Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session V48: Readout and Trajectories in Superconducting Circuits |
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Sponsoring Units: GQI Chair: Kater Murch, Washington University, St. Louis Room: 349 |
Thursday, March 17, 2016 2:30PM - 2:42PM |
V48.00001: Probing the Speed Limits of Transmon Dispersive Readout Theo Walter, Philipp Kurpiers, Mintu Mondal, Marek Pechal, Andreas Wallraff, S. Gasparinetti In circuit QED, faster and more accurate measurement of a qubit's state is necessary to achieve better feedback control, to accomplish more complex quantum algorithms and simulations, and to cross the threshold for fault tolerant quantum computing. In this talk, we discuss our experimental progress to minimize the time needed to readout the state of a dispersively coupled transmon qubit with high fidelity. We outline a signal-to-noise ratio model, illuminate the constraints and find optimal parameters for maximizing measurement speed, while maintaining high readout fidelity. Utilizing a Purcell Filter increases the generality of our results as it becomes possible to reach these speeds with a broader set of system parameters. [Preview Abstract] |
Thursday, March 17, 2016 2:42PM - 2:54PM |
V48.00002: Fast Quantum Nondemolition Readout by Parametric Modulation of Longitudinal Qubit-Oscillator Interaction J\'er\^ome Bourassa, Nicolas Didier, Alexandre Blais For quantum information processing, qubit readout must be fast, of high-fidelity and ideally quantum non-demolition (QND). To rapidly reuse the measured qubit, fast reset of the measurement pointer states is also needed. Combining these characteristics is essential to meet the stringent requirements of fault-tolerant quantum computation. For superconducting qubits, a common strategy is the dispersive readout where the qubit is coupled to an oscillator acting as pointer. In this talk, we present an alternative strategy based on parametric modulation of longitudinal qubit-oscillator interaction. We show that compared to dispersive readout it leads to a faster, high-fidelity and ideally QND qubit readout with a simple reset mechanism [1]. We moreover show how to exponentially improve the signal-to-noise ratio (SNR) of this measurement with the help of single-mode squeezed input state on the oscillator. We present an implementation of this longitudinal parametric readout in circuit quantum electrodynamics along with results using realistic experimental parameters. [1] N. Didier, J. Bourassa and A. Blais, Phys. Rev. Lett., In Print (2015) [Preview Abstract] |
Thursday, March 17, 2016 2:54PM - 3:06PM |
V48.00003: Non-QNDness of Dispersive Measurement in Superconducting Qubits, Part I: Theory Mostafa Khezri, Daniel Sank, Zijun Chen, Rami Barends, Yu Chen, Austin Fowler, Robert Graff, Evan Jeffrey, Julian Kelly, Erik Lucero, Anthony Megrant, Josh Mutus, Pedram Roushan, Ted White, Matthew Neeley, Brooks Campbell, Benjamin Chiaro, Andrew Dunsworth, Charles Neill, Peter O'Malley, Christopher Quintana, Amit Vainsencher, James Wenner, John M. Martinis, Alexander N. Korotkov We theoretically analyze the dispersive measurement of an Xmon qubit in the circuit QED setup at moderately high power, so that the number of photons in the resonator exceeds the so-called critical number by up to an order of magnitude. Our results show an abrupt change of the qubit state when the number of photons reaches a certain threshold, which depends on the detuning between the qubit and the resonator. The simulation results are in agreement with experimental findings for Xmon measurement at moderately high power. We will discuss the physical mechanism causing an abrupt deterioration of the measurement QNDness at the threshold. [Preview Abstract] |
Thursday, March 17, 2016 3:06PM - 3:18PM |
V48.00004: Non-QNDness of Dispersive Measurement in Superconducting Qubits, Part II: Experiment Daniel Sank, Z. Chen, M. Khezri, R. Barends, B. Campbell, Y. Chen, B. Chiaro, A. Dunsworth, A. Fowler, R. Graff, E. Jeffrey, J. Kelly, E. Lucero, A. Megrant, J. Mutus, M. Neeley, C. Neill, P. J. J. O'Malley, C. Quintana, P. Roushan, A. Vainsencher, J. Wenner, T. White, A. Korotkov, J. M. Martinis Modern quantum state measurement in transmon qubits uses the interaction between the qubit and a harmonic oscillator. In the dispersive limit of the interaction, the coupling operator $n \, \sigma_z$ commutes with the qubit Hamiltonian and should be perfectly QND. However, previous experiments have indicated that sufficiently high resonator drive power causes unwanted qubit state transitions, producing errors. We investigate these errors in detail, connect the results with theory, and comment on the implications for quantum computer design. [Preview Abstract] |
Thursday, March 17, 2016 3:18PM - 3:30PM |
V48.00005: Dependence of transmon qubit relaxation rate on readout drive power S.O. Mundhada, S. Shankar, A. Narla, E. Zalys-Geller, S.M. Girvin, M.H. Devoret In circuit QED experiments, microwave drives are applied to the readout mode for qubit measurement, control and to realize various multi-photon processes. These microwave drives have been observed to detrimentally affect the qubit mode by increasing the qubit relaxation rates for both upward and downward transitions. These transitions demolish the qubit state during a measurement, limiting the maximum measurement strength and thus the readout fidelity and speed. Here, we experimentally investigate this effect for transmon qubits coupled to different realizations of the readout mode: 3-dimensional microwave cavities, strip-line resonators and nonlinear readout modes in a waveguide. [Preview Abstract] |
Thursday, March 17, 2016 3:30PM - 3:42PM |
V48.00006: A balanced, superconducting multiplier circuit for fast-switching and multiplexed qubit readout: Design and modeling Eric I. Rosenthal, Benjamin J. Chapman, Brad A. Moores, Joseph Kerckhoff, K. W. Lehnert Superconducting qubits hold great promise for the development of new quantum-information technology. Coherence times of individual transmon qubits in microwave cavities are consistently improving. While qubits are becoming well developed tools, scaling qubit readout for many-qubit architectures remains prohibitively complex and expensive. Here, we present a concept for a multipurpose device that enables time or code domain multiplexing of qubit readout. It is a two-port, microwave device that can be switched rapidly between three modes of operation: transmission, reflection and inversion. The design is based on a Wheatstone bridge-like structure of tunable inductors, which we realize with arrays of SQUIDs. A single bias line modulates the flux through the SQUIDs, and hence the imbalance of the bridge, putting the device in one of its three modes of operation. This talk will discuss the theory, design and layout behind the device and its potential use for multiplexing of qubit networks. The device is designed to operate over a broad bandwidth (4-8 GHz), and to have low dissipation, appropriate for integration with superconducting qubit networks. [Preview Abstract] |
Thursday, March 17, 2016 3:42PM - 3:54PM |
V48.00007: A balanced, superconducting multiplier circuit for fast-switching and multiplexed qubit readout: Performance and demonstration Brad A. Moores, Benjamin J. Chapman, Eric I. Rosenthal, Joseph Kerckhoff, K. W. Lehnert A major challenge of scaling the promising transmon qubits into a quantum information processing machine is the classical hardware burden required to readout many qubits. Within the cavity QED architecture, qubit states are measured by detecting the transmission through microwave cavities. A multiplexing scheme could allow the classical hardware burden of generating and measuring a readout tone to be shared among several cavity-qubit systems. In this talk, we will present measurements of a recently designed superconducting multiplier circuit intended to accomplish time and code domain multiplexed readout. In particular, we characterize three modes of microwave operation: transmission, reflection and inversion. The device can be switched between these modes approximately 100 times faster than typical qubit coherence times. Exploiting this performance, we demonstrate a code domain multiplexing scheme with classical signals created to simulate typical qubit signals. The scheme operates with near unity fidelity at microwave powers comparable to typical qubit tones. [Preview Abstract] |
Thursday, March 17, 2016 3:54PM - 4:06PM |
V48.00008: Frequency and sensitivity tunable microresonator array for high-speed quantum processor readout Emile Hoskinson, J. D. Whittaker, L. J. Swenson, M. H. Volkmann, P. Spear, F. Altomare, A. J. Berkley, B. Bumble, P. Bunyk, P. K. Day, B. H. Eom, R. Harris, J. P. Hilton, M. W. Johnson, A. Kleinsasser, E. Ladizinsky, T. Lanting, T. Oh, I. Perminov, E. Tolkacheva, J. Yao Frequency multiplexed arrays of superconducting microresonators have been used as detectors in a variety of applications. The degree of multiplexing achievable is limited by fabrication variation causing non-uniform shifts in resonator frequencies. We have designed, implemented and characterized a superconducting microresonator readout that incorporates two tunable inductances per detector, allowing independent control of each detector frequency and sensitivity. The tunable inductances are adjusted using on-chip programmable digital-to-analog flux converters, which are programmed with a scalable addressing scheme that requires few external lines. [Preview Abstract] |
Thursday, March 17, 2016 4:06PM - 4:18PM |
V48.00009: Simple quantum trajectories for transmon measurement with moderate bandwidth Alexander N. Korotkov So far, most experiments on the continuous quantum measurement of superconducting qubits in a circuit QED setup have been well described by the Quantum Bayesian formalism, which assumes the "bad cavity limit": the resonator bandwidth is assumed to be much larger than the measurement-induced dephasing. However, in some experiments this assumption is not applicable, and then the Quantum Bayesian formalism should be extended. We discuss a relatively simple generalization to the case of arbitrary resonator bandwidth, which can be applied when there is no significant qubit evolution due to Rabi oscillations. We also discuss how to include Rabi oscillations and energy relaxation into the simulation of quantum trajectories. [Preview Abstract] |
Thursday, March 17, 2016 4:18PM - 4:30PM |
V48.00010: Mapping quantum state dynamics of spontaneous emission Mahdi Naghiloo, Neda Frouzani, Dian Tan, Kater Murch Controlling the dynamics of spontaneous emission for quantum emitters is relevant to many novel applications in quantum computation and quantum optics. In this work we use homodyne measurements of the spontaneous emission of a superconducting qubit to track its quantum evolution. A resonant pulse is used to prepare the qubit in the excited state and the emission from the system into this environment is monitored with a near-quantum-limited Josephson parametric amplifier acting as a homodyne detector. By an appropriately chosen phase of amplification, we execute weak measurements in the $\sigma_x$ basis of the qubit. We use the measurement results to track individual quantum trajectories as the qubit evolves from its excited to ground state, revealing rich dynamics that occur in the process of spontaneous emission. [Preview Abstract] |
Thursday, March 17, 2016 4:30PM - 4:42PM |
V48.00011: Tracking Multi-State Quantum Jumps in a Superconducting Circuit Neda Forouzani, Dian Tan, Mahdi Naghiloo, Kater Murch Quantum measurements are known to be crucial for quantum error-correction and state initialization. Continuous measurements can be used for state tracking and real-time quantum feedback. If the measurements are strong, the state dynamics are described by quantum jumps between states. Using continuous measurements, we track the quantum state of a transmon circuit initially in its lowest energy state. We observe spurious jumps between five observable states of the circuit and use a Bayesian update formalism to estimate state occupation probabilities as well as transition rates over time. Our analysis reveals switching between different quantum jump statistics. Resolving the energy distribution of spurious jumps will help characterize this decoherence process. [Preview Abstract] |
Thursday, March 17, 2016 4:42PM - 4:54PM |
V48.00012: Theory and practice of dressed coherent states in circuit QED Frank Wilhelm, Luke C.G. Govia In the dispersive regime of qubit-cavity coupling, classical cavity drive populates the cavity, but leaves the qubit state unaffected. However, the dispersive Hamiltonian is derived after both a frame transformation and an approximation. Therefore, to connect to external experimental devices, the inverse frame transformation from the dispersive frame back to the lab frame is necessary. We show that in the lab frame the system is best described by an entangled state known as the dressed coherent state, and thus even in the dispersive regime, entanglement is generated between the qubit and the cavity. Also, we show that further qubit evolution depends on both the amplitude and phase of the dressed coherent state. This provides a limitation to readout in the dispersive regime. We show that only in the limit of infinite measurement time is this protocol QND, as the formation of a dressed coherent state in the qubit-cavity system applies an effective rotation to the qubit state. We show how this rotation can be corrected by a unitary operation, leading to improved qubit initialization by measurement and unitary feedback.] L.C.G. Govia and F.K. Wllhelm Phys. Rev. Applied 4, 054001 (2015) LC.G. Govia and F.K. Wilhelm, arXiv: 1506.04997 [Preview Abstract] |
Thursday, March 17, 2016 4:54PM - 5:06PM |
V48.00013: Design and initial tests of a superconducting circulator for quantum microwave systems Benjamin J. Chapman, Eric I. Rosenthal, Brad A. Moores, Joseph Kerckhoff, Kevin Lalumi\`{e}re, Alexandre Blais, K.W. Lehnert Microwave circulators enforce a single propagation direction for signals in an electrical network.~ Unfortunately, commercial circulators are bulky, lossy, and cannot be integrated close to superconducting circuits because they emit large stray magnetic fields.~ Here we report progress toward the development of a lossless, on-chip, active circulator for superconducting microwave circuits in the 4-8 GHz band.~ Non-reciprocity is achieved by actively modulating circuit elements on a slow time scale (10 -- 100 ns). Our circulator's active components are dynamically tunable inductors constructed with arrays of dc-SQUIDs in series. The array inductance is tuned by varying the magnetic flux through the SQUIDs with fields weaker than 1 Oe. Initial tests show that the device exhibits non-reciprocity, but performance is degraded by trapped magnetic flux in the circuit. Nevertheless, the device meets many design goals including a tunable center frequency between 4-8 GHz and a high (-93 dBm) saturation power.~ ~This presentation will describe these tests and a new layout designed to avoid trapped flux. [Preview Abstract] |
Thursday, March 17, 2016 5:06PM - 5:18PM |
V48.00014: Squeezing-enhanced superconducting qubit measurement using driven nonlinear resonators Luke C. G. Govia, Benjamin Levitan, Aashish A. Clerk Dispersive measurement of a superconducting qubit is a key ingredient in many contemporary protocols in circuit quantum electrodynamics, and high measurement fidelity has recently been achieved. However, as the number of qubits on chip and the complexity of protocols increases, so too does the required measurement fidelity. To reach higher fidelity, it has been proposed that squeezed microwave fields injected into the resonator can be used to reduce the noise in the measured field quadrature\footnote{Phys. Rev. Lett. 115, 093604 (2015)}\footnote{Phys. Rev. B 90, 134515 (2014)}. However, creating, preserving, and injecting a squeezed microwave field is a technologically challenging task. Here, we theoretically analyze the dispersive measurement of a qubit coupled to one or more driven nonlinear resonators, which provide an in situ source of microwave field squeezing. This is potentially a more flexible way of harnessing the physics that leads to the increase in measurement fidelity seen for both single-mode and two-mode squeezed states\footnotemark[1], without the drawback of having to independently create and inject these states. [Preview Abstract] |
Thursday, March 17, 2016 5:18PM - 5:30PM |
V48.00015: Closing a quantum feedback loop for a superconducting qubit inside a cryostat Christian Kraglund Andersen, Joseph Kerckhoff, Konrad W Lehnert, Benjamin J Chapman, Klaus Mølmer Several quantum information protocols relies upon efficient feedback (or feed-forward) schemes. Recently, within the field of superconducting qubits, many experiments have shown tremendous progress towards high fidelity quantum feedback scheme. Some experiments work by traditional measurement based schemes where the classical output is processed on a classical "computer" before a signal is fed back to the qubits. Other approaches are working in a continuous coherent manner, where the full quantum description of the system creates an effective bath that relaxes the system into the desired state. This talk will present a different approach that aims to close a measurement based feedback loop inside a cryostat and, thus, the scheme works completely autonomous. This approach sidesteps many of the inefficiencies inherent in two-way communication between temperature stages in typical systems with room temperature controllers, and avoids increasing the cryogenic heat load. This controller may find a broad range of uses in multi-qubit systems, but here I analyze two specific demonstrative cases in single qubit-control and show simulations of the time evolution for the full system dynamics. [Preview Abstract] |
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