Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Q28: Focus Session: Superconducting Qubits: Trajectories & Measurement |
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Sponsoring Units: GQI Chair: Andrew Cross, IBM Room: 601 |
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q28.00001: Observing interferences between past and future quantum states in resonance fluorescence Invited Speaker: Landry Bretheau In quantum physics, measurement results are random but their statistics can be predicted at any time assuming some knowledge about the system in the past. Additional knowledge from a future measurement deeply changes these statistics in the present and leads to purely quantum features. In particular conditioned average outcomes of a weak measurement, so-called weak values, were shown to go beyond the conventional range, give a way to directly measure complex values, and can be used to enhance the sensitivity of quantum meters. Recently, these concepts have been considered in the general case of open quantum systems where decoherence occurs. Then, what are the properties of weak values for the unavoidable measurement associated to decoherence, the one performed by the environment? Here, we answer this question in the simplest open quantum system: a quantum bit in presence of a relaxation channel. We continuously monitor the fluorescence emitted by a superconducting qubit driven at resonance. Conditioned on initial preparation and final single shot measurement outcome of the qubit state, we probe weak values displaying all the above properties. The fluorescence signal exhibits interferences between oscillations associated to past or future quantum states. The measured data are in excellent agreement with a recently developed formalism. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q28.00002: Observing single quantum trajectories of a superconducting qubit: introduction Kater Murch, Steven Weber, Chris Macklin, Irfan Siddiqi The length of time that a quantum system can exist in a coherent superposition is determined by how strongly it interacts with its environment. Unmonitored environmental fluctuations can be viewed as a source of noise, causing random evolution of the quantum system from an initially pure state into a statistical mixture. However, by accurately measuring the environment in real time, the quantum system can be maintained in a pure state and its time evolution described by a ``quantum trajectory'' determined by the measurement outcome. We use weak measurements to monitor a microwave cavity embedding a superconducting qubit and track the individual quantum trajectories of the system. We perform quantum state tomography at discrete times along the trajectory to verify that we have faithfully tracked the state of the quantum system as it diffuses on the surface of the Bloch sphere. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q28.00003: Observing single quantum trajectories of a superconducting qubit: ensemble properties and driven dynamics Steven Weber, K.W. Murch, A. Chantasri, J. Dressel, A.N. Jordan, I. Siddiqi We use weak measurements to track individual quantum trajectories of a superconducting qubit embedded in a microwave cavity. Using a near-quantum-limited parametric amplifier, we selectively measure either the phase or amplitude of the cavity field, and thereby confine trajectories to either the equator or a meridian of the Bloch sphere. We analyze ensembles of trajectories to determine statistical properties such as the most likely path and most likely time connecting pre and post-selected quantum states. We compare our results with theoretical predictions derived from an action principle for continuous quantum measurement. Furthermore, by introducing a qubit drive, we investigate the interplay between unitary state evolution and non-unitary measurement dynamics. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q28.00004: Backtracking quantum trajectories with analog feedback G. de Lange$^{*}$, D. Rist\`e$^{*}$, M.J. Tiggelman, C. Eichler, L. Tornberg, G. Johansson, A. Wallraff, R.N. Schouten, L. DiCarlo Circuit quantum electrodynamics offers a nearly ideal platform for the fundamental study of continuous quantum measurement. A nondemolition measurement of a superconducting qubit can be performed via homodyne detection of microwave transmission through a dispersively coupled cavity. By boosting the homodyne signal with a nearly noiseless phase-sensitive parametric amplifier, we experimentally show that a form of measurement backaction, consisting of stochastic quantum phase kicks on the measured qubit, is highly correlated with the fluctuations in the continuous homodyne record. We demonstrate a real-time analog feedback scheme that counteracts these phase kicks and thereby reduces measurement-induced dephasing. We develop a numerical optimization technique to overcome the bandwidth limitations of the amplification chain and provide a theoretical model for the optimization result. A quantum efficiency of 50\% is extracted for the complete analog feedback loop. Finally, we discuss the integration of this analog feedback technique to improve performance in our recent demonstration [1] of entanglement by dispersive parity measurement. $^{*}$equal contribution. [1] D. Rist\`e {\it et al.}, Nature 502, 350 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q28.00005: Waveguide implementation of a fluxonium qubit W.C. Smith, A. Kou, I.M. Pop, R.J. Schoelkopf, M.H. Devoret Enclosing a fluxonium qubit in a 3D cavity has recently been shown to provide remarkable qubit relaxation times of order 1 ms. This is realized by inductively coupling the qubit to an on-chip antenna, itself electromagnetically coupled to a microwave readout cavity. In order to improve measurement contrast, we propose reading out the on-chip antenna directly by embedding the substrate in a rectangular waveguide impedance-matched to open coaxial lines. This scheme allows for greater flexibility in engineering the dispersive shift of the readout resonator. Independently, we can optimize the readout resonator external quality factor to maximize measurement contrast. Results of dispersive shift calculations, electromagnetic simulations, and experimental design will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q28.00006: Measurement of a fluxonium qubit embedded in a waveguide A. Kou, W.C. Smith, I.M. Pop, R.J. Schoelkopf, M.H. Devoret The fluxonium qubit in a 3D cavity has been shown to have energy relaxation times of order 1 ms. The qubit readout contrast, however, was only approximately 10 degrees. High qubit readout contrast and fast operations on a qubit state are necessary ingredients for any implementation of a quantum computer. We have designed and measured a fluxonium qubit embedded in a waveguide in order to achieve these goals. We present measurements of the dispersive shift on a Josephson junction resonator inductively coupled to the fluxonium qubit as well as the relaxation times of the fluxonium qubit in this new architecture. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q28.00007: Quantum jumps of a fluxonium qubit U. Vool, I.M. Pop, K. Sliwa, B. Abdo, T. Brecht, S. Shankar, M. Hatridge, R.J. Schoelkopf, M. Mirrahimi, L. Glazman, M.H. Devoret The fluxonium qubit has recently been shown to have energy relaxation time (T1) of the order of 1 ms, limited by quasiparticle dissipation. With the addition of a Josephson Parametric Converter (JPC) to the experiment, trajectories corresponding to quantum jumps between the ground and 1st excited state can be measured, thus allowing the observation of the qubit decay in real time instead of that of an ensemble average. Our measurement fidelity with the JPC is in excess of 98{\%} for an acquisition time of 5 us and we can thus continuously monitor the quantum jumps of the qubit in equilibrium with its environment in a time much shorter than its average relaxation time. We observe in our sample a jump statistics that varies from being completely Poissonian with a long (500 us) mean time in the ground state to being highly non-Poissonian with short (100 us) mean time in the ground state. The changes between these regimes occur on time scales of seconds, minutes and even hours. We have studied this effect and its relation to quasiparticle dynamics by injecting quasiparticles with a short intense microwave pulse and by seeding quasiparticle-trapping vortices with magnetic field. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q28.00008: Quasi-particle population of superconducting islands probed by quantum jumps of a fluxonium qubit I.M. Pop, U. Vool, K. Sliwa, B. Abdo, T. Brecht, R.J. Schoelkopf, M. Mirrahimi, L. Glazman, M.H. Devoret The origin and the dynamics of nonequlibrium quasiparticles in superconducting circuits remain an open problem. One of the most sensitive systems that could be used to measure quasiparticles is the fluxonium qubit. Recently, this artificial atom has demonstrated relaxation times on the order of 1 ms, limited by quasiparticle dissipation. Moreover, the sensitivity to quasiparticle loss can be tuned in situ by applying a magnetic flux. By using a quantum limited amplifier (a Josephson Parametric Converter) we can observe quantum jumps between the 0 and 1 states of a fluxonium 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. Our data is entirely consistent with the hypothesis that our system is sensitive to single quasiparticle excitations. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q28.00009: Fluxon readout for superconducting qubits Kirill Fedorov, Anastasia Shcherbakova, Alexey Ustinov We demonstrate an experiment on coupling of a single Josephson vortex (fluxon) in a long annular Josephson junction (AJJ) with a flux qubit. Using a possibility to measure the microwave radiation induced by a fluxon oscillations in the AJJ, the interaction of a current dipole generated by the flux qubit and the propagating fluxon was studied. We discuss relativistic dynamics of the Josephson vortex scattering on the current dipole. We detected specific periodic variations of the fluxon oscillation frequency versus magnetic flux through the qubit. We found that quantum states of the flux qubit can be distinguished by measuring small frequency shifts of the coupled fluxon oscillations. The fluxon readout for the superconducting flux qubit was experimentally performed by measuring of a characteristic energy spectrum of the latter. The demonstrated approach is compatible with the existing low-temperature digital RSFQ (Rapid Single Flux Quantum) electronics and may be useful as a scalable interface between classical computers and respective quantum counterparts. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q28.00010: Generating entanglement via measurement between two remote superconducting qubits N. Roch, M.E. Schwarzt, F. Motzoi, C. Macklin, R. Vijay, A.W. Eddins, A.N. Korotkov, B. Whaley, M. Sarovar, I. Siddiqi Measurement has traditionally been viewed as a mechanism for restoring classical behavior: a quantum superposition, once observed, transforms into a single classical state. However, for some quantum systems it is possible to design a measurement that probabilistically projects onto an entangled state, thereby purifying, rather than destroying, quantum correlations. We use a joint dispersive readout to entangle two superconducting qubits, in individual cavities, separated by more than a meter of coaxial cable. We obtain a concurrence of 0.35, which is consistent with transmission losses and detector efficiency. The intensity of the readout pulse can be continuously varied, enabling us to monitor the dynamics of entanglement generation. The data agree with both a Bayesian model and a full master equation treatment. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q28.00011: Continuous measurement of two spatially separated superconducting qubits: Quantum trajectories and feedback M.E. Schwartz, N. Roch, F. Motzoi, B. Whaley, A.N. Korotkov, M. Sarovar, I. Siddiqi Measurement can be harnessed to probabilistically generate entanglement in the absence of local interactions, for example between spatially separated quantum objects. Continuous weak measurement allows us to observe the dynamics associated with this process. In particular, we perform joint dispersive readout of two superconducting transmon qubits separated by one meter of coaxial cable. We track the evolution of a joint quantum state under the influence of measurement, both as an ensemble and as a set of individual quantum trajectories. We analyze the statistics of such quantum trajectories and find good agreement with a Bayesian formalism for a two-body quantum system. Such tracking opens the door to continuous feedback-stabilized remote entanglement. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q28.00012: Quantum Zeno effect in a strongly measured superconducting qubit D.H. Slichter, R. Vijay, S.J. Weber, C. Mueller, A. Blais, I. Siddiqi A qubit undergoing strong measurement is continuously projected into an eigenstate of the measured observable. A simultaneous resonant qubit drive will give rise to transitions between qubit states, but the presence of measurement slows the rate of these transitions -- a phenomenon known as the quantum Zeno effect. We observe this effect in a transmon qubit using linear circuit QED readout and a near-quantum-limited following amplifier. The experimental measurement record, consisting of a series of quantum jumps between states, is analyzed to extract qubit transition rates. We study the dependence of these rates on measurement strength and qubit drive amplitude and compare with theoretical predictions. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q28.00013: Local comb generation in nonlinear TiN superconducting resonators David Pappas, Michael R. Vissers, Robert Erickson, Martin Sandberg, Jiansong Gao Low loss superconducting nonlinear resonators are extensively used for qubit readout as well as photon detectors. These devices are typically capacitively coupled to a launch line. When driven at high power, a shift in resonant frequency is observed due to the kinetic inductance of the TiN superconductor. At higher power, the resonant frequency mixes with the drive tone to produce a series of peaks that are observed to be equally spaced at the detuning frequency, i.e. a ``local comb.'' The full circuit analysis of this system is derived. The renormalized resonant frequency is obtained and the local comb is derived from a first order successive approximation. [Preview Abstract] |
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