Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Z39: Focus Session: Superconducting Qubits and Junctions |
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Sponsoring Units: GQI Chair: William Oliver, MIT Lincoln Laboratory Room: 213AB |
Friday, March 6, 2015 11:15AM - 11:27AM |
Z39.00001: Measurement of recombination and single-vortex trapping of quasiparticles in a superconducting qubit Chen Wang, Y.Y. Gao, I.M. Pop, U. Vool, C. Axline, T. Brecht, R.W. Heeres, L. Frunzio, G. Catelani, M.H. Devoret, L.I. Glazman, R.J. Schoelkopf We measure the dynamics of quasiparticle relaxation over multiple orders of magnitude in density in superconducting transmon qubits using a contactless injection technique by microwave pulses. We demonstrate the power-law decay characteristics for quasiparticle recombination and exponential decay for single quasiparticle loss due to trapping effects, and find both mechanisms play a significant role in quasiparticle relaxation depending on device geometry. We observe quantized changes in quasiparticle trapping rate due to individual vortices, and thus quantitatively measure the interaction between non-equilibrium quasiparticles and a single vortex in a superconducting aluminum film. These results are described in Ref. [1]. [1] C. Wang et al. arXiv:1406.7300 [quant-ph] [Preview Abstract] |
Friday, March 6, 2015 11:27AM - 11:39AM |
Z39.00002: Suppressing decoherence of superconducting qubits by trapping non-equilibrium quasiparticles Yvonne Gao, Chen Wang, I.M. Pop, U. Vool, C. Axline, T. Brecht, R.W. Heeres, L. Frunzio, M.H. Devoret, G. Catelani, L.I. Glazman, R.J. Schoelkopf We report a counter-intuitive observation that vortices can improve the coherence of superconducting qubits by suppressing non-equilibrium quasiparticles. This effect is systematically studied by measuring the magnetic-field dependence of qubit coherence times and quasiparticle lifetimes in transmons with different geometries in a 3D cQED architecture. Varying quasiparticle dynamics by vortices allows separation of dissipation mechanisms and measurement of the stray generation rate of quasiparticles in our devices. More details are described in Ref [1]. Our results indicate that quasiparticles contribute significantly to qubit decoherence. Hence suppression of quasiparticle density in the device is essential for further improvement of coherence times of superconducting qubits and we will present recent results aimed at alleviating decoherence due to quasiparticles. [1] C.Wang, Y.Y.Gao et al arXiv:1406.7300 [Preview Abstract] |
Friday, March 6, 2015 11:39AM - 12:15PM |
Z39.00003: Coherent Suppression of Quasiparticle Dissipation in Superconducting Artificial Atom Invited Speaker: Ioan M. Pop We demonstrate immunity to quasiparticle dissipation in a Josephson junction. At the foundation of this protection rests a prediction by Brian Josephson from fifty years ago: the particle-hole interference of superconducting quasiparticles when tunneling across a Josephson junction [1]. The junction under study is the central element of a fluxonium artificial atom, which we place in an extremely low loss environment and measure using radio-frequency dispersive techniques [2]. Furthermore, by using a quantum limited amplifier (a Josephson Parametric Converter) we can observe quantum jumps between the 0 and 1 states of the qubit in thermal equilibrium with the environment. The distribution of the times in-between the quantum jumps reveals quantitative information about the population and dynamics of quasiparticles [3]. The data is entirely consistent with the hypothesis that our system is sensitive to single quasiparticle excitations, which opens new perspectives for quasiparticle monitoring in low temperature devices. [1] B. D. Josephson, Physics Letters 1, 251 (1962)~; [2] I. M. Pop et al., Nature 508 (2014)~; [3] U. Vool et al., PRL (in press 2014); [Preview Abstract] |
Friday, March 6, 2015 12:15PM - 12:27PM |
Z39.00004: From quantum jumps to quasiparticle population U. Vool, I.M. Pop, K. Sliwa, B. Abdo, C. Wang, Y.Y. Gao, A. Kou, W.C. Smith, T. Brecht, S. Shankar, M. Hatridge, G. Catelani, L. Frunzio, R.J. Schoelkopf, L. Glazman, M. Mirrahimi, M.H. Devoret Superconducting quasiparticles (QP) play a dominant role in the relaxation of the fluxonium qubit in the vicinity of the half-flux-quantum bias point. Recent experiments integrating the fluxonium with a quantum-limited amplifier have measured quantum jump trajectories between the ground state and the first excited state. These trajectories show a change in the characteristic lifetime of the fluxonium qubit as a function of time, arising from a change in the number of QP's in the sample [1]. Using a simple model of QP dynamics and their effect on the fluxonium qubit, we can access the QP population with temporal resolution better than a 100 microsecond. Such rapid monitoring of QP dynamics is essential for understanding the sources of QP's and ultimately suppressing them.\\[4pt] [1] ``Non-Poissonian quantum jumps of a fluxonium qubit due to quasiparticle excitations,'' U. Vool, I.M. Pop et al. to be published in PRL 2014. [Preview Abstract] |
Friday, March 6, 2015 12:27PM - 12:39PM |
Z39.00005: Simulation and measurement of a fluxonium qubit inductively coupled to a readout resonator W.C. Smith, A. Kou, U. Vool, I.M. Pop, R.J. Schoelkopf, M.H. Devoret Prototypical circuit QED experiments can be performed using a fluxonium qubit that shares a portion of its superinductance with an on-chip LC oscillator, dubbed an ``antenna,'' that is used as a readout resonator. However, the complete fluxonium-antenna artificial atom had not been previously understood in all coupling regimes. We have measured Hamiltonian parameters and decay rates by channeling microwave pulses into a rectangular waveguide containing the antenna-qubit system. Accurate modeling of energy spectra, dispersive shifts, and Purcell loss is achieved by diagonalizing the effective circuit Hamiltonian. We will present spectroscopy data, coherence times, and simulation results. [Preview Abstract] |
Friday, March 6, 2015 12:39PM - 12:51PM |
Z39.00006: Quantum and Ionic Transport Across Superconductor-based Heterostructures Osama Nayfeh, Son Dinh, Benjamin Taylor, Marcio de Andrade, Paul Swanson, Bruce Offord, Anna Leese de Escobar, Stephanie Claussen, Sam Kassegne We present analysis of quantum and ionic transport across superconductor/barrier/ionic/barrier/superconductor (SBIBS) heterostructures. Calculations for various ionic configurations demonstrate modification of the quantum transport coherence length and energy profile with moderate ionic transport away from the superconductor-barrier interface. The effect of electric field and cryogenic temperature on the stability of the ionic configurations for quantum information state storage is examined. Characterization and analysis of constructed Al and Nb-based device structures are presented. Acknowledgements: We acknowledge the support of the SSC Pacific In-house Laboratory Independent Research Science and Technology Program managed by Dr. Dave Rees, the Naval Innovative Science and Engineering Program managed by Mr. Robin Laird, and the ONR Summer Faculty Research Program. Interactions with Dr. Van Vechten (ONR) and Dr. Manheimer (IARPA) are appreciated. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of SPAWAR or the U.S. Government. Approved for Public Release; distribution is unlimited. [Preview Abstract] |
Friday, March 6, 2015 12:51PM - 1:03PM |
Z39.00007: Correlations of microwave photons emitted by inelastic Cooper pair tunneling Alexander Grimm, Salha Jebari, Dibyendu Hazra, Carles Altimiras, Olivier Parlavecchio, Fabien Portier, Max Hofheinz \\ A simple DC voltage-bias on a small Josephson junction leads to emission of microwave radiation via inelastic Cooper-pair tunneling. In this process a tunneling Cooper pair emits one or several microwave photons with a total energy of 2eV. The observed average photon emission rate is well explained within the so-called P(E) theory, but this theory does not make any predictions about the statistics of the emitted photons. \\ Recent theory indicates that these statistics can be highly nontrivial. Depending on the bias conditions and the impedance of the circuit in which the junction is embedded, correlations can range from strongly bunched to anti-bunched. I will present experiments investigating photon correlations in circuits with specifically engineered environments. \\ This type of devices might offer a new way of generating useful photon states for circuit quantum optics experiments, without the need of carefully calibrated control pulses. Moreover, the frequency of the emitted radiation is only limited by the gap of the superconductor. We are building our devices using NbN-MgO-NbN tunnel junctions which should in principle allow operation up to the THz regime. [Preview Abstract] |
Friday, March 6, 2015 1:03PM - 1:15PM |
Z39.00008: Can a strain yield a qubit? Colin Benjamin 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 $\pi $-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. [Preview Abstract] |
Friday, March 6, 2015 1:15PM - 1:27PM |
Z39.00009: Nonequilibrium noise and current fluctuations at the superconducting phase transition Dmitry Bagrets, Alex Levchenko We study non-Gaussian out-of-equilibrium current fluctuations in a mesoscopic NSN circuit at the point of a superconducting phase transition. The setup consists of a voltage-biased thin film nanobridge superconductor (S) connected to two normal-metal (N) leads by tunnel junctions. We find that above a critical temperature fluctuations of the superconducting order parameter associated with the preformed Cooper pairs mediate inelastic electron scattering that promotes strong current fluctuations. Though the conductance is suppressed due to the depletion of the quasiparticle density of states, higher cumulants of current fluctuations are parametrically enhanced. We identify experimentally relevant transport regime where excess current noise may reach or even exceed the level of the thermal noise. [Preview Abstract] |
Friday, March 6, 2015 1:27PM - 1:39PM |
Z39.00010: Manipulating the Quantum State of a Single Cooper Pair in a One-Atom Contact Cristian Urbina, Camille Janvier, Leandro Tosi, \c{C}a\u{g}lar Girit, Michael Stern, Patrice Bertet, Denis Vion, Philippe Joyez, Daniel Esteve, Marcelo Goffman, Hugues Pothier Superconducting qubits presently used in quantum information experiments are based on Josephson tunnel junctions. Nevertheless, these circuits exploit only partially the richness of the Josephson effect, as they overlook the existence of an internal, spin-like degree of freedom, inherent to all Josephson structures. Each conduction channel of a weak-link gives rise to a doublet of discrete subgap states (the Andreev bound states), which represents the two possible states of a localized Cooper pair. We spotlight these doublets with experiments on the simplest Josephson weak-link: a one-atom contact between two superconductors. The atomic contact is inserted in a superconducting loop coupled to a microwave resonator. This standard circuit-QED architecture allows performing single shot measurements of the state of a localized Cooper pair, and to manipulate coherently its quantum state, as illustrated by Rabi oscillations, Ramsey fringes and spin echoes. [Preview Abstract] |
Friday, March 6, 2015 1:39PM - 1:51PM |
Z39.00011: Flux Solitons Studied for Energy-Conserving Reversible Computing Kevin D. Osborn, Waltraut Wustmann On-chip logic is desired for controlling superconducting qubits. Since qubits are very sensitive to photon field noise, it is desirable to develop an energy-conserving reversible logic, i.e. one which can compute without substantial energy dissipation or applied drive fields. With this goal in mind, simulations on discretized long Josephson junctions (DLJJs) have been performed, where the flux soliton is studied as a potential information carrier. Undriven soliton propagation is studied as a function of discreteness, dissipation, and uncertainty in the junction critical current. The perturbing parameters are low in the simulations such that the solitons fit well to an ideal Sine-Gordon soliton. Surprisingly, using realizable parameters a single flux soliton in a DLJJ is found to travel hundreds of Josephson penetration depths without backscattering in the absence of a driving force. In addition, even with a non-ideal launch, solitons are found to propagate predictably such that they show potential for synchronous routing into reversible logic gates. [Preview Abstract] |
Friday, March 6, 2015 1:51PM - 2:03PM |
Z39.00012: Flux Soliton Interactions in Coupled Long Josephson Junctions Waltraut Wustmann, Kevin D. Osborn Flux solitons and long Josephson junctions are being studied as structures for energy-conserving reversible computing. The solitons in long Josephson junctions are generally described by the Sine-Gordon equation. Simulations have been performed on discrete long Josephson junctions (DLJJs), where the soliton extends over at least a few unit cells. We will report on the dissipation and mutual interaction from two solitons in separate DLJJs. Single fluxon dissipation in DLJJs is found to arise from junction damping as well as radiation loss created by discreteness and perturbed soliton oscillations. Dissipation of interacting solitons in coupled DLJJs will be compared to the single fluxon case. [Preview Abstract] |
Friday, March 6, 2015 2:03PM - 2:15PM |
Z39.00013: Long Time Electrical Stability of Plasma Oxidized Aluminum Tunnel Barriers Zachary Barcikowski, Josh Pomeroy By measuring resistance-area product values over time, we assess the electrical stability of tunnel junctions with plasma oxidized AlOx tunnel barriers. AlOx is a commonly used material in the superconducting qubit community due to its ease of fabrication, but often has a high density of electrically active defects. We believe that plasma oxidation, as opposed to the standard thermal oxidation, can lead to tunnel barriers with lower defect densities and improved electrical properties. This talk will present measurements taken on tunnel barrier devices taken over a period of months and correlate observed stability/instability with the process conditions used to fabricate them. [Preview Abstract] |
Friday, March 6, 2015 2:15PM - 2:27PM |
Z39.00014: Fluctuation-dissipation relations of a tunnel junction driven by a quantum circuit Daniel Esteve, Olivier Parlavecchio, Carles Altimiras, Jean-Rene Souquet, Pascal Simon, Ines Safi, Philippe Joyez, Denis Vion, Patrice Roche, Fabien Portier We derive fluctuation-dissipation relations for a tunnel junction driven by a high impedance microwave resonator, displaying strong quantum fluctuations. We find that the fluctuation-dissipation relations derived for classical forces hold, provided the effect of the circuit's quantum fluctuations is incorporated into a modified non-linear I(V) curve. We also demonstrate that all quantities measured under a coherent time-dependent bias can be reconstructed from their dc counterpart with a photo-assisted tunneling relation. We confirm these predictions by implementing the circuit and measuring the dc current through the junction, its high frequency admittance and its current noise at the frequency of the resonator. Results avaiable in arXiv:1409.6696. [Preview Abstract] |
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