubit entanglement. The U-duality orbits resulting from timelike reduction of string theory from D=4 to D=3 yield 31 entanglement families, which reduce to nine up to permutation of the four qubits.

  9. Four-qubit entanglement classification from string theory.

    PubMed

    Borsten, L; Dahanayake, D; Duff, M J; Marrani, A; Rubens, W

    2010-09-01

    We invoke the black-hole-qubit correspondence to derive the classification of four-qubit entanglement. The U-duality orbits resulting from timelike reduction of string theory from D=4 to D=3 yield 31 entanglement families, which reduce to nine up to permutation of the four qubits. PMID:20867503

  10. Bidirectional Quantum Teleportation by Using Five-qubit Cluster State

    NASA Astrophysics Data System (ADS)

    Sang, Ming-huang

    2016-03-01

    We propose a scheme for bidirectional quantum teleportation by using a five-qubit cluster state. In our scheme, Alice can transmit an arbitrary two-qubit entangled state to Bob and at the same time Bob can teleport an arbitrary single-qubit state to Alice.

  11. Optimal post-processing for a generic single-shot qubit readout

    NASA Astrophysics Data System (ADS)

    D'Anjou, Benjamin; Coish, William A.

    2014-03-01

    We analyze three different post-processing methods applied to a single-shot qubit readout: the average-signal (boxcar filter), peak-signal, and maximum-likelihood methods. In contrast to previous work, we account for a stochastic turn-on time ti associated with the leading edge of a pulse signaling one of the qubit states. This model is relevant to spin-qubit readouts based on spin-to-charge conversion and would be generically reached in the limit of large signal-to-noise ratio r for several other physical systems, including fluorescence-based readouts of ion-trap qubits and nitrogen-vacancy center spins. We find that the peak-signal method outperforms the boxcar filter significantly when ti is stochastic, but is only marginally better for deterministic ti. We generalize the theoretically optimal maximum-likelihood method to stochastic ti and show numerically that a stochastic turn-on time ti will always result in a larger single-shot error rate. Based on this observation, we propose a general strategy to improve the quality of single-shot readouts by forcing ti to be deterministic. We acknowledge finanical support from NSERC, CIFAR, FQRNT and INTRIQ.

  12. One-qubit quantum gates in a circular graphene quantum dot: genetic algorithm approach

    NASA Astrophysics Data System (ADS)

    Amparán, Gibrán; Rojas, Fernando; Pérez-Garrido, Antonio

    2013-05-01

    The aim of this work was to design and control, using genetic algorithm (GA) for parameter optimization, one-charge-qubit quantum logic gates σ x, σ y, and σ z, using two bound states as a qubit space, of circular graphene quantum dots in a homogeneous magnetic field. The method employed for the proposed gate implementation is through the quantum dynamic control of the qubit subspace with an oscillating electric field and an onsite (inside the quantum dot) gate voltage pulse with amplitude and time width modulation which introduce relative phases and transitions between states. Our results show that we can obtain values of fitness or gate fidelity close to 1, avoiding the leakage probability to higher states. The system evolution, for the gate operation, is presented with the dynamics of the probability density, as well as a visualization of the current of the pseudospin, characteristic of a graphene structure. Therefore, we conclude that is possible to use the states of the graphene quantum dot (selecting the dot size and magnetic field) to design and control the qubit subspace, with these two time-dependent interactions, to obtain the optimal parameters for a good gate fidelity using GA.

  13. Optimal partial deterministic quantum teleportation of qubits

    SciTech Connect

    Mista, Ladislav Jr.; Filip, Radim

    2005-02-01

    We propose a protocol implementing optimal partial deterministic quantum teleportation for qubits. This is a teleportation scheme realizing deterministically an optimal 1{yields}2 asymmetric universal cloning where one imperfect copy of the input state emerges at the sender's station while the other copy emerges at receiver's possibly distant station. The optimality means that the fidelities of the copies saturate the asymmetric cloning inequality. The performance of the protocol relies on the partial deterministic nondemolition Bell measurement that allows us to continuously control the flow of information among the outgoing qubits. We also demonstrate that the measurement is optimal two-qubit operation in the sense of the trade-off between the state disturbance and the information gain.

  14. Soft decoding of a qubit readout apparatus.

    PubMed

    D'Anjou, B; Coish, W A

    2014-12-01

    Qubit readout is commonly performed by thresholding a collection of analog detector signals to obtain a sequence of single-shot bit values. The intrinsic irreversibility of the mapping from analog to digital signals discards soft information associated with an a posteriori confidence that can be assigned to each bit value when a detector is well characterized. Accounting for soft information, we show significant improvements in enhanced state detection with the quantum repetition code as well as quantum state or parameter estimation. These advantages persist in spite of non-Gaussian features of realistic readout models, experimentally relevant small numbers of qubits, and finite encoding errors. These results show useful and achievable advantages for a wide range of current experiments on quantum state tomography, parameter estimation, and qubit readout. PMID:25526105

  15. A singlet - triplet T+ based qubit

    NASA Astrophysics Data System (ADS)

    Ribeiro, Hugo; Petta, Jason; Burkard, Guido

    2010-03-01

    We theoretically show that the electronic two-spin states singlet and triplet T+ are promising candidates for the implementation of a qubit in GaAs double quantum dots (DQD). A coherent superposition of the two-spin states is obtained by finite time Landau-Zener-St"uckelberg interferometry [1] and the single qubit rotations are performed by means of an external magnetic field with a typical amplitude of about 100 mT. In such a system, the coherent manipulation of the qubit takes place in a time scale of about 1 ns. We also study the nuclear induced decoherence, mainly due to hyperfine contact coupling between the electronic and nuclear spins, and compute the decoherence time T2^* ˜10 ns. [4pt] [1] H. Ribeiro and G. Burkard, Phys. Rev. Lett. 102, 216802 (2009)

  16. Optimal cloning of qubits from replicas of a qubit and its orthogonal states

    SciTech Connect

    Kato, Go

    2010-09-15

    We consider the situation where s replicas of a qubit with an unknown state and its orthogonal k replicas are given as an input, and we try to make c clones of the qubit with the unknown state. As a function of s, k, and c, we obtain the optimal fidelity between the qubit with an unknown state and the clone by explicitly giving a completely positive trace-preserving (CPTP) map that represents a cloning machine. We discuss dependency of the fidelity on the values of the parameters s, k, and c.

  17. Suppressing Leakage in High Fidelity Single Qubit Gates for Superconducting Qubits

    NASA Astrophysics Data System (ADS)

    Chen, Z.; Kelly, J.; Quintana, C.; Barends, R.; Campbell, B.; Chen, Y.; Chiaro, B.; Dunsworth, A.; Fowler, A. G.; Lucero, E.; Jeffrey, E.; Megrant, A.; Mutus, J.; Neeley, M.; Neill, C.; O'Malley, P. J. J.; Roushan, P.; Sank, D.; Vainsencher, A.; Wenner, J.; White, T.; Korotkov, A. N.; Martinis, J. M.

    Recent results show that superconducting qubits are approaching the threshold for fault tolerant quantum error correction. However, leakage into non-qubit states remains a significant hurdle because leakage errors are highly detrimental for error correction schemes such as the surface code. I will demonstrate that with a simple addition to DRAG pulse shaping, leakage can be suppressed to the 10-5 level while simultaneously maintaining 10-3 gate fidelity. I will also show that the remaining leakage errors are due to heating of the qubit, suggesting further avenues for improvement. The work was supported by Google Inc., and by the NSFGRF under Grant No. DGE 1144085.

  18. Macroscopic optomechanical superposition via periodic qubit flipping

    NASA Astrophysics Data System (ADS)

    Ge, Wenchao; Zubairy, M. Suhail

    2015-01-01

    We propose a scheme to generate macroscopic superpositions of well-distinguishable coherent states in an optomechanical system via periodic qubit flipping. Our scheme does not require the single-photon strong-coupling rate of an optomechanical system. The generated mechanical superposition state can be reconstructed using mechanical quantum-state reconstruction. The proposed scheme relies on recycling of an atom, fast atomic qubit flipping, and coherent state mapping between a single-photon superposition state and an atomic superposition state. We discuss the experimental feasibility of our proposal under current technology.

  19. Entangling unstable optically active matter qubits

    SciTech Connect

    Matsuzaki, Yuichiro; Fitzsimons, Joseph; Benjamin, Simon C.

    2011-06-15

    In distributed quantum computation, small devices composed of a single or a few qubits are networked together to achieve a scalable machine. Typically, there is an optically active matter qubit at each node, so that photons are exploited to achieve remote entanglement. However, in many systems the optically active states are unstable or poorly defined. We report a scheme to perform a high-fidelity entanglement operation even given severe instability. The protocol exploits the existence of optically excited states for phase acquisition without actually exciting those states; it functions with or without cavities and does not require number-resolving detectors.

  20. Entanglement and the geometry of two qubits

    SciTech Connect

    Avron, J.E. Kenneth, O.

    2009-02-15

    Two qubits is the simplest system where the notions of separable and entangled states and entanglement witnesses first appear. We give a three-dimensional geometric description of these notions. This description, however, carries no quantitative information on the measure of entanglement. A four-dimensional description captures also the entanglement measure. We give a neat formula for the Bell states which leads to a slick proof of the fundamental teleportation identity. We describe optimal distillation of two qubits geometrically and present a simple geometric proof of the Peres-Horodecki separability criterion.

  1. Photonic qubits for remote quantum information processing

    NASA Astrophysics Data System (ADS)

    Maunz, P.; Olmschenk, S.; Hayes, D.; Matsukevich, D. N.; Duan, L.-M.; Monroe, C.

    2009-05-01

    Quantum information processing between remote quantum memories relies on a fast and faithful quantum channel. Recent experiments employed both, the photonic polarization and frequency qubits, in order to entangle remote atoms [1, 2], to teleport quantum information [3] and to operate a quantum gate between distant atoms. Here, we compare the dierent schemes used in these experiments and analyze the advantages of the dierent choices of atomic and photonic qubits and their coherence properties. [4pt] [1] D. L. Moehring et al. Nature 449, 68 (2007).[0pt] [2] D. N. Matsukevich et al. Phys. Rev. Lett. 100, 150404 2008).[0pt] [3] S. Olmschenk et al. Science, 323, 486 (2009).

  2. Spin blockade qubit in a superconducting junction

    NASA Astrophysics Data System (ADS)

    Padurariu, C.; Nazarov, Yu. V.

    2012-12-01

    We interpret a recent pioneering experiment (Zgirski M. et al., Phys. Rev. Lett., 106 (2011) 257003) on quasiparticle manipulation in a superconducting break junction in terms of spin blockade drawing analogy with spin qubits. We propose a novel qubit design that exploits the spin state of two trapped quasiparticles. We detail the coherent control of all four spin states by resonant quantum manipulation and compute the corresponding Rabi frequencies. The read-out technique is based on the spin blockade that inhibits quasiparticle recombination in triplet states. We provide extensive microscopic estimations of the parameters of our model.

  3. Deterministic quantum computation with one photonic qubit

    NASA Astrophysics Data System (ADS)

    Hor-Meyll, M.; Tasca, D. S.; Walborn, S. P.; Ribeiro, P. H. Souto; Santos, M. M.; Duzzioni, E. I.

    2015-07-01

    We show that deterministic quantum computing with one qubit (DQC1) can be experimentally implemented with a spatial light modulator, using the polarization and the transverse spatial degrees of freedom of light. The scheme allows the computation of the trace of a high-dimension matrix, being limited by the resolution of the modulator panel and the technical imperfections. In order to illustrate the method, we compute the normalized trace of unitary matrices and implement the Deutsch-Jozsa algorithm. The largest matrix that can be manipulated with our setup is 1080 ×1920 , which is able to represent a system with approximately 21 qubits.

  4. Bursting behaviour in coupled Josephson junctions.

    PubMed

    Hongray, Thotreithem; Balakrishnan, J; Dana, Syamal K

    2015-12-01

    We report an interesting bow-tie shaped bursting behaviour in a certain parameter regime of two resistive-capacitative shunted Josephson junctions, one in the oscillatory and the other in the excitable mode and coupled together resistively. The burst emerges in both the junctions and they show near-complete synchronization for strong enough couplings. We discuss a possible bifurcation scenario to explain the origin of the burst. An exhaustive study on the parameter space of the system is performed, demarcating the regions of bursting from other solutions. PMID:26723143

  5. On Chip Josephson Junction Microwave Switch

    NASA Astrophysics Data System (ADS)

    Naaman, Ofer; Abutaleb, Mohamed; Kirby, Chris; Rennie, Michael

    We report on the design and measurement of a reflective single-pole single-throw microwave switch based on a superconducting circuit containing a single Josephson junction. The device has no internal power dissipation, minimal insertion loss, and is controlled by Φ0-level base-band signals. The data demonstrates the device operation with 2 GHz instantaneous bandwidth centered at 10 GHz and better than 20 dB on/off ratio for input powers up to -100 dBm.

  6. On-chip Josephson junction microwave switch

    NASA Astrophysics Data System (ADS)

    Naaman, O.; Abutaleb, M. O.; Kirby, C.; Rennie, M.

    2016-03-01

    The authors report on the design and measurement of a reflective single-pole single-throw microwave switch with no internal power dissipation, based on a superconducting circuit containing a single Josephson junction. The data demonstrate the switch operation with 2 GHz instantaneous bandwidth centered at 10 GHz, low insertion loss, and better than 20 dB on/off ratio. The switch's measured performance agrees well with simulations for input powers up to -100 dBm. An extension of the demonstrated circuit to implement a single-pole double-throw switch is shown in simulation.

  7. Bursting behaviour in coupled Josephson junctions

    NASA Astrophysics Data System (ADS)

    Hongray, Thotreithem; Balakrishnan, J.; Dana, Syamal K.

    2015-12-01

    We report an interesting bow-tie shaped bursting behaviour in a certain parameter regime of two resistive-capacitative shunted Josephson junctions, one in the oscillatory and the other in the excitable mode and coupled together resistively. The burst emerges in both the junctions and they show near-complete synchronization for strong enough couplings. We discuss a possible bifurcation scenario to explain the origin of the burst. An exhaustive study on the parameter space of the system is performed, demarcating the regions of bursting from other solutions.

  8. Measurement of Decoherence Time in a Flux Qubit

    NASA Astrophysics Data System (ADS)

    Harrabi, K.; Yoshihara, F.; Nakamura, Y.; Tsai, J. S.

    2006-09-01

    We present a measurement of the relaxation and the dephasing times in a flux qubit. In order to improve coherence of the qubit, two external parameters were optimized: the applied flux through the qubit loop and the bias current of the SQUID which serves as a readout device of the qubit state. At the optimal point the dephasing time measured with spin-echo technique was twice longer than the energy relaxation time. By changing one of the two bias parameters while keeping the other at the optimal value, one can separate the contribution of the noise in each parameter to the decoherence of the qubit.

  9. Detecting highly entangled states with a joint qubit readout

    SciTech Connect

    Chow, J. M.; DiCarlo, L.; Nunnenkamp, A.; Bishop, Lev S.; Frunzio, L.; Devoret, M. H.; Girvin, S. M.; Schoelkopf, R. J.; Gambetta, J. M.

    2010-06-15

    A single-channel joint readout is used to analyze highly entangled two-qubit states in a circuit quantum electrodynamics architecture. The measurement model for the readout is fully characterized, demonstrating a large sensitivity to two-qubit correlations. We quantify the high degree of entanglement by measuring a violation of the Clauser-Horne-Shimony-Holt inequality with a value of 2.61{+-}0.04, without optimizing the preparation of the two-qubit state. In its present form, this joint readout can resolve improvements to the fidelity of two-qubit operations and be extended to three or four qubits.

  10. Coplanar waveguide flux qubit suitable for quantum annealing

    NASA Astrophysics Data System (ADS)

    Quintana, Chris; Chen, Yu; Sank, D.; Kafri, D.; Megrant, A.; White, T. C.; Shabani, A.; Barends, R.; Campbell, B.; Chen, Z.; Chiaro, B.; Dunsworth, A.; Fowler, A.; Jeffrey, E.; Kelly, J.; Lucero, E.; Mutus, J. Y.; Neeley, M.; Neill, C.; O'Malley, P. J. J.; Roushan, P.; Vainsencher, A.; Wenner, J.; Martinis, J. M.

    We introduce the ''fluxmon'' flux qubit, designed with the goal of practical quantum annealing. The qubit's capacitance and linear inductance are provided by a coplanar waveguide on a low loss substrate, minimizing dielectric dissipation and in principle allowing for GHz-scale inter-qubit coupling in a highly connected tunable architecture. Utilizing a dispersive microwave readout scheme, we characterize single-qubit noise and dissipation, and present a simple tunable inter-qubit coupler. We discuss tradeoffs between coherence and coupling in a quantum annealing architecture. This work was supported by Google Inc. and by the NSF GRFP.

  11. Three-qubit thermal entanglement via entanglement swapping on two-qubit Heisenberg XY chains

    SciTech Connect

    Kao, Zi Chong; Ng, Jezreel; Yeo, Ye

    2005-12-15

    In this paper, we consider the generation of a three-qubit Greenberger-Horne-Zeilinger-like thermal state by applying the entanglement swapping scheme of Zukowski et al. [Ann. N. Y. Acad. Sci. 755, 91 (1995)] to three pairs of two-qubit Heisenberg XY chains. The quality of the resulting three-qubit entanglement is studied by analyzing the teleportation fidelity, when it is used as a resource in the teleportation protocol of Karlsson et al. [Phys. Rev. A 58, 4394 (1998)]. We show that even though thermal noise in the original two-qubit states is amplified by the entanglement swapping process, we are still able to achieve nonclassical fidelities for the anisotropic Heisenberg XY chains at finitely higher and higher temperatures by adjusting the strengths of an external magnetic field. This has a positive implication on the solid-state realization of a quantum computer.

  12. Entanglement universality of two-qubit X-states

    SciTech Connect

    Mendonça, Paulo E.M.F.; Marchiolli, Marcelo A.; Galetti, Diógenes

    2014-12-15

    We demonstrate that for every two-qubit state there is a X-counterpart, i.e., a corresponding two-qubit X-state of same spectrum and entanglement, as measured by concurrence, negativity or relative entropy of entanglement. By parametrizing the set of two-qubit X-states and a family of unitary transformations that preserve the sparse structure of a two-qubit X-state density matrix, we obtain the parametric form of a unitary transformation that converts arbitrary two-qubit states into their X-counterparts. Moreover, we provide a semi-analytic prescription on how to set the parameters of this unitary transformation in order to preserve concurrence or negativity. We also explicitly construct a set of X-state density matrices, parametrized by their purity and concurrence, whose elements are in one-to-one correspondence with the points of the concurrence versus purity (CP) diagram for generic two-qubit states. - Highlights: • Parametrization of separable, entangled and rank-specific two-qubit X-states. • Construction of a set of two-qubit X-states exhausting a two-qubit CP-diagram. • Parametrization of a disentangling unitary transformation for any two-qubit X-state. • Unitary transformation of any two-qubit state into a X-state of same entanglement.

  13. Decoherence Patterns of Topological Qubits from Majorana Modes

    NASA Astrophysics Data System (ADS)

    Chao, Sung Po; Ho, Shih Hao; Chou, Chung Hsien; Lin, Feng Li

    2015-03-01

    We investigate the decoherence patterns of topological qubits in contact with the environment. Each topological qubit is made of two Majorana modes of a 1D Kitaev's chain. These two Majorana modes weakly interact with the fermionic/bosonic environments. We find the topological qubits decohere completely in the Ohmic and sub-Ohmic environments but not in the super-Ohmic ones. Though the fermion parities of the topological qubits cannot prevent the qubit states from decoherence in the sub-Ohmic environments, it can prevent the qubits turning into Gibbs state. We also study the cases in which each Majorana mode couples to different Ohmic-like environments and the time dependence of concurrence for two topological qubits. FLL is supported by Taiwan's NSC Grants (Grant NO. 100-2811-M-003-011 and 100-2918-I-003-008). All authors acknowledge the support by NCTS.

  14. Universal quantum computation with hybrid spin-Majorana qubits

    NASA Astrophysics Data System (ADS)

    Hoffman, Silas; Schrade, Constantin; Klinovaja, Jelena; Loss, Daniel

    2016-07-01

    We theoretically propose a set of universal quantum gates acting on a hybrid qubit formed by coupling a quantum-dot spin qubit and Majorana fermion qubit. First, we consider a quantum dot that is tunnel coupled to two topological superconductors. The effective spin-Majorana exchange facilitates a hybrid cnot gate for which either qubit can be the control or target. The second setup is a modular scalable network of topological superconductors and quantum dots. As a result of the exchange interaction between adjacent spin qubits, a cnot gate is implemented that acts on neighboring Majorana qubits and eliminates the necessity of interqubit braiding. In both setups, the spin-Majorana exchange interaction allows for a phase gate, acting on either the spin or the Majorana qubit, and for a swap or hybrid swap gate which is sufficient for universal quantum computation without projective measurements.

  15. Entangling distant resonant exchange qubits via circuit quantum electrodynamics

    NASA Astrophysics Data System (ADS)

    Srinivasa, Vanita; Taylor, Jacob M.; Tahan, Charles

    Enabling modularity within a quantum information processing device relies on robust entanglement of coherent qubits at macroscopic distances. To address this challenge, we investigate theoretically a hybrid quantum system consisting of spatially separated resonant exchange qubits, defined in three-electron semiconductor triple quantum dots, that are coupled via a superconducting transmission line resonator. By analyzing three specific approaches drawn from circuit quantum electrodynamics and Hartmann-Hahn double resonance techniques for implementing resonator-mediated two-qubit entangling gates in both dispersive and resonant regimes, we show that methods for entangling superconducting qubits map directly to resonant exchange qubits. We also calculate the rate of relaxation via phonons for resonant exchange qubits in silicon triple dots and show that such an implementation is particularly well-suited to achieving the strong coupling regime. Our approach combines the robustness of encoded spin qubits in silicon with the rapid and robust long-range entanglement provided by circuit QED systems.

  16. Josephson effects in condensates of excitons and exciton polaritons

    SciTech Connect

    Shelykh, I. A.; Solnyshkov, D. D.; Pavlovic, G.; Malpuech, G.

    2008-07-15

    We analyze theoretically the phenomena related to the Josephson effect for exciton and polariton condensates, taking into account their specific spin degrees of freedom. We distinguish between two types of Josephson effects: the extrinsic effect, related to the coherent tunneling of particles with the same spin between two spatially separated potential traps, and the intrinsic effect, related to the 'tunneling' between different spinor components of the condensate within the same trap. We show that the Josephson effect in the nonlinear regime can lead to nontrivial polarization dynamics and produce spontaneous separation of the condensates with opposite polarization in real space.

  17. The critical current of point symmetric Josephson tunnel junctions

    NASA Astrophysics Data System (ADS)

    Monaco, Roberto

    2016-06-01

    The physics of Josephson tunnel junctions drastically depends on their geometrical configurations. The shape of the junction determines the specific form of the magnetic-field dependence of its Josephson current. Here we address the magnetic diffraction patterns of specially shaped planar Josephson tunnel junctions in the presence of an in-plane magnetic field of arbitrary orientations. We focus on a wide ensemble of junctions whose shape is invariant under point reflection. We analyze the implications of this type of isometry and derive the threshold curves of junctions whose shape is the union or the relative complement of two point symmetric plane figures.

  18. Enhancement of Shapiro-like steps in multiterminal Josephson structures

    NASA Astrophysics Data System (ADS)

    Savinov, D. A.

    2016-08-01

    The distinctive features of current-voltage characteristics are studied for mesoscopic multiterminal structures effected by external irradiation. Considering a simple model of applied dc+ac voltage, we calculate Shapiro-like steps in Josephson systems with several weakly coupled superconducting electrodes. Owing to the action of an external alternating signal, the dc current is found to be rather increased than it appears in the same multiterminal Josephson node without the radiation. The possible applications of our results for the experimental observation of the Shapiro-like steps in such Josephson structures are discussed.

  19. Interplay between electron overheating and ac Josephson effect

    NASA Astrophysics Data System (ADS)

    De Cecco, A.; Le Calvez, K.; Sacépé, B.; Winkelmann, C. B.; Courtois, H.

    2016-05-01

    We study the response of high-critical-current proximity Josephson junctions to a microwave excitation. Electron overheating in such devices is known to create hysteretic dc voltage-current characteristics. Here we demonstrate that it also strongly influences the ac response. The interplay of electron overheating and ac Josephson dynamics is revealed by the evolution of the Shapiro steps with the microwave drive amplitude. Extending the resistively shunted Josephson junction model by including a thermal balance for the electronic bath coupled to phonons, a strong electron overheating is obtained.

  20. Embedding qubits into fermionic Fock space: Peculiarities of the four-qubit case

    NASA Astrophysics Data System (ADS)

    Lévay, Péter; Holweck, Frédéric

    2015-06-01

    We give a fermionic Fock space description of embedded entangled qubits. Within this framework the problem of classification of pure state entanglement boils down to the problem of classifying spinors. The usual notion of separable states turns out to be just a special case of the one of pure spinors. By using the notion of single, double and mixed occupancy representation with intertwiners relating them a natural physical interpretation of embedded qubits is found. As an application of these ideas one can make a physical sound meaning of some of the direct sum structures showing up in the context of the so-called black-hole/qubit correspondence. We discuss how the usual invariants for qubits serving as measures of entanglement can be obtained from invariants for spinors in an elegant manner. In particular a detailed case study for recovering the invariants for four-qubits within a spinorial framework is presented. We also observe that reality conditions on complex spinors defining Majorana spinors for embedded qubits boil down to self-conjugate states under the Wootters spin flip operation. Finally we conduct a study on the explicit structure of Spin(16 ,C ) invariant polynomials related to the structure of possible measures of entanglement for fermionic systems with eight modes. Here we find an algebraically independent generating set of the generalized stochastic local operations and classical communication invariants and calculate their restriction to the dense orbit. We point out the special role the largest exceptional group E8 is playing in these considerations.

  1. Broadband Josephson parametric amplifiers: Beyond the standard gain-bandwidth product

    NASA Astrophysics Data System (ADS)

    Roy, Tanay; Kundu, Suman; Chand, Madhavi; Vadiraj, A. M.; Ranadive, A.; Nehra, N.; Patankar, Meghan P.; Aumentado, J.; Clerk, A. A.; Vijay, R.

    Recent development of multiplexed qubit measurement schemes demand broadband quantum-limited amplifiers to enable high fidelity readout with minimal resources. We present a simple technique to enhance the bandwidth of a resonator based Josephson Parametric Amplifier (JPA) beyond the standard gain-bandwidth product. This is achieved by introducing a positive linear slope in the imaginary component of the input impedance seen by the JPA using a λ / 2 transformer. Our theoretical model predicts an extremely flat gain profile with a bandwidth enhancement proportional to the square root of the amplitude gain. Experimentally, we achieved a nearly flat 20 dB gain profile over a 640 MHz band, with a mean 1-dB compression point of -110 dBm along with nearly quantum-limited noise performance. The results are in excellent agreement with our theoretical model. We will then discuss strategies to further enhance the performance in terms of bandwidth and dynamic range of the JPA. Finally, we will consider the applicability of our technique to different parametric pumping methods and other parametric amplifier designs as well.

  2. Detection of Coherent Terahertz Radiation from a High-Temperature Superconductor Josephson Junction by a Semiconductor Quantum-Dot Detector

    NASA Astrophysics Data System (ADS)

    Shaikhaidarov, R.; Antonov, V. N.; Casey, A.; Kalaboukhov, A.; Kubatkin, S.; Harada, Y.; Onomitsu, K.; Tzalenchuk, A.; Sobolev, A.

    2016-02-01

    We examine the application of Josephson radiation emitters to spectral calibration of single-photon-resolving detectors. Josephson junctions are patterned in a YBCO film on a bicrystal sapphire substrate and are voltage controlled to generate radiation in the frequency range of 0.05-1 THz. The detector used in this work consists of a gate-defined quantum-dot photon-to-charge transducer coupled to a single-electron transistor. Both the emitter and the detector are equipped with a matching on-chip wide-band antenna. The combination of a tuneable emitter and detector allows us to determine the efficacy of the YBCO emitter and also to analyze the elementary processes involved in the detection.

  3. Heralded Quantum Entanglement between Distant Matter Qubits

    PubMed Central

    Yang, Wen-Juan; Wang, Xiang-Bin

    2015-01-01

    We propose a scheme to realize heralded quantum entanglement between two distant matter qubits using two Λ atom systems. Our proposal does not need any photon interference. We also present a general theory of outcome state of non-monochromatic incident light and finite interaction time. PMID:26041259

  4. Gaussianity and localization of N -qubit states

    NASA Astrophysics Data System (ADS)

    Gaeta, M.; Muñoz, C.; Klimov, A. B.

    2016-06-01

    We analyze collective properties of N -qubit states. In particular, we exhaustively discuss the localization aspect of distributions in the measurement space and introduce the concept of Gaussian states in the macroscopic limit. The effect of local shifts on the localization and Gaussianity is analyzed.

  5. Optimal Compression for Identically Prepared Qubit States.

    PubMed

    Yang, Yuxiang; Chiribella, Giulio; Hayashi, Masahito

    2016-08-26

    We establish the ultimate limits to the compression of sequences of identically prepared qubits. The limits are determined by Holevo's information quantity and are attained through use of the optimal universal cloning machine, which finds here a novel application to quantum Shannon theory. PMID:27610836

  6. Fabrication of Niobium Nanobridge Josephson Junctions

    NASA Astrophysics Data System (ADS)

    Tachiki, T.; Horiguchi, K.; Uchida, T.

    2014-05-01

    To realize antenna-coupled Josephson detectors for microwave and millimeter-wave radiation, planar-type Nb nanobridge Josephson junctions were fabricated. Nb thin films whose thickness, the root mean square roughness and the critical temperature were 20.0 nm, 0.109 nm and 8.4 K, respectively were deposited using a DC magnetron sputtering at a substrate temperature of 700°C. Nanobridges were obtained from the film using 80-kV electron beam lithography and reactive ion-beam etching in CF4 (90%) + O2 (10%) gases. The minimum bridge area was 65 nm wide and 60 nm long. For the nanobridge whose width and length were less than 110 nm, an I-V characteristic showed resistively-shunted-junction behaviour near the critical temperature. Moreover, Shapiro steps were observed in the nanobridge with microwave irradiation at a frequency of 6 - 30 GHz. The Nb nanobridges can be used as detectors in the antenna-coupled devices.

  7. Josephson current in parallel SFS junctions

    NASA Astrophysics Data System (ADS)

    Ioselevich, Pavel; Ostrovsky, Pavel; Fominov, Yakov; Feigelman, Mikhail

    We study a Josephson junction between superconductors connected by two parallel ferromagnetic arms. If the ferromagnets are fully polarised, supercurrent can only flow via Cooper pair splitting between the differently polarised arms. The disorder-average current is suppressed, but mesoscopic fluctuations lead to a significant typical current. We extract the typical current from a current-current correlator. The current is proportional to sin2 α / 2 , where α is the angle between the polarisations of the two arms, revealing the spin dependence of crossed Andreev reflection. Compared to an SNS device of the same geometry, the typical SFS current is small by a factor determined by the properties of the superconducting leads alone. The current is insensitive to the flux threading the area between the ferromagnetic arms of the junction. However, if the ferromagnetic arms are replaced by metal with magnetic impurities, or partially polarised ferromagnets, the Josephson current starts depending on the flux with a period of h / e , i.e. twice the superconducting flux quantum.

  8. Nanoscale phase engineering of thermal transport with a Josephson heat modulator.

    PubMed

    Fornieri, Antonio; Blanc, Christophe; Bosisio, Riccardo; D'Ambrosio, Sophie; Giazotto, Francesco

    2016-03-01

    Macroscopic quantum phase coherence has one of its pivotal expressions in the Josephson effect, which manifests itself both in charge and energy transport. The ability to master the amount of heat transferred through two tunnel-coupled superconductors by tuning their phase difference is the core of coherent caloritronics, and is expected to be a key tool in a number of nanoscience fields, including solid-state cooling, thermal isolation, radiation detection, quantum information and thermal logic. Here, we show the realization of the first balanced Josephson heat modulator designed to offer full control at the nanoscale over the phase-coherent component of thermal currents. Our device provides magnetic-flux-dependent temperature modulations up to 40 mK in amplitude with a maximum of the flux-to-temperature transfer coefficient reaching 200 mK per flux quantum at a bath temperature of 25 mK. Foremost, it demonstrates the exact correspondence in the phase engineering of charge and heat currents, breaking ground for advanced caloritronic nanodevices such as thermal splitters, heat pumps and time-dependent electronic engines. PMID:26641530

  9. Nanoscale phase engineering of thermal transport with a Josephson heat modulator

    NASA Astrophysics Data System (ADS)

    Fornieri, Antonio; Blanc, Christophe; Bosisio, Riccardo; D'Ambrosio, Sophie; Giazotto, Francesco

    2016-03-01

    Macroscopic quantum phase coherence has one of its pivotal expressions in the Josephson effect, which manifests itself both in charge and energy transport. The ability to master the amount of heat transferred through two tunnel-coupled superconductors by tuning their phase difference is the core of coherent caloritronics, and is expected to be a key tool in a number of nanoscience fields, including solid-state cooling, thermal isolation, radiation detection, quantum information and thermal logic. Here, we show the realization of the first balanced Josephson heat modulator designed to offer full control at the nanoscale over the phase-coherent component of thermal currents. Our device provides magnetic-flux-dependent temperature modulations up to 40 mK in amplitude with a maximum of the flux-to-temperature transfer coefficient reaching 200 mK per flux quantum at a bath temperature of 25 mK. Foremost, it demonstrates the exact correspondence in the phase engineering of charge and heat currents, breaking ground for advanced caloritronic nanodevices such as thermal splitters, heat pumps and time-dependent electronic engines.

  10. Interplay between Coulomb blockade and Josephson effect in a topological superconductor-quantum dot device

    NASA Astrophysics Data System (ADS)

    Lee, Yu-Li; Lee, Yu-Wen

    2016-05-01

    We study the behavior of a topological Josephson junction in which two topological superconductors are coupled through a quantum dot. We focus on the case with the bulk superconducting gap being the largest energy scale. Two parameter regimes are investigated: a weak tunneling between the dot and the superconductors, with the dot near its charge degeneracy point, and a strong-tunneling regime in which the transmission between the dot and the superconductors is nearly perfect. We show that in the former situation, the Andreev spectrum for each sector with fixed fermion parity consists of only two levels, which gives rise to the nontrivial current-phase relation. Moreover, we study the Rabi oscillation between the two levels and indicate that the corresponding frequency is a 4 π -periodic function of the phase difference between the two superconductors, which is immune to the quasiparticle poisoning. In the latter case, we find that the Coulomb charging energy enhances the effect of backscattering at the interfaces between the dot and the superconductors. Both the temperature and the gate-voltage dependence of the critical Josephson current are examined.

  11. Multiphoton Coherent Manipulation in Large Spin Qubits

    NASA Astrophysics Data System (ADS)

    Chiorescu, Irinel

    2009-03-01

    Manipulation of quantum information allows certain algorithms to be performed at unparalleled speeds. Photons are an ideal choice to manipulate qubits as they interact with quantum systems in predictable ways. They are a versatile tool for manipulating, reading/coupling qubits and for encoding/transferring quantum information over long distances. Spin-based qubits have well known behavior under photon driving and can be potentially operated up to room temperature. When diluted enough to avoid uncontrolled spin-spin interactions, a variety of spin qubits show long coherence times, e.g. the nitrogen vacancies in pure diamonds (1,2), nitrogen atoms trapped in a C60 cage (3), Ho3+ and Cr5+ ions (4,5) and molecular magnets (6,7). We have used large spin Mn2+ ions (S=5/2) to realize a six level system that can be operated by means of single as well as multi-photon coherent Rabi oscillations (8). This spin system has a very small anisotropy whose effect can be tuned in-situ to turn the system into a multi-level harmonic system. This offer new ways of manipulating, reading and resetting a spin qubit. Decoherence effects are strongly reduced by the quasi-isotropic electron interaction with the crystal field and with the 55Mn nuclear spins. [0pt] 1. R. Hanson et al., Science 320, 352 (2008). [0pt] 2. M.V. Gurudev Dutt et al., Science 316, 1312 (2007). [0pt] 3. G.W. Morley et al., Phys. Rev. Lett. 98, 220501 (2007). [0pt] 4. S. Bertaina et al., Nat. Nanotech. 2, 39 (2007). [0pt] 5. S. Nellutla et al., Phys. Rev. Lett. 99, 137601 (2007). [0pt] 6. A. Ardavan et al., Phys. Rev. Lett. 98, 057201 (2007). [0pt] 7. S. Bertaina et al., Nature 453, 203,(2008). [0pt] 8. S. Bertaina et al., submitted.

  12. Two different types of optical hybrid qubits for teleportation in a lossy environment

    NASA Astrophysics Data System (ADS)

    Kim, Hoyong; Lee, Seung-Woo; Jeong, Hyunseok

    2016-08-01

    We investigate the performance of quantum teleportation under a lossy environment using two different types of optical hybrid qubits. One is the hybrid of a polarized single-photon qubit and a coherent-state qubit (type-I logical qubit), and the other is the hybrid of a qubit of the vacuum and the single-photon and a coherent-state qubit (type-II logical qubit). We show that type-II hybrid qubits are generally more robust to photon loss effects compared to type-I hybrid qubits with respect to fidelities and success probabilities of quantum teleportation.

  13. Self-consistent solution for proximity effect and Josephson current in ballistic graphene SNS Josephson junctions

    SciTech Connect

    Black-Schaffer, Annica M.

    2010-04-06

    We use a tight-binding Bogoliubov-de Gennes (BdG) formalism to self-consistently calculate the proximity effect, Josephson current, and local density of states in ballistic graphene SNS Josephson junctions. Both short and long junctions, with respect to the superconducting coherence length, are considered, as well as different doping levels of the graphene. We show that self-consistency does not notably change the current-phase relationship derived earlier for short junctions using the non-selfconsistent Dirac-BdG formalism but predict a significantly increased critical current with a stronger junction length dependence. In addition, we show that in junctions with no Fermi level mismatch between the N and S regions superconductivity persists even in the longest junctions we can investigate, indicating a diverging Ginzburg-Landau superconducting coherence length in the normal region.

  14. Fluxons in a triangular set of coupled long Josephson junctions

    NASA Astrophysics Data System (ADS)

    Yukon, Stanford P.; Malomed, Boris A.

    2015-09-01

    We report results of an analysis of the dynamics of magnetic flux solitons in the system of three long Josephson junctions between three bulk superconductors that form a prism. The system is modeled by coupled sine-Gordon equations for the phases of the junctions. The Aharonov-Bohm constraint takes into account the axial magnetic flux enclosed by the prism and reduces the system from three independent phases to two. The equations of motion for the phases include dissipative terms, and a control parameter δ which accounts for the deviation of the enclosed flux from half a quantum. Analyzing the effective potential of the coupled equations, we identify different species of topological and non-topological phase solitons (fluxons) in this system. In particular, subkinks with fractional topological charges ±1/3 and ±2/3, confined inside integer-charge fluxons, may be mapped onto the root diagrams for mesons and baryons in the original quark model of hadrons. Solutions for straight-line kinks and for two types of non-topological solitons are obtained in an explicit analytical form. Numerical tests demonstrate that the former species is unstable against breakup into pairs of separating single-fluxon kinks. The non-topological kinks feature metastability, eventually breaking up into fluxon-antifluxon pairs. Free fractional-fluxon kinks, that connect different potential minima and are, accordingly, pulled by the potential difference, are also considered. Using the momentum-balance method, we predict the velocity at which these kinks should move in the presence of the dissipation. Numerical tests demonstrate that the analysis predicts the velocity quite closely. Higher-energy static solutions for all of the stable kink types mentioned above, as well as kinks connecting false vacua, are found by means of the shooting method. Inelastic collisions among the stable fractional and single-fluxon kinks are investigated numerically.

  15. Fluxons in a triangular set of coupled long Josephson junctions

    SciTech Connect

    Yukon, Stanford P.; Malomed, Boris A.

    2015-09-15

    We report results of an analysis of the dynamics of magnetic flux solitons in the system of three long Josephson junctions between three bulk superconductors that form a prism. The system is modeled by coupled sine-Gordon equations for the phases of the junctions. The Aharonov-Bohm constraint takes into account the axial magnetic flux enclosed by the prism and reduces the system from three independent phases to two. The equations of motion for the phases include dissipative terms, and a control parameter δ which accounts for the deviation of the enclosed flux from half a quantum. Analyzing the effective potential of the coupled equations, we identify different species of topological and non-topological phase solitons (fluxons) in this system. In particular, subkinks with fractional topological charges ±1/3 and ±2/3, confined inside integer-charge fluxons, may be mapped onto the root diagrams for mesons and baryons in the original quark model of hadrons. Solutions for straight-line kinks and for two types of non-topological solitons are obtained in an explicit analytical form. Numerical tests demonstrate that the former species is unstable against breakup into pairs of separating single-fluxon kinks. The non-topological kinks feature metastability, eventually breaking up into fluxon-antifluxon pairs. Free fractional-fluxon kinks, that connect different potential minima and are, accordingly, pulled by the potential difference, are also considered. Using the momentum-balance method, we predict the velocity at which these kinks should move in the presence of the dissipation. Numerical tests demonstrate that the analysis predicts the velocity quite closely. Higher-energy static solutions for all of the stable kink types mentioned above, as well as kinks connecting false vacua, are found by means of the shooting method. Inelastic collisions among the stable fractional and single-fluxon kinks are investigated numerically.

  16. Bidirectional Quantum Controlled Teleportation via a Maximally Seven-qubit Entangled State

    NASA Astrophysics Data System (ADS)

    Duan, Ya-Jun; Zha, Xin-Wei; Sun, Xin-Mei; Xia, Jia-Fan

    2014-08-01

    A bidirectional quantum controlled teleportation scheme using a seven-qubit maximally entangled state as quantum channel is proposed. This means that Alice can transmit an arbitrary single qubit state of qubit a to Bob and Bob can transmit an arbitrary single qubit state of qubit b to Alice via the control of the supervisor Charlie.

  17. Bidirectional Controlled Quantum Teleportation by Using Five-Qubit Entangled State

    NASA Astrophysics Data System (ADS)

    Chen, Yan

    2014-05-01

    We propose a scheme for bidirectional controlled quantum teleportation by using a genuine five-qubit entangled state. In our scheme, Alice may transmit an arbitrary single qubit state of qubit A to Bob and at the same time, Bob may transmit an arbitrary single qubit state of qubit B to Alice via the control of the supervisor Charlie.

  18. Giant magnetic effects and oscillations in antiferromagnetic Josephson weak links

    SciTech Connect

    Gorkov, L.; Kresin, Vladimir

    2001-04-01

    Josephson junctions with an antiferromagnetic metal as a link are described. The junction can be switched off by a relatively small magnetic field. The amplitude of the current oscillates as a function of the field.

  19. Higher-order nonlinear effects in a Josephson parametric amplifier

    NASA Astrophysics Data System (ADS)

    Kochetov, Bogdan A.; Fedorov, Arkady

    2015-12-01

    Nonlinearity of the current-phase relationship of a Josephson junction is the key resource for a Josephson parametric amplifier (JPA) as well as for a Josephson traveling-wave parametric amplifier, the only devices in which the quantum limit for added noise has so far been approached at microwave frequencies. A standard approach to describe JPA takes into account only the lowest order (cubic) nonlinearity resulting in a Duffing-like oscillator equation of motion or in a Kerr-type nonlinearity term in the Hamiltonian. In this paper we derive the quantum expression for the gain of JPA including all orders of the Josephson junction nonlinearity in the linear response regime. We then analyze gain saturation effect for stronger signals within a semiclassical approach. Our results reveal nonlinear effects of higher orders and their implications for operation of a JPA.

  20. Josephson effects in a Bose–Einstein condensate of magnons

    SciTech Connect

    Troncoso, Roberto E.; Núñez, Álvaro S.

    2014-07-15

    A phenomenological theory is developed, that accounts for the collective dynamics of a Bose–Einstein condensate of magnons. In terms of such description we discuss the nature of spontaneous macroscopic interference between magnon clouds, highlighting the close relation between such effects and the well known Josephson effects. Using those ideas, we present a detailed calculation of the Josephson oscillations between two magnon clouds, spatially separated in a magnonic Josephson junction. -- Highlights: •We presented a theory that accounts for the collective dynamics of a magnon-BEC. •We discuss the nature of macroscopic interference between magnon-BEC clouds. •We remarked the close relation between the above phenomena and Josephson’s effect. •We remark the distinctive oscillations that characterize the Josephson oscillations.

  1. Josephson junction spectrum analyzer for millimeter and submillimeter wavelengths

    SciTech Connect

    Larkin, S.Y.; Anischenko, S.E.; Khabayev, P.V.

    1994-12-31

    A prototype of the Josephson-effect spectrum analyzer developed for the millimeter-wave band is described. The measurement results for spectra obtained in the frequency band from 50 to 250 GHz are presented.

  2. 1D Josephson quantum interference grids: diffraction patterns and dynamics

    NASA Astrophysics Data System (ADS)

    Lucci, M.; Badoni, D.; Corato, V.; Merlo, V.; Ottaviani, I.; Salina, G.; Cirillo, M.; Ustinov, A. V.; Winkler, D.

    2016-02-01

    We investigate the magnetic response of transmission lines with embedded Josephson junctions and thus generating a 1D underdamped array. The measured multi-junction interference patterns are compared with the theoretical predictions for Josephson supercurrent modulations when an external magnetic field couples both to the inter-junction loops and to the junctions themselves. The results provide a striking example of the analogy between Josephson phase modulation and 1D optical diffraction grid. The Fiske resonances in the current-voltage characteristics with voltage spacing {Φ0}≤ft(\\frac{{\\bar{c}}}{2L}\\right) , where L is the total physical length of the array, {Φ0} the magnetic flux quantum and \\bar{c} the speed of light in the transmission line, demonstrate that the discrete line supports stable dynamic patterns generated by the ac Josephson effect interacting with the cavity modes of the line.

  3. Bosonic Josephson effect in the Fano-Anderson model

    NASA Astrophysics Data System (ADS)

    Engelhardt, G.; Schaller, G.; Brandes, T.

    2016-07-01

    We investigate the coherent dynamics of a noninteracting Bose-Einstein condensate in a system consisting of two bosonic reservoirs coupled via a spatially localized mode. We describe this system by a two-terminal Fano-Anderson model and investigate analytically the time evolution of observables such as the Josephson current. In doing so, we find that the Josephson current sensitively depends on the on-site energy of the localized mode. This facilitates using this setup as a transistor for a Bose-Einstein condensate. We identify two regimes. In one regime, the system exhibits well-behaved long-time dynamics with a slowly oscillating and undamped Josephson current. In a second regime, the Josephson current is a superposition of an extremely weakly damped slow oscillation and an undamped fast oscillation. Our results are confirmed by finite-size simulations.

  4. Josephson Junction spectrum analyzer for millimeter and submillimeter wavelengths

    NASA Technical Reports Server (NTRS)

    Larkin, S. Y.; Anischenko, S. E.; Khabayev, P. V.

    1995-01-01

    A prototype of the Josephson-effect spectrum analyzer developed for the millimeter wave band is described. The measurement results for spectra obtained in the frequency band from 50 to 250 GHz are presented.

  5. Sign reversal of ac Josephson current in a ferromagnetic Josephson junction

    NASA Astrophysics Data System (ADS)

    Hikino, Shin-Ichi; Mori, Michiyasu; Takahashi, Saburo; Maekawa, Sadamichi

    2009-03-01

    It is known that in a superconductor/insulator/superconductor (SIS) junction, when a finite voltage is applied, the Josephson current shows a logarithmic divergence, i.e., the so-called Riedel peak(RP) at the gap voltage, V=2δ/e, (δ is a superconducting gap). In a double barrier Josephson junction such as SXS junction, on the other hand, the voltage dependence of Ic has not been investigated so far, where X is a normal metal (N) or a ferromagnet (F). We study the voltage dependence of Josephson critical current (Ic) in a variety of SXS junctions. In a SNS junction, Ic shows the RP at the gap voltage similar to a SIS junction. On the other hand, in a SFS junction, Ic shows a damped oscillation with the alternation of sign as a function of thickness (d) of F due to 0-π transition. The RP exhibits a strong dependence on d, and changes its sign. It is predicted that the RP disappears at the 0-π transition in the SFS junction.

  6. On the electrodynamics of Josephson effect in anisotropic superconductors

    SciTech Connect

    Mints, R.G.

    1989-01-01

    Specificities of Josephson effect electrodynamics in anisotropic superconductors are of considerable interest for the study of high temperature superconductors with strongly anisotropic layered structure. In this paper the authors give the calculation for the tunnel Josephson contact of an isolated vortex, the law of dispersion of its low-amplitude oscillations, the critical field H/sub cl/ for the penetration of magnetic flux, and the maximum current across a rectangular contact.

  7. Josephson effect and quasiparticle states in d-wave superconductors

    SciTech Connect

    Tanaka, Yukio; Kashiwaya, Satoshi

    1996-12-31

    A general formula for the Josephson current in a d-wave/insulator/d-wave-superconductor junction is presented by taking account of the zero-energy states formed around the interfaces. For a fixed phase difference between the two superconductors, the current component becomes either positive or negative depending on the injection angle of the quasiparticle. Anomalous temperature dependences are predicted in the maximum Josephson current and in the free energy minima.

  8. Sensitivity of Josephson-effect millimeter-wave radiometer

    NASA Technical Reports Server (NTRS)

    Ohta, H.; Feldman, M. J.; Parrish, P. T.; Chiao, R. Y.

    1974-01-01

    The noise temperature and the minimum detectable temperature of a Josephson junction in video detection of microwave and millimeter-wave radiation has been calculated. We use the well-known method based on a Fokker-Planck equation. The noise temperature can be very close to ambient temperature. Because its predetection bandwidth is very wide, a Josephson-effect radio telescope receiver can have a minimum detectable temperature better than that of a traveling-wave maser.

  9. Ballistic transport in InSb Josephson junctions

    NASA Astrophysics Data System (ADS)

    Damasco, John Jeffrey; Gill, Stephen; Car, Diana; Bakkers, Erik; Mason, Nadya

    We present transport measurements on Josephson junctions consisting of InSb nanowires contacted by Al at various junction lengths. Junction behavior as a function of gate voltage, electric field, and magnetic field is discussed. We show that short junctions behave as 1D quantum wires, exhibiting quantized conductance steps. In addition, we show how Josephson behavior changes as transport evolves from ballistic to diffusive as a function of contact spacing.

  10. Topological phase transition of a Josephson junction and its dynamics

    NASA Astrophysics Data System (ADS)

    Hutasoit, Jimmy; Marciani, Marco; Tarasinski, Brian; Beenakker, Carlo

    A Josephson junction formed by a superconducting ring interrupted by a semiconductor nanowire can realize a zero-dimensional class D topological superconductor. By coupling the Josephson junction to a ballistic wire and altering the strength of the coupling, one can drive this topological superconductor through a topological phase transition. We study the compressibility of the junction as a probe of the topological phase transition. We also study the dynamics of the phase transition by studying the current pulse injected into the wire.

  11. Suppression of dephasing by qubit motion in superconducting circuits

    NASA Astrophysics Data System (ADS)

    Averin, D. V.; Hu, K.; Zhong, Y. P.; Song, C.; Wang, H.; Han, S.

    We suggest and demonstrate a protocol which suppresses dephasing due to the low-frequency noise by qubit motion, i.e., transfer of the logical qubit of information in a system of n >= 2 physical qubits. The protocol requires only the nearest-neighbor coupling and is applicable to different qubit structures. Motion of a logical qubit limits the correlation time of the effective noise seen by this qubit and suppresses its decoherence rate. This effect is qualitatively similar to the dynamic decoupling, but relies on the different resource: additional physical qubits, not extra control pulses. In this respect, suggested protocol can serve as the basis for an alternative approach to scalable quantum circuits. We further analyze its effectiveness against noises with arbitrary correlations. Our analysis, together with experiments using up to three superconducting qubits, shows that for the realistic uncorrelated noises, qubit motion increases the dephasing time of the logical qubit as √{ n}. In general, the protocol provides a diagnostic tool for measurements of the noise correlations. This work was supported by the National Basic Research Program of China (2014CB921200, 2012CB927404), US NSF Grants PHY-1314758 and PHY-1314861, the National Natural Science Foundation of China, and Zhejiang Provincial Natural Science Foundation.

  12. Extracting entangled qubits from Majorana fermions in quantum dot chains through the measurement of parity.

    PubMed

    Dai, Li; Kuo, Watson; Chung, Ming-Chiang

    2015-01-01

    We propose a scheme for extracting entangled charge qubits from quantum-dot chains that support zero-energy edge modes. The edge mode is composed of Majorana fermions localized at the ends of each chain. The qubit, logically encoded in double quantum dots, can be manipulated through tunneling and pairing interactions between them. The detailed form of the entangled state depends on both the parity measurement (an even or odd number) of the boundary-site electrons in each chain and the teleportation between the chains. The parity measurement is realized through the dispersive coupling of coherent-state microwave photons to the boundary sites, while the teleportation is performed via Bell measurements. Our scheme illustrates localizable entanglement in a fermionic system, which serves feasibly as a quantum repeater under realistic experimental conditions, as it allows for finite temperature effect and is robust against disorders, decoherence and quasi-particle poisoning. PMID:26062033

  13. Quantum waveguide theory of the Josephson effect in multiband superconductors

    NASA Astrophysics Data System (ADS)

    Nappi, C.; Romeo, F.; Sarnelli, E.; Citro, R.

    2015-12-01

    We formulate a quantum waveguide theory of the Josephson effect in multiband superconductors, with special emphasis on iron-based materials. By generalizing the boundary conditions of the scattering problem, we first determine the Andreev levels spectrum and then derive an explicit expression for the Josephson current which generalizes the formula of the single-band case. In deriving the results, we provide a second quantization field theory, allowing us to evaluate the current-phase relation and the Josephson current fluctuations in multiband systems. We present results for two different order parameter symmetries, namely s± and s++, which are relevant in multiband systems. The obtained results show that the s± symmetry can support π states which are absent in the s++ case. We also argue that there is a certain fragility of the Josephson current against phase fluctuations in the s++ case. The temperature dependence of the Josephson critical current is also analyzed and we find, for both the order parameter symmetries, remarkable violations of the Ambegaokar-Baratoff relation. The results are relevant in view of possible experiments aimed at investigating the order parameter symmetry of multiband superconductors using mesoscopic Josephson junctions.

  14. Josephson currents and spin-transfer torques in ballistic SFSFS nanojunctions

    NASA Astrophysics Data System (ADS)

    Halterman, Klaus; Alidoust, Mohammad

    2016-05-01

    Utilizing a full microscopic Bogoliubov–de Gennes (BdG) approach, we study the equilibrium charge and spin currents in ballistic SFSFS Josephson systems, where F is a uniformly magnetized ferromagnet and S is a conventional s-wave superconductor. From the spatially varying spin currents, we also calculate the associated equilibrium spin-transfer torques. Through variations in the relative phase differences between the three S regions, and magnetization orientations of the ferromagnets, our study demonstrates tunability and controllability of the spin and charge supercurrents. The spin-transfer torques are shown to reveal details of the proximity effects that play a crucial role in these types of hybrid system. The proposed SFSFS nanostructure is discussed within the context of a superconducting magnetic torque transistor.

  15. Quantum heat transport of a two-qubit system: Interplay between system-bath coherence and qubit-qubit coherence

    SciTech Connect

    Kato, Akihito Tanimura, Yoshitaka

    2015-08-14

    We consider a system consisting of two interacting qubits that are individually coupled to separate heat baths at different temperatures. The quantum effects in heat transport are investigated in a numerically rigorous manner with a hierarchial equations of motion (HEOM) approach for non-perturbative and non-Markovian system-bath coupling cases under non-equilibrium steady-state conditions. For a weak interqubit interaction, the total system is regarded as two individually thermostatted systems, whereas for a strong interqubit interaction, the two-qubit system is regarded as a single system coupled to two baths. The roles of quantum coherence (or entanglement) between the two qubits (q-q coherence) and between the qubit and bath (q-b coherence) are studied through the heat current calculated for various strengths of the system-bath coupling and interqubit coupling for high and low temperatures. The same current is also studied using the time convolutionless (TCL) Redfield equation and using an expression derived from the Fermi golden rule (FGR). We find that the HEOM results exhibit turnover behavior of the heat current as a function of the system-bath coupling strength for all values of the interqubit coupling strength, while the results obtained with the TCL and FGR approaches do not exhibit such behavior, because they do not possess the capability of treating the q-b and q-q coherences. The maximum current is obtained in the case that the q-q coherence and q-b coherence are balanced in such a manner that coherence of the entire heat transport process is realized. We also find that the heat current does not follow Fourier’s law when the temperature difference is very large, due to the non-perturbative system-bath interactions.

  16. Charge of a quasiparticle in a superconductor

    PubMed Central

    Ronen, Yuval; Cohen, Yonatan; Kang, Jung-Hyun; Haim, Arbel; Rieder, Maria-Theresa; Heiblum, Moty; Mahalu, Diana; Shtrikman, Hadas

    2016-01-01

    Nonlinear charge transport in superconductor–insulator–superconductor (SIS) Josephson junctions has a unique signature in the shuttled charge quantum between the two superconductors. In the zero-bias limit Cooper pairs, each with twice the electron charge, carry the Josephson current. An applied bias VSD leads to multiple Andreev reflections (MAR), which in the limit of weak tunneling probability should lead to integer multiples of the electron charge ne traversing the junction, with n integer larger than 2Δ/eVSD and Δ the superconducting order parameter. Exceptionally, just above the gap eVSD ≥ 2Δ, with Andreev reflections suppressed, one would expect the current to be carried by partitioned quasiparticles, each with energy-dependent charge, being a superposition of an electron and a hole. Using shot-noise measurements in an SIS junction induced in an InAs nanowire (with noise proportional to the partitioned charge), we first observed quantization of the partitioned charge q = e*/e=n, with n = 1–4, thus reaffirming the validity of our charge interpretation. Concentrating next on the bias region eVSD∼2Δ, we found a reproducible and clear dip in the extracted charge to q ∼0.6, which, after excluding other possibilities, we attribute to the partitioned quasiparticle charge. Such dip is supported by numerical simulations of our SIS structure. PMID:26831071

  17. Charge of a quasiparticle in a superconductor.

    PubMed

    Ronen, Yuval; Cohen, Yonatan; Kang, Jung-Hyun; Haim, Arbel; Rieder, Maria-Theresa; Heiblum, Moty; Mahalu, Diana; Shtrikman, Hadas

    2016-02-16

    Nonlinear charge transport in superconductor-insulator-superconductor (SIS) Josephson junctions has a unique signature in the shuttled charge quantum between the two superconductors. In the zero-bias limit Cooper pairs, each with twice the electron charge, carry the Josephson current. An applied bias VSD leads to multiple Andreev reflections (MAR), which in the limit of weak tunneling probability should lead to integer multiples of the electron charge ne traversing the junction, with n integer larger than 2Δ/eVSD and Δ the superconducting order parameter. Exceptionally, just above the gap eVSD ≥ 2Δ, with Andreev reflections suppressed, one would expect the current to be carried by partitioned quasiparticles, each with energy-dependent charge, being a superposition of an electron and a hole. Using shot-noise measurements in an SIS junction induced in an InAs nanowire (with noise proportional to the partitioned charge), we first observed quantization of the partitioned charge q = e*/e = n, with n = 1-4, thus reaffirming the validity of our charge interpretation. Concentrating next on the bias region eVSD ~ 2Δ, we found a reproducible and clear dip in the extracted charge to q ~ 0.6, which, after excluding other possibilities, we attribute to the partitioned quasiparticle charge. Such dip is supported by numerical simulations of our SIS structure. PMID:26831071

  18. High dynamic range Josephson parametric amplifiers

    NASA Astrophysics Data System (ADS)

    Roch, Nicolas; Murch, Kater W.; Vijay, Rajamani

    Josephson parametric amplifiers (JPAs) have become the technology of choice to amplify small amplitude microwave signals since they show noise performances close to the quantum limit of amplification. An important challenge that faces this technology is the low dynamic range of current devices, which limits the number of measurements that can be performed concurrently and the rate of information acquisition for single measurements. We have fabricated and tested novel parametric amplifiers based on arrays of up to 100 SQUIDS. The amplifiers produce gain in excess of 20 dB over a large bandwidth and match the dynamic range achieved with traveling wave devices. Compared to the latter devices they are fabricated in a single lithography step and we will show that their bandwidth performance can be further extended using a recently developed impedance matching technique.

  19. AC Josephson effect applications in microwave systems

    NASA Astrophysics Data System (ADS)

    Larkin, Serguey Y.

    1996-12-01

    analysis allow to get the picture of temperature distribution along the plasma cord diameter in accordance with dynamics of thermonuclear process development. Modem raclioastronomic research gives scientists the unique information on the world tructure. It is also necessary to analyze Space microwave radiation providing exclusive sensitivity of the equipment. In both cases equipment is required to be superwide band, to have high sensitivity and ability to operate at more than 300 GHz frequencies. Today all these requirements are met by the devices using the ac Josephson effect. The Josephson junctions are used as an active transforming element in such devices. At the end of 20 century the sphere of their utilization embraces medicine, communications, radiophysics, space exploration, ecology, military use, etc. The State Research Center "Fonon" ( SRC "Fonon") of the State Committee on Science and Technology of Ukraine was founded in 1991. The main aim of its creation was to concentrate the scientific and financial efforts for development and production of unique devices based on the results of fundamental study in physics of high T superconductivity. First of all we were interested in technological research on the obtaining of low impedance Josephson junctions out of the High T thin films. Using such junctions in combination with our original techniques developed in our Center we have succeed in creating the following new generation equipment: industrial set-up of the frequency meter in the range of 60 ... 600 GHz; experimental set-up of the spectrum analyzer operating in the range of 50 250 GHz; experimental model of radiometric receiver in 180...260 GHz range. All the above devices are based on the using ac Josephson effect for the receiving and processing mm- and submm- microwave signals.

  20. Synchronized Switching in a Josephson Junction Crystal

    NASA Astrophysics Data System (ADS)

    Leib, Martin; Hartmann, Michael J.

    2014-06-01

    We consider a superconducting coplanar waveguide resonator where the central conductor is interrupted by a series of uniformly spaced Josephson junctions. The device forms an extended medium that is optically nonlinear on the single photon level with normal modes that inherit the full nonlinearity of the junctions but are nonetheless accessible via the resonator ports. For specific plasma frequencies of the junctions, a set of normal modes clusters in a narrow band and eventually becomes entirely degenerate. Upon increasing the intensity of a red detuned drive on these modes, we observe a sharp and synchronized switching from low-occupation quantum states to high-occupation classical fields, accompanied by a pronounced jump from low to high output intensity.

  1. Josephson vortex lattice in layered superconductors

    SciTech Connect

    Koshelev, A. E.; Dodgson, M. J. W.

    2013-09-15

    Many superconducting materials are composed of weakly coupled conducting layers. Such a layered structure has a very strong influence on the properties of vortex matter in a magnetic field. This review focuses on the properties of the Josephson vortex lattice generated by the magnetic field applied in the direction of the layers. The theoretical description is based on the Lawrence-Doniach model in the London limit, which takes only the phase degree of freedom of the superconducting order parameter into account. In spite of its simplicity, this model leads to an amazingly rich set of phenomena. We review in detail the structure of an isolated vortex line and various properties of the vortex lattice, in both dilute and dense limits. In particular, we extensively discuss the influence of the layered structure and thermal fluctuations on the selection of lattice configurations at different magnetic fields.

  2. Dissipation and traversal time in Josephson junctions

    SciTech Connect

    Cacciari, Ilaria; Ranfagni, Anedio; Moretti, Paolo

    2010-05-01

    The various ways of evaluating dissipative effects in macroscopic quantum tunneling are re-examined. The results obtained by using functional integration, while confirming those of previously given treatments, enable a comparison with available experimental results relative to Josephson junctions. A criterion based on the shortening of the semiclassical traversal time tau of the barrier with regard to dissipation can be established, according to which DELTAtau/tau > or approx. N/Q, where Q is the quality factor of the junction and N is a numerical constant of order unity. The best agreement with the experiments is obtained for N=1.11, as it results from a semiempirical analysis based on an increase in the potential barrier caused by dissipative effects.

  3. Synchronized switching in a josephson junction crystal.

    PubMed

    Leib, Martin; Hartmann, Michael J

    2014-06-01

    We consider a superconducting coplanar waveguide resonator where the central conductor is interrupted by a series of uniformly spaced Josephson junctions. The device forms an extended medium that is optically nonlinear on the single photon level with normal modes that inherit the full nonlinearity of the junctions but are nonetheless accessible via the resonator ports. For specific plasma frequencies of the junctions, a set of normal modes clusters in a narrow band and eventually becomes entirely degenerate. Upon increasing the intensity of a red detuned drive on these modes, we observe a sharp and synchronized switching from low-occupation quantum states to high-occupation classical fields, accompanied by a pronounced jump from low to high output intensity. PMID:24949766

  4. Characterization of single- and two-qubit gates in a 2D neutral atom qubit array

    NASA Astrophysics Data System (ADS)

    Xia, Tian; Maller, Kara; Lichtman, Martin; Piotrowicz, Michal; Carr, Alex; Isenhower, Larry; Saffman, Mark

    2015-05-01

    We have developed a 2D array of optically trapped single atom qubits for quantum computation experiments. We characterize single qubit Clifford gate operations with randomized benchmarking achieving global and site selected gates with fidelities close to fault tolerance thresholds for quantum computation. An average fidelity of 0.9983, limited by the qubit T2 coherence time, is measured for global microwave driven gates applied to a 49 qubit array. Single site gates are implemented with a focused laser beam to Stark shift the microwaves into resonance at a selected site. At Stark selected single sites we observe fidelities of 0.9923 and an average spin flip crosstalk error at other sites of 0.002. A two-qubit Rydberg blockade interaction provides a CNOT gate which is used to create entangled Bell pairs. The fidelity is characterized with parity oscillation measurements. The influence of two-photon Stark shifts on the gate matrix and fidelity is studied. We show how to select excitation parameters to suppress the ground-Rydberg differential Stark shift. Work supported by the IARPA MQCO program and ARO.

  5. Final Report on DE-FG02-04ER46107: Glasses, Noise and Phase Transitions

    SciTech Connect

    Yu, Clare C.

    2011-12-31

    We showed that noise has distinct signatures at phase transitions in spin systems. We also studied charge noise, critical current noise, and flux noise in superconducting qubits and Josephson junctions.

  6. Experimental Quantum Randomness Processing Using Superconducting Qubits.

    PubMed

    Yuan, Xiao; Liu, Ke; Xu, Yuan; Wang, Weiting; Ma, Yuwei; Zhang, Fang; Yan, Zhaopeng; Vijay, R; Sun, Luyan; Ma, Xiongfeng

    2016-07-01

    Coherently manipulating multipartite quantum correlations leads to remarkable advantages in quantum information processing. A fundamental question is whether such quantum advantages persist only by exploiting multipartite correlations, such as entanglement. Recently, Dale, Jennings, and Rudolph negated the question by showing that a randomness processing, quantum Bernoulli factory, using quantum coherence, is strictly more powerful than the one with classical mechanics. In this Letter, focusing on the same scenario, we propose a theoretical protocol that is classically impossible but can be implemented solely using quantum coherence without entanglement. We demonstrate the protocol by exploiting the high-fidelity quantum state preparation and measurement with a superconducting qubit in the circuit quantum electrodynamics architecture and a nearly quantum-limited parametric amplifier. Our experiment shows the advantage of using quantum coherence of a single qubit for information processing even when multipartite correlation is not present. PMID:27419550

  7. Experimental Quantum Randomness Processing Using Superconducting Qubits

    NASA Astrophysics Data System (ADS)

    Yuan, Xiao; Liu, Ke; Xu, Yuan; Wang, Weiting; Ma, Yuwei; Zhang, Fang; Yan, Zhaopeng; Vijay, R.; Sun, Luyan; Ma, Xiongfeng

    2016-07-01

    Coherently manipulating multipartite quantum correlations leads to remarkable advantages in quantum information processing. A fundamental question is whether such quantum advantages persist only by exploiting multipartite correlations, such as entanglement. Recently, Dale, Jennings, and Rudolph negated the question by showing that a randomness processing, quantum Bernoulli factory, using quantum coherence, is strictly more powerful than the one with classical mechanics. In this Letter, focusing on the same scenario, we propose a theoretical protocol that is classically impossible but can be implemented solely using quantum coherence without entanglement. We demonstrate the protocol by exploiting the high-fidelity quantum state preparation and measurement with a superconducting qubit in the circuit quantum electrodynamics architecture and a nearly quantum-limited parametric amplifier. Our experiment shows the advantage of using quantum coherence of a single qubit for information processing even when multipartite correlation is not present.

  8. Measurement strategy for spatially encoded photonic qubits

    SciTech Connect

    Solis-Prosser, M. A.; Neves, L.

    2010-11-15

    We propose a measurement strategy which can, probabilistically, reproduce the statistics of any observable for spatially encoded photonic qubits. It comprises the implementation of a two-outcome positive operator-valued measure followed by a detection in a fixed transverse position, making the displacement of the detection system unnecessary, unlike previous methods. This strategy generalizes a scheme recently demonstrated by one of us and co-workers, restricted to measurement of observables with equatorial eigenvectors only. The method presented here can be implemented with the current technology of programmable multipixel liquid-crystal displays. In addition, it can be straightforwardly extended to high-dimensional qudits and may be a valuable tool in optical implementations of quantum information protocols with spatial qubits and qudits.

  9. Integrated photonic quantum gates for polarization qubits

    PubMed Central

    Crespi, Andrea; Ramponi, Roberta; Osellame, Roberto; Sansoni, Linda; Bongioanni, Irene; Sciarrino, Fabio; Vallone, Giuseppe; Mataloni, Paolo

    2011-01-01

    The ability to manipulate quantum states of light by integrated devices may open new perspectives both for fundamental tests of quantum mechanics and for novel technological applications. However, the technology for handling polarization-encoded qubits, the most commonly adopted approach, is still missing in quantum optical circuits. Here we demonstrate the first integrated photonic controlled-NOT (CNOT) gate for polarization-encoded qubits. This result has been enabled by the integration, based on femtosecond laser waveguide writing, of partially polarizing beam splitters on a glass chip. We characterize the logical truth table of the quantum gate demonstrating its high fidelity to the expected one. In addition, we show the ability of this gate to transform separable states into entangled ones and vice versa. Finally, the full accessibility of our device is exploited to carry out a complete characterization of the CNOT gate through a quantum process tomography. PMID:22127062

  10. Generation and efficient measurement of single photons from fixed-frequency superconducting qubits

    NASA Astrophysics Data System (ADS)

    Kindel, William F.; Schroer, M. D.; Lehnert, K. W.

    2016-03-01

    We demonstrate and evaluate an on-demand source of single itinerant microwave photons. Photons are generated using a highly coherent, fixed-frequency qubit-cavity system, and a protocol where the microwave control field is far detuned from the photon emission frequency. By using a Josephson parametric amplifier (JPA), we perform efficient single-quadrature detection of the state emerging from the cavity. We characterize the imperfections of the photon generation and detection, including detection inefficiency and state infidelity caused by measurement back-action over a range of JPA gains from 17 to 33 dB. We observe that both detection efficiency and undesirable back-action increase with JPA gain. We find that the density matrix has its maximum single-photon component ρ11=0.36 ±0.01 at 29 dB JPA gain. At this gain, back-action of the JPA creates cavity photon number fluctuations that we model as a thermal distribution with an average photon number n ¯=0.041 ±0.003 .

  11. Scalable quantum computation via local control of only two qubits

    SciTech Connect

    Burgarth, Daniel; Maruyama, Koji; Murphy, Michael; Montangero, Simone; Calarco, Tommaso; Nori, Franco; Plenio, Martin B.

    2010-04-15

    We apply quantum control techniques to a long spin chain by acting only on two qubits at one of its ends, thereby implementing universal quantum computation by a combination of quantum gates on these qubits and indirect swap operations across the chain. It is shown that the control sequences can be computed and implemented efficiently. We discuss the application of these ideas to physical systems such as superconducting qubits in which full control of long chains is challenging.

  12. The atomic details of the interfacial interaction between the bottom electrode of Al/AlO{sub x}/Al Josephson junctions and HF-treated Si substrates

    SciTech Connect

    Zeng, L. J.; Nik, S.; Olsson, E.; Krantz, P.; Delsing, P.

    2015-04-28

    The interface between the Al bottom contact layer and Si substrates in Al based Josephson junctions is believed to have a significant effect on the noise observed in Al based superconducting devices. We have studied the atomic structure of it by transmission electron microscopy. An amorphous layer with a thickness of ∼5 nm was found between the bottom Al electrode and HF-treated Si substrate. It results from intermixing between Al, Si, and O. We also studied the chemical bonding states among the different species using energy loss near edge structure. The observations are of importance for the understanding of the origin of decoherence mechanisms in qubits based on these junctions.

  13. Quantum gambling using mesoscopic ring qubits

    NASA Astrophysics Data System (ADS)

    Pakuła, Ireneusz

    2007-07-01

    Quantum Game Theory provides us with new tools for practising games and some other risk related enterprices like, for example, gambling. The two party gambling protocol presented by Goldenberg {\\it et al} is one of the simplest yet still hard to implement applications of Quantum Game Theory. We propose potential physical realisation of the quantum gambling protocol with use of three mesoscopic ring qubits. We point out problems in implementation of such game.

  14. Entanglement monogamy relations of qubit systems

    NASA Astrophysics Data System (ADS)

    Zhu, Xue-Na; Fei, Shao-Ming

    2014-08-01

    We investigate the monogamy relations related to the concurrence and the entanglement of formation. General monogamy inequalities given by the αth power of concurrence and entanglement of formation are presented for N-qubit states. The monogamy relation for entanglement of assistance is also established. Based on these general monogamy relations, the residual entanglement of concurrence and entanglement of formation are studied. Some relations among the residual entanglement, entanglement of assistance, and three tangle are also presented.

  15. EDITORIAL: PLASMA 2006: The 5th International Conference on the Intrinsic Josephson Effect and Plasma Oscillations in High-TC Superconductors

    NASA Astrophysics Data System (ADS)

    Warburton, Paul; Yurgens, August

    2007-02-01

    The 5th International Conference on the Intrinsic Josephson Effect and Plasma Oscillations in High-TC Superconductors (known as `PLASMA' for short) took place in London from July 17th to 19th 2006. The meeting was organised jointly by the Superconductivity Group of the Institute of Physics and the European Science Foundation network `Arrays of Quantum Dots and Josephson Junctions' (AQDJJ). It was sponsored by the UK Engineering and Physical Sciences Research Council, AQDJJ, the Japan Society for the Promotion of Physics and the National Institute of Materials Science (NIMS). The meeting was chaired by Paul Warburton of University College London who wishes to put on record his thanks to the conference sponsors for their generosity, without which the conference could not have taken place. Since the previous PLASMA conference in Tsukuba in 2004 the most significant advance in intrinsic Josephson junction (IJJ) research has arguably been the observation of macroscopic quantum tunnelling in IJJs. At the time of the conference this had been observed by both the RIEC/NIMS/AIST collaboration in Japan and by Paul M\\"uller's group in Erlangen. We therefore felt that the conference presented an ideal and timely opportunity for the IJJ community to learn from the more established community of researchers on macroscopic quantum phenomena in low-TC superconductors---and indeed vice versa. As a result a number of leading researchers from the field of low-TC Josephson qubit devices gave several illuminating presentations. Other sessions included those on Josephson vortex dynamics in layered systems and terahertz oscillations in IJJs, in addition to a lively poster session on the first evening. The conference was rounded off by an excellent summary of the highlights of the meeting given by Professor Hu-Jong Lee. The conference organisers would like to thank all those who made the meeting possible and contributed to its smooth running. In addition to the international organising

  16. Bidirectional Quantum Controlled Teleportation by Using a Genuine Six-qubit Entangled State

    NASA Astrophysics Data System (ADS)

    Chen, Yan

    2015-01-01

    A bidirectional quantum controlled teleportation scheme by using a genuine six-qubit entangled state is proposed. In our scheme, such a six-qubit entangled state is employed as the quantum channel linking three legitimate participants. And Alice may transmit an arbitrary single qubit state of qubit A to Bob and Bob may transmit an arbitrary single qubit state of qubit B to Alice via the control of the supervisor Charlie. This bidirectional quantum controlled teleportation is deterministic.

  17. Quantum interface to charged particles in a vacuum

    NASA Astrophysics Data System (ADS)

    Okamoto, Hiroshi

    2015-11-01

    A superconducting qubit device suitable for interacting with a flying electron has recently been proposed [Okamoto and Nagatani, Appl. Phys. Lett. 104, 062604 (2014), 10.1063/1.4865244]. Either a clockwise or counterclockwise directed loop of half magnetic flux quantum encodes a qubit, which naturally interacts with any single charged particle with arbitrary kinetic energy. Here, the device's properties, sources of errors, and possible applications are studied in detail. In particular, applications include detection of a charged particle essentially without applying a classical force to it. Furthermore, quantum states can be transferred between an array of the proposed devices and the charged particle.

  18. Entanglement and the power of one qubit

    SciTech Connect

    Datta, Animesh; Flammia, Steven T.; Caves, Carlton M.

    2005-10-15

    The 'power of one qubit' refers to a computational model that has access to only one pure bit of quantum information, along with n qubits in the totally mixed state. This model, though not as powerful as a pure-state quantum computer, is capable of performing some computational tasks exponentially faster than any known classical algorithm. One such task is to estimate with fixed accuracy the normalized trace of a unitary operator that can be implemented efficiently in a quantum circuit. We show that circuits of this type generally lead to entangled states, and we investigate the amount of entanglement possible in such circuits, as measured by the multiplicative negativity. We show that the multiplicative negativity is bounded by a constant, independent of n, for all bipartite divisions of the n+1 qubits, and so becomes, when n is large, a vanishingly small fraction of the maximum possible multiplicative negativity for roughly equal divisions. This suggests that the global nature of entanglement is a more important resource for quantum computation than the magnitude of the entanglement.

  19. Dual species entanglement of Rb and Cs qubits with Rydberg blockade for crosstalk free qubit measurements

    NASA Astrophysics Data System (ADS)

    Baker, Kevin; Yu, Zhaoning; Ebert, Matthew; Sun, Yuan; Saffman, Mark

    2016-05-01

    One of the outstanding challenges facing neutral atom qubit approaches to quantum computation is suppression of crosstalk between proximal qubits due to scattered light that is generated during optical pumping and measurement operations. We have recently proposed a dual species approach to solving this challenge whereby computational qubits encoded in Cs atoms are entangled with Rb atoms via an interspecies Rydberg interaction. The quantum state of a Cs atom can then be readout by measuring the state of a Rb atom. The difference in resonant wavelengths of the two species effectively suppresses crosstalk. We will present progress towards experimental demonstration of dual species entanglement using Rb and Cs atoms cotrapped in a single beam optical trap. Work supported by the ARL CDQI.

  20. Entanglement and Metrology with Singlet-Triplet Qubits

    NASA Astrophysics Data System (ADS)

    Shulman, Michael Dean

    Electron spins confined in semiconductor quantum dots are emerging as a promising system to study quantum information science and to perform sensitive metrology. Their weak interaction with the environment leads to long coherence times and robust storage for quantum information, and the intrinsic tunability of semiconductors allows for controllable operations, initialization, and readout of their quantum state. These spin qubits are also promising candidates for the building block for a scalable quantum information processor due to their prospects for scalability and miniaturization. However, several obstacles limit the performance of quantum information experiments in these systems. For example, the weak coupling to the environment makes inter-qubit operations challenging, and a fluctuating nuclear magnetic field limits the performance of single-qubit operations. The focus of this thesis will be several experiments which address some of the outstanding problems in semiconductor spin qubits, in particular, singlet-triplet (S-T0) qubits. We use these qubits to probe both the electric field and magnetic field noise that limit the performance of these qubits. The magnetic noise bath is probed with high bandwidth and precision using novel techniques borrowed from the field of Hamiltonian learning, which are effective due to the rapid control and readout available in S-T 0 qubits. These findings allow us to effectively undo the undesired effects of the fluctuating nuclear magnetic field by tracking them in real-time, and we demonstrate a 30-fold improvement in the coherence time T2*. We probe the voltage noise environment of the qubit using coherent qubit oscillations, which is partially enabled by control of the nuclear magnetic field. We find that the voltage noise bath is frequency-dependent, even at frequencies as high as 1MHz, and it shows surprising and, as of yet, unexplained temperature dependence. We leverage this knowledge of the voltage noise environment, the

  1. Multiplication of qubits in a doubly resonant bichromatic field

    NASA Astrophysics Data System (ADS)

    Saiko, A. P.; Fedaruk, R.

    2010-06-01

    Multiplication of spin qubits arises at double resonance in a bichromatic field when the frequency of the radio-frequency (rf) field is close to that of the Rabi oscillation in the microwave field, provided its frequency equals the Larmor frequency of the initial qubit. We show that the operational multiphoton transitions of dressed qubits can be selected by the choice of both the rotating frame and the rf phase. In order to enhance the precision of dressed qubit operations in the strong-field regime, the counter-rotating component of the rf field is taken into account.

  2. Geometry and Symmetric Coherent States of Three Qubits Systems

    NASA Astrophysics Data System (ADS)

    Guo, Xiao-Kan

    2016-06-01

    In this paper, we first generalize the previous results that relate 1- and 2-qubit geometries to complex and quaternionic Möbius transformations respectively, to the case of 3-qubit states under octonionic Möbius transformations. This completes the correspondence between the qubit geometries and the four normed division algebras. Thereby, new systems of symmetric coherent states with 2 and 3 qubits can be constructed by mapping the spin coherent states to their antipodal symmetric ponits on the generalized Bloch spheres via Möbius transformations in corresponding dimensions. Finally, potential applications of the normed division algebras in physics are discussed.

  3. Purification and switching protocols for dissipatively stabilized entangled qubit states

    NASA Astrophysics Data System (ADS)

    Hein, Sven M.; Aron, Camille; Türeci, Hakan E.

    2016-06-01

    Pure dephasing processes limit the fidelities achievable in driven-dissipative schemes for stabilization of entangled states of qubits. We propose a scheme which, combined with already existing entangling methods, purifies the desired entangled state by driving out of equilibrium auxiliary dissipative cavity modes coupled to the qubits. We lay out the specifics of our scheme and compute its efficiency in the particular context of two superconducting qubits in a cavity-QED architecture, where the strongly coupled auxiliary modes provided by collective cavity excitations can drive and sustain the qubits in maximally entangled Bell states with fidelities reaching 90% for experimentally accessible parameters.

  4. Controllable gaussian-qubit interface for extremal quantum state engineering.

    PubMed

    Adesso, Gerardo; Campbell, Steve; Illuminati, Fabrizio; Paternostro, Mauro

    2010-06-18

    We study state engineering through bilinear interactions between two remote qubits and two-mode gaussian light fields. The attainable two-qubit states span the entire physically allowed region in the entanglement-versus-global-purity plane. Two-mode gaussian states with maximal entanglement at fixed global and marginal entropies produce maximally entangled two-qubit states in the corresponding entropic diagram. We show that a small set of parameters characterizing extremally entangled two-mode gaussian states is sufficient to control the engineering of extremally entangled two-qubit states, which can be realized in realistic matter-light scenarios. PMID:20867288

  5. Catch-disperse-release readout for superconducting qubits.

    PubMed

    Sete, Eyob A; Galiautdinov, Andrei; Mlinar, Eric; Martinis, John M; Korotkov, Alexander N

    2013-05-24

    We analyze a single-shot readout for superconducting qubits via the controlled catch, dispersion, and release of a microwave field. A tunable coupler is used to decouple the microwave resonator from the transmission line during the dispersive qubit-resonator interaction, thus circumventing damping from the Purcell effect. We show that, if the qubit frequency tuning is sufficiently adiabatic, a fast high-fidelity qubit readout is possible, even in the strongly nonlinear dispersive regime. Interestingly, the Jaynes-Cummings nonlinearity leads to the quadrature squeezing of the resonator field below the standard quantum limit, resulting in a significant decrease of the measurement error. PMID:23745846

  6. Logical operations realized on the Ising chain of N qubits

    SciTech Connect

    Asano, Masanari; Tateda, Norihiro; Ishii, Chikara

    2004-08-01

    Multiqubit logical gates are proposed as implementations of logical operations on N qubits realized physically by the local manipulation of qubits before and after the one-time evolution of an Ising chain. This construction avoids complicated tuning of the interactions between qubits. The general rules of the action of multiqubit logical gates are derived by decomposing the process into the product of two-qubit logical operations. The formalism is demonstrated by the construction of a special type of multiqubit logical gate that is simulated by a quantum circuit composed of controlled-NOT gates.

  7. Optimal design of two-qubit quantum circuits

    NASA Technical Reports Server (NTRS)

    Vatan, F.; Williams, C.

    2004-01-01

    In order to demonstrate non-trivial quantum computations experimentally, such as the synthesis of arbitrary entangled states, it will be useful to nderstand how to decompose a desired quantum computation into the shortest possible sequence of one-qubit and two-qubit gates. We contribute to this effort by providing a method to construct an optimal quantum circuit for a general two-qubit gate that requires at most 3 CNOT gates and 15 elementary one qubit gates. We then prove that these constructions are optimal with respect to the family of CNOT, y-rotation, z-rotation, and phase gates.

  8. Testing tripartite Mermin inequalities by spectral joint measurements of qubits

    SciTech Connect

    Huang, J. S.; Oh, C. H.; Wei, L. F.

    2011-06-15

    Using spectral joint measurements of the qubits, we propose a scheme to test the tripartite Mermin inequality with three qubits dispersively coupled to a driven cavity. First, we show how to generate a three-qubit Greenberger-Horne-Zeilinger (GHZ) state by only one-step quantum operation. Then spectral joint measurements are introduced to directly confirm such tripartite entanglement. Assisted by a series of single-qubit operations, these measurements are further utilized to test the Mermin inequality. The feasibility of the proposal is robustly demonstrated by the present numerical experiments.

  9. The Quantum Socket: Wiring for Superconducting Qubits - Part 3

    NASA Astrophysics Data System (ADS)

    Mariantoni, M.; Bejianin, J. H.; McConkey, T. G.; Rinehart, J. R.; Bateman, J. D.; Earnest, C. T.; McRae, C. H.; Rohanizadegan, Y.; Shiri, D.; Penava, B.; Breul, P.; Royak, S.; Zapatka, M.; Fowler, A. G.

    The implementation of a quantum computer requires quantum error correction codes, which allow to correct errors occurring on physical quantum bits (qubits). Ensemble of physical qubits will be grouped to form a logical qubit with a lower error rate. Reaching low error rates will necessitate a large number of physical qubits. Thus, a scalable qubit architecture must be developed. Superconducting qubits have been used to realize error correction. However, a truly scalable qubit architecture has yet to be demonstrated. A critical step towards scalability is the realization of a wiring method that allows to address qubits densely and accurately. A quantum socket that serves this purpose has been designed and tested at microwave frequencies. In this talk, we show results where the socket is used at millikelvin temperatures to measure an on-chip superconducting resonator. The control electronics is another fundamental element for scalability. We will present a proposal based on the quantum socket to interconnect a classical control hardware to a superconducting qubit hardware, where both are operated at millikelvin temperatures.

  10. High-fidelity gates in quantum dot spin qubits

    PubMed Central

    Koh, Teck Seng; Coppersmith, S. N.; Friesen, Mark

    2013-01-01

    Several logical qubits and quantum gates have been proposed for semiconductor quantum dots controlled by voltages applied to top gates. The different schemes can be difficult to compare meaningfully. Here we develop a theoretical framework to evaluate disparate qubit-gating schemes on an equal footing. We apply the procedure to two types of double-dot qubits: the singlet–triplet and the semiconducting quantum dot hybrid qubit. We investigate three quantum gates that flip the qubit state: a DC pulsed gate, an AC gate based on logical qubit resonance, and a gate-like process known as stimulated Raman adiabatic passage. These gates are all mediated by an exchange interaction that is controlled experimentally using the interdot tunnel coupling g and the detuning ϵ, which sets the energy difference between the dots. Our procedure has two steps. First, we optimize the gate fidelity (f) for fixed g as a function of the other control parameters; this yields an that is universal for different types of gates. Next, we identify physical constraints on the control parameters; this yields an upper bound that is specific to the qubit-gate combination. We show that similar gate fidelities should be attainable for singlet-triplet qubits in isotopically purified Si, and for hybrid qubits in natural Si. Considerably lower fidelities are obtained for GaAs devices, due to the fluctuating magnetic fields ΔB produced by nuclear spins. PMID:24255105

  11. Milestones toward Majorana-based quantum computing: Fusion rule detection and topological qubit validation

    NASA Astrophysics Data System (ADS)

    Mishmash, Ryan V.; Aasen, David; Hell, Michael; Higginbotham, Andrew; Danon, Jeroen; Leijnse, Martin; Jespersen, Thomas S.; Folk, Joshua A.; Marcus, Charles M.; Flensberg, Karsten; Alicea, Jason

    We introduce a scheme for preparation, manipulation, and readout of Majorana zero modes in semiconducting wires coated with mesoscopic superconducting islands. Our approach synthesizes recent advances in materials growth with tools commonly used in quantum-dot experiments, including gate-control of tunnel barriers and Coulomb effects, charge sensing, and charge pumping. Recently, we have outlined a sequence of relatively modest milestones which interpolate between zero-mode detection and longer term quantum computing applications. In this talk, I will discuss two of these milestones: (1) detection of fusion rules for non-Abelian anyons using either proximal charge sensing or Majorana-mediated charge pumping and (2) validation of a prototype topological qubit via unconventional scaling relations between the time-averaged qubit splitting and its decoherence times T1 and T2. Both of these proposed experiments require only a single wire with two islands--a hardware configuration already available in the laboratory. Furthermore, these pre-braiding experiments can be adapted to other manipulation and readout schemes as well.

  12. Josephson-like spin current in junctions composed of antiferromagnets and ferromagnets

    NASA Astrophysics Data System (ADS)

    Moor, A.; Volkov, A. F.; Efetov, K. B.

    2012-01-01

    We study Josephson-like junctions formed by materials with antiferromagnetic (AF) order parameters. As an antiferromagnet, we consider a two-band material in which a spin density wave (SDW) arises. This could be Fe-based pnictides in the temperature interval Tc≤T≤TN, where Tc and TN are the critical temperatures for the superconducting and antiferromagnetic transitions, respectively. The spin current jSp in AF/F/AF junctions with a ballistic ferromagnetic layer and in tunnel AF/I/AF junctions is calculated. It depends on the angle between the magnetization vectors in the AF leads in the same way as the Josephson current depends on the phase difference of the superconducting order parameters in S/I/S tunnel junctions. It turns out that in AF/F/AF junctions, two components of the SDW order parameter are induced in the F layer. One of them oscillates in space with a short period ξF,b˜ℏv/H, while the other decays monotonously from the interfaces over a long distance of the order ξN,b=ℏv/2πT (where v, H, and T are the Fermi velocity, the exchange energy, and the temperature, respectively; the subindex “b” denotes the ballistic case). This is a clear analogy with the case of Josephson S/F/S junctions with a nonhomogeneous magnetization where short- and long-range condensate components are induced in the F layer. However, in contrast to the charge Josephson current in S/F/S junctions, the spin current in AF/F/AF junctions is not constant in space, but oscillates in the ballistic F layer. We also calculate the dependence of jSp on the deviation from the ideal nesting in the AF/I/AF junctions. The spin current is maximal in the insulating phase of the AF and decreases in the metallic phase. It turns to zero at the Neel point when the amplitude of the SDW is zero and changes sign for certain values of the detuning parameter.

  13. Teleportation of Three-Qubit State via Six-qubit Cluster State

    NASA Astrophysics Data System (ADS)

    Yu, Li-zhi; Sun, Shao-xin

    2015-05-01

    A scheme of probabilistic teleportation was proposed. In this scheme, we took a six-qubit nonmaximally cluster state as the quantum channel to teleport an unknown three-qubit entangled state. Based on Bob's three times Bell state measurement (BSM) results, the receiver Bob can by introducing an auxiliary particle and the appropriate transformation to reconstruct the initial state with a certain probability. We found that, the successful transmission probability depend on the absolute value of coefficients of two of six particle cluster state minimum.

  14. Interplay of classical and quantum capacitance in a one-dimensional array of Josephson junctions

    NASA Astrophysics Data System (ADS)

    Ribeiro, Pedro; García-García, Antonio M.

    2014-02-01

    Even in the absence of Coulomb interactions, phase fluctuations induced by quantum size effects become increasingly important in superconducting nanostructures as the mean level spacing becomes comparable with the bulk superconducting gap. Here we study the role of these fluctuations, termed "quantum capacitance," in the phase diagram of a one-dimensional ring of ultrasmall Josephson junctions at zero temperature by using path-integral techniques. Our analysis also includes dissipation due to quasiparticle tunneling and Coulomb interactions through a finite mutual and self-capacitance. The resulting phase diagram has several interesting features: A finite quantum capacitance can stabilize superconductivity even in the limit of only a finite mutual-capacitance energy, which classically leads to breaking of phase coherence. In the case of vanishing charging effects, relevant in cold-atom settings where Coulomb interactions are absent, we show analytically that superfluidity is robust to small quantum finite-size fluctuations and identify the minimum grain size for phase coherence to exist in the array. We have also found that the renormalization group results are in some cases very sensitive to relatively small changes of the instanton fugacity. For instance, a certain combination of capacitances could lead to a nonmonotonic dependence of the superconductor-insulator transition on the Josephson coupling.

  15. Quantum decay of the persistent current in a Josephson junction ring

    NASA Astrophysics Data System (ADS)

    Garanin, D. A.; Chudnovsky, E. M.

    2016-03-01

    We study the persistent current in a ring consisting of N ≫1 Josephson junctions threaded by the magnetic flux. When the dynamics of the ring is dominated by the capacitances of the superconducting islands the system is equivalent to the x y spin system in 1 +1 dimensions at the effective temperature T*=√{2 J U } , with J being the Josephson energy of the junction and U being the charging energy of the superconducting island. The numerical problem is challenging due to the absence of thermodynamic limit and slow dynamics of the Kosterlitz-Thouless transition. It is investigated on lattices containing up to ×106 sites. At T*≪J the quantum phase slips are frozen. The low-T* dependence of the persistent current computed numerically agrees quantitatively with the analytical formula provided by the spin-wave approximation. The high-T* behavior depends strongly on the magnetic flux and on the number of superconducting islands N . We present a detailed numerical study of the unbinding of vortex-antivortex pairs responsible for the phase slips, the superconductor-insulator transition, and evolution of the persistent current in a finite-size system.

  16. Gate-Tunable Superconductor-Insulator Transition in Bilayer-Graphene Josephson Junctions

    NASA Astrophysics Data System (ADS)

    Jeong, Dongchan; Lee, Gil-Ho; Doh, Yong-Joo; Lee, Hu-Jong

    2012-02-01

    Bilayer graphene shows opening of electric-field-induced band gap, the size of which is proportional to the intensity of the electric field. We report electronic transport measurements on superconducting proximity effect in planar dual-gated bilayer-graphene Josephson junction with Pb0.93In0.07 (PbIn) electrodes (δPbIn ˜ 1.1meV, Tc = 7.0 K). The junction resistance along the charge-neutral point (CNP) increases as we modulate top- and back-gate voltages away from the zero-gap CNP. The resistive state near the CNP shows a variable-range-hopping-type insulating behavior in R-T curve with lowering temperature crossing the superconducting transition of PbIn electrodes. However, a highly doped regime shows metallic R-T behavior and junction becomes superconducting below Tc. Moreover, magnetic-field-induced Fraunhofer supercurrent modulation, microwave-induced Shapiro steps, and multiple Andreev reflection (MAR) are observed, which indicate the formation of genuine Josephson coupling across the planar junctions below Tc with sufficiently transparent superconductor-bilayer graphene interface. The separatrix of the superconductor-insulator transition corresponds to the square junction conductance of Gsq˜ 6-8e^2/h.

  17. Initialization of a spin qubit in a site-controlled nanowire quantum dot

    NASA Astrophysics Data System (ADS)

    Lagoudakis, Konstantinos G.; McMahon, Peter L.; Fischer, Kevin A.; Puri, Shruti; Müller, Kai; Dalacu, Dan; Poole, Philip J.; Reimer, Michael E.; Zwiller, Val; Yamamoto, Yoshihisa; Vučković, Jelena

    2016-05-01

    A fault-tolerant quantum repeater or quantum computer using solid-state spin-based quantum bits will likely require a physical implementation with many spins arranged in a grid. Self-assembled quantum dots (QDs) have been established as attractive candidates for building spin-based quantum information processing devices, but such QDs are randomly positioned, which makes them unsuitable for constructing large-scale processors. Recent efforts have shown that QDs embedded in nanowires can be deterministically positioned in regular arrays, can store single charges, and have excellent optical properties, but so far there have been no demonstrations of spin qubit operations using nanowire QDs. Here we demonstrate optical pumping of individual spins trapped in site-controlled nanowire QDs, resulting in high-fidelity spin-qubit initialization. This represents the next step towards establishing spins in nanowire QDs as quantum memories suitable for use in a large-scale, fault-tolerant quantum computer or repeater based on all-optical control of the spin qubits.

  18. High-Fidelity Two-Qubit Gates in a Surface Ion Trap

    NASA Astrophysics Data System (ADS)

    Lobser, Daniel; Blain, Matthew; Blume-Kohout, Robin; Fortier, Kevin; Mizrahi, Jonathan; Nielsen, Erik; Rudinger, Kenneth; Sterk, Jonathan; Stick, Daniel; Maunz, Peter

    2016-05-01

    Microfabricated surface traps are capable of supporting a variety of exotic trapping geometries and provide a scalable system for trapped ion Quantum Information Processing (QIP). However, the feasibility of using surface traps for QIP has long been a point of contention because the close proximity of the ions to trap electrodes increases heating rates and might lead to laser-induced charging of the trap. As surface traps continue to evolve at a remarkable rate, their performance is rapidly approaching that of macroscopic electrode traps. Using Sandia's High-Optical-Access surface trap, we demonstrate robust single-qubit gates, both laser- and microwave-based. Our gates are accurately characterized by Gate Set Tomography (GST) and we report the first diamond norm measurements near the fault-tolerance threshold. Extending these techniques, we've realized a Mølmer-Sørensen two-qubit gate that is stable for several hours. This stability has allowed us to perform the first GST measurements of a two-qubit gate, yielding a process fidelity of 99.58(6)%. This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia National Laboratories.

  19. Composite Sequences for Triple-dot Qubits that Compensate for Miscalibration and Hyperfine Gradients

    NASA Astrophysics Data System (ADS)

    Ladd, Thaddeus

    2014-03-01

    Exchange-only qubits defined in triple quantum dots form a promising means for all-electrical semiconductor quantum control, but they suffer from both charge noise and random magnetic field gradients. Low-frequency noise sources can be compensated using composite sequences, but the development of such sequences is constrained by the fact that exchange energies are always positive and the control axes are non-orthogonal. Here, we present the results of both analytical approaches and computational searches for composite pulse sequences, which compensate for simultaneous low-frequency miscalibration (due to fixed random electric fields) and hyperfine effects (due to nuclear magnetic fields) in a single triple-dot qubit. We also present compensation sequences for multi-qubit gates. These results can substantially improve the working fidelity of quantum operations in semiconductor quantum dot devices. Sponsored by United States Department of Defense. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the United States Department of Defense or the U.S. Government.

  20. Subgap Structures in High-Tc Intrinsic Josephson Junctions

    NASA Astrophysics Data System (ADS)

    Müller, Paul

    1998-03-01

    Due to their extremely short coherence length many high-Tc superconductors form natural superconducting multilayers. Adjacent superconducting layers are weakly coupled by the Josephson effect. As a result single crystals act intrinsically as vertical stacks of hundreds of Josephson junctions. We start by summarizing our present state of knowledge, including recent observations of Cherenkov radiation from moving fluxons (G. Hechtfischer, R. Kleiner, A.V. Ustinov, P. Müller, Phys. Rev. Lett. 79, 1365 (1997), and this conference.), and the direct measurement of the (collective) Josephson plasma frequency. We then report on pronounced structures in the current-voltage characteristics of Bi_2Sr_2CaCu_2O8 single crystals, and of Tl_2Ba_2Ca_2Cu_3O_10 thin films. These structures appear well below the superconducting gap, independent on magnetic field and temperatures up to 0.5 T_c(K. Schlenga, G. Hechtfischer, R. Kleiner, W. Walkenhorst, P. Müller, Phys. Rev. Lett. 76, 4943 (1996).). We explain these features by coupling between c-axis phonons and Josephson oscillations(Ch. Helm, Ch. Preis, F. Forsthofer, J. Keller, K. Schlenga, R. Kleiner, P. Müller, Phys. Rev. Lett. 79, 737 (1997).). C-axis lattice vibrations between adjacent superconducting layers are exited by the rf Josephson currents in the resistive state. Our results correspond well to the frequencies of longitudinal c-axis phonons.