Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session R48: Decoherence in Superconducting Qubits: Junctions and FluxoniumFocus Session
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Sponsoring Units: GQI Chair: Britton Plourde, Syracuse University Room: 349 |
Thursday, March 17, 2016 8:00AM - 8:36AM |
R48.00001: Novel Josephson circuit elements for high magnetic field parity detection Invited Speaker: Maja Cassidy Evidence for Majorana modes in semiconductor nanowires to date has relied on DC transport measurements that probe their zero-energy characteristics. However, in order to unambiguously demonstrate the non-Abelian nature of Majoranas, it is necessary to braid them and measure their parity. Superconducting transmon qubits have been shown to be sensitive parity detectors, however traditional designs are incompatible with the strong magnetic fields required for the creation of Majoranas in nanowires. In this talk I will discuss our development of novel superconducting circuit elements such as CPW resonators, tunnel junctions, transmon qubits and on chjp microwave sources that survive magnetic fields in excess of 1T. [Preview Abstract] |
Thursday, March 17, 2016 8:36AM - 8:48AM |
R48.00002: Decoherence and Decay of Two-level Systems due to Non-equilibrium Quasiparticles Sebastian Zanker, Michael Marthaler, Gerd Schön It is frequently observed that even at very low temperatures the number of quasiparticles in superconducting materials is higher than predicted by standard BCS-theory. These quasiparticles can interact with two-level systems, such as superconducting qubits or two-level systems (TLS) in the amorphous oxide layer of a Josephson junction. This interaction leads to decay and decoherence of the TLS, with specific results, such as the time dependence, depending on the distribution of quasiparticles and the form of the interaction. We study the resulting decay laws for different experimentally relevant protocols. [Preview Abstract] |
Thursday, March 17, 2016 8:48AM - 9:00AM |
R48.00003: Superconducting resonators with trapped vortices under direct injection of quasiparticles Ibrahim Nsanzineza, Umesh Patel, K. R. Dodge, R. F. McDermott, B. L. T. Plourde Nonequilibrium quasiparticles and trapped magnetic flux vortices can significantly impact the performance of superconducting microwave resonant circuits and qubits at millikelvin temperatures. Quasiparticles result in excess loss, reducing resonator quality factors and qubit lifetimes. Vortices trapped near regions of large microwave currents also contribute excess loss. However, vortices located in current-free areas in the resonator or in the ground plane of a device can actually trap quasiparticles and lead to a reduction in the quasiparticle loss. We will describe experiments involving the controlled trapping of vortices in superconducting resonators with direct injection of quasiparticles using Normal metal-Insulator-Superconductor (NIS)-tunnel junctions. [Preview Abstract] |
Thursday, March 17, 2016 9:00AM - 9:12AM |
R48.00004: Robustness of superconducting quantum modes against direct quasiparticle injection U. Patel, I. Nsanzineza, M. G. Vavilov, B. L. T. Plourde, R. McDermott Classical Josephson digital logic based on Single Flux Quantum (SFQ) pulses offers a path to high-fidelity coherent control of large-scale superconducting quantum machines. However, an SFQ pulse driver generates nonequilibrium quasiparticles that contribute to qubit relaxation, and steps must be taken to protect the qubit from this decoherence channel. Here we describe experiments to characterize the robustness of high-Q superconducting linear resonators and qubits against direct quasiparticle injection. We use NIS junctions and SFQ elements to controllably inject quasiparticles into the groundplane of superconducting resonator and qubit chips, and we characterize the quasiparticle contribution to dissipation. We examine the effectiveness of groundplane cuts, normal metal quasiparticle traps, and spatially-varying superconducting gaps at protecting the quantum modes against quasiparticle loss. Finally, we discuss strategies for the integration of multiqubit circuits with on-chip SFQ control elements. [Preview Abstract] |
Thursday, March 17, 2016 9:12AM - 9:24AM |
R48.00005: Normal Metal Quasiparticle Traps in 3D-Transmon Qubits Luke D. Burkhart, Yvonne Y. Gao, Chen Wang, Kyle Serniak, Gijs de Lange, Yiwen Chu, Uri Vool, Luigi Frunzio, Michel H. Devoret, Gianluigi Catelani, Leonid I. Glazman, Robert J. Schoelkopf 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. [Preview Abstract] |
Thursday, March 17, 2016 9:24AM - 9:36AM |
R48.00006: Gap-engineered quasiparticle traps in the fluxonium artificial atom K. Serniak, G. de Lange, U. Vool, M. Hays, L.D. Burkhart, Y.Y. Gao, C. Wang, K.M. Sliwa, I.M. Pop, L. Frunzio, L.I. Glazman, R.J. Schoelkopf, M.H. Devoret Recent experiments have shown that the density of quasiparticles in superconducting quantum circuits exceeds the expected thermal density. In Josephson junction based superconducting qubits, these non-equilibrium quasiparticles can tunnel through the junctions of the circuit, causing decoherence. Quasiparticle traps aim to reduce the density of quasiparticles near the junctions, and therefore the rate of energy loss and dephasing due to tunneling events. These traps must be designed to not introduce any additional losses in the qubit. In this talk we will discuss recent progress in the design and implementation of quasiparticle traps in the fluxonium artificial atom. [Preview Abstract] |
Thursday, March 17, 2016 9:36AM - 9:48AM |
R48.00007: Spectroscopy and decoherence of plasmons and fluxons in superconducting fluxonium qubit. Long Nguyen, Yen-Hsiang Lin, Nicholas Grabon, Natalya Solovyeva, Vladimir Manucharyan Transition spectrum of a fluxonium circuit changes drastically with respect to the p$=$EJ/EC parameter of the small junction, remaining charge-insensitive at all values of p. At larger values of p, the spectrum consists of exponentially decoupled fluxon and plasmon transitions. At smaller values, fluxons no longer exist, and plasmons reside mostly in the array inductance. We present spectroscopy of tunable fluxoniums and discuss our findings from the decoherence measurements of various transitions. [Preview Abstract] |
Thursday, March 17, 2016 9:48AM - 10:00AM |
R48.00008: The fluxonium as a lambda system U. Vool, A. Kou, W.C. Smith, K. Serniak, S. Shankar, S.M. Girvin, M.H. Devoret A lambda system is a 3-level system in which two low-energy states can transition to a third higher-energy state by a coherent drive but not to each other. Lambda systems are commonly implemented in systems relying on atomic transitions. In the field of superconducting quantum circuits, the fluxonium qubit, an artificial atom consisting of a Josephson junction shunted by a super-inductance, is a unique artificial atom with highly non-linear energy levels. At half-flux quantum it has two low-energy states with a long energy lifetime, but it is difficult to perform fast quantum gates in this manifold. Employing the higher 2nd excited state as an intermediate level would be much more efficient. However, selection rules in the fluxonium qubit prohibit transitions between low-energy states and higher-energy states of the same parity. In this talk, we will introduce a way to create formerly forbidden transitions between levels of the fluxonium qubit - thus creating a more interesting artificial atom and a useful tool for future superconducting quantum circuits. [Preview Abstract] |
Thursday, March 17, 2016 10:00AM - 10:12AM |
R48.00009: Simultaneous monitoring of fluxonium qubits in a waveguide A. Kou, W.C. Smith, U. Vool, I.M. Pop, K.M. Sliwa, M. Hatridge, R.J. Schoelkopf, M.H. Devoret 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. [Preview Abstract] |
Thursday, March 17, 2016 10:12AM - 10:24AM |
R48.00010: A disordered kinetic superinductor M. Hays, G. de Lange, K. Serniak, Z. Wang, U. Vool, L. Frunzio, M.H. Devoret The superinductance is a superconducting circuit element whose reactance exceeds the resistance quantum at the relevant microwave operation frequencies of quantum circuits. It must also be as non-dissipative as possible. Such an element is key to the fluxonium artificial atom, a highly anharmonic, charge insensitive superconducting qubit that has been proposed as the detection circuit for Majorana Fermions. So far fluxonium qubits are made exclusively from arrays of Al-AlOx-Al Josephson junctions. However, aluminium is difficult to employ in conjunction with the strong magnetic fields required in Majorana Fermion experiments. The large kinetic inductance of highly resistive disordered superconducting alloys, such as NbTiN, is currently explored as an alternative material for superinductance in quantum electronic circuits. We report the results of measurement of quality factors and phase-slip rates of high-impedance resonators made from thin-film NbTiN. [Preview Abstract] |
Thursday, March 17, 2016 10:24AM - 10:36AM |
R48.00011: Fabrication and characterization of low loss and high inductance Josephson tunnel junction chains for quantum circuits Nicholas Grabon, Natalya Solovyeva, Long Nguyen, Yen-Hsiang Lin, Vladimir Manucharyan 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 [1]. We report fabrication and microwave characterization of linear Al/AlOx/Al Josephson tunnel junction chains and discuss their advantages and limitations as superinductors. [1]: 10.1103/PhysRevA.87.052306 [Preview Abstract] |
Thursday, March 17, 2016 10:36AM - 10:48AM |
R48.00012: Extending quantum coherence of superconducting flux Fei Yan, Archana Kamal, Terry Orlando, Simon Gustavsson, William Oliver We present the design of a superconducting qubit with multiple Josephson junctions. The design starts with a capacitively shunted flux qubit, and it incorporates particular junction parameter choices for the purpose of simultaneously optimizing over transition frequency, anharmonicity, flux- and charge-noise sensitivity around flux degeneracy. By studying the scaling properties with design parameters, we identify directions to extend coherence substantially. [Preview Abstract] |
Thursday, March 17, 2016 10:48AM - 11:00AM |
R48.00013: Quantum memristor in a superconducting circuit Juha Salmilehto, Mikel Sanz, Massimiliano Di Ventra, Enrique Solano Memristors, resistive elements that retain information of their past, have garnered interest due to their paradigm-changing potential in information processing and electronics. The emergent hysteretic behaviour allows for novel architectural applications and has recently been classically demonstrated in a simplified superconducting setup using the phase-dependent conductance in the tunnel-junction-microscopic model[1]. In this contribution, we present a truly quantum model for a memristor constructed using established elements and techniques in superconducting nanoelectronics, and explore the parameters for feasible operation as well as refine the methods for quantifying the memory retention. In particular, the memristive behaviour is shown to arise from quasiparticle-induced tunneling in the full dissipative model and can be observed in the phase-driven tunneling current. The relevant hysteretic behaviour should be observable using current state-of-the-art measurements for detecting quasiparticle excitations. Our theoretical findings constitute the first quantum memristor in a superconducting circuit and act as the starting point for designing further circuit elements that have non-Markovian characteristics. [1] S. Peotta and M. Di Ventra, Phys. Rev. Applied 2, 034011 (2014). [Preview Abstract] |
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