Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session R35: Readout and Measurement of Superconducting QubitsFocus

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Sponsoring Units: DQI Chair: Zlatko Minev, Yale Univ Room: BCEC 205B 
Thursday, March 7, 2019 8:00AM  8:12AM 
R35.00001: Correcting measurement errors in multiqubit circuits Sarah Sheldon, Sergey Bravyi, Abhinav Kandala, David Christopher McKay, Jay Gambetta Reducing measurement errors is critical for performing any quantum algorithm, especially those requiring feedback. In certain situations, e.g. in quantum experiments that only require averaged measurements, errors can be corrected using postprocessing techniques. These experiments include any that only require operator expectation value measurements, such as tomography or variational quantum eigensolvers. Here we will present one such method, which involves calibrating these errors via measurements of prepared computational states which are used to feed a neural network. In this way we reduce the experimental overhead from exponential to polynomial scaling. We will present experimental demonstrations of this technique on IBM Q devices. 
Thursday, March 7, 2019 8:12AM  8:24AM 
R35.00002: Fast dispersive readout of superconducting qubits for faulttolerant quantum computing Cornelis Christiaan Bultink, Rene Vollmer, Nandini Muthusubramanian, Marc Beekman, Michiel Adriaan Rol, Brian M Tarasinski, Leonardo DiCarlo Faulttolerant quantum computing requires repetitive highfidelity quantum parity measurements. In quantum processors based on circuit QED, the fidelity of indirect quantum parity measurements using an ancillary qubit is compromised by errors induced in the coherent interaction step and in the ancilla measurement. Here, we improve upon the state of the art by combining two techniques to reduce ancilla measurement time and measurementinduced crossdephasing of data qubits: dedicated Purcell filtering for each qubit and active photon depletion. We find the parity measurement speed limit by minimizing the parity measurement error rate as a function of the cycle time. We individually quantify the error contributions of one and twoqubit gates, residual interactions, crosstalk, parasitic measurementinduced dephasing, and quantum demolition. 
Thursday, March 7, 2019 8:24AM  8:36AM 
R35.00003: Crosstalk between transmons during multiqubit readout. Zhenyi Qi, Maxim Vavilov, Kostyantyn Kechedzhi Highfidelity readout of the state of a qubit is an indispensable part of a quantum computer. Progress in implementation of controllable largescale quantum computers calls for implementation of a scalable high fidelity multi qubit readout scheme. In a typical transmon superconducting qubit, measurement is implemented by applying a microwave drive to a readout resonator dispersively coupled to the qubit. Recent experiments demonstrated that despite detuning between the drive and qubit frequencies, measurement renormalizes the spectrum of the combined qubitresonator system and induces crosstalk between nearby qubits resulting in reduced fidelity. A starting point is to develop an optimized readout scheme for a twoqubit system. With both numerical and approximate analytical analysis, we were able to identify the source of spurious transitions and explore qubit parameters optimal for scalable multiqubit readout scheme. 
Thursday, March 7, 2019 8:36AM  9:12AM 
R35.00004: Quantum measurement in superconducting qubits Invited Speaker: Michael Hatridge Highfidelity, quantum nondemolition qubit measurement is a vital prerequisite for robust, largescale quantum machines. In superconducting quantum circuits, the typical information carriers for qubit readout are coherent states of light, which must be amplified before they can be efficiently recorded in roomtemperature electronics. Typically, these amplifiers consist of one, or two, microwave modes linked by a parametrically driven coupling. Such amplifiers regularly approach the quantum limit for amplification, allowing us to closely track qubits’ states. However, conventional parametric amplifiers lack almost every other desirable property, including high saturation power, large bandwidth, and directional operation. I'll discuss our recent efforts to address these shortcomings by suppressing unwanted terms in the device's Hamiltonian and combining multiple, simultaneous parametric drives between a pair of microwave modes. In a single device by varying the parametric drives, we can produce desired behaviors including transmissiononly phasesensitive amplification, input match, and gainindependent bandwidth. We have used observation of quantum jumps and weak measurements of superconducting qubits to benchmark the device's performance. I will discuss the prospects for adding the final desired property, directionality, via further parametric couplings to a third microwave mode. Another route to higher measurement fidelity is to replace coherent states with squeezed light as the information carrier. I will present data from a recent experiment which uses an interferometric scheme for qubit readout with twomode squeezed light, achieving a voltage signaltonoise ratio improvement of ~25 % versus coherent state readout. I will also discuss the prospects for using twomode squeezed light to remotely entangle distant qubits. 
Thursday, March 7, 2019 9:12AM  9:24AM 
R35.00005: A tunable Purcell filter design for multiplexed qubit readout Gabriel ÉthierMajcher, Alireza NajafiYazdi High fidelity and multiplexed qubit readout is a key ingredient to the realization of a quantum processor. One factor limiting the readout fidelity of a superconducting qubit is photon leakage into the readout line which reduces the qubit lifetime ($T_1$). To prevent this effect, Purcell filters can be used to suppress readout line modes at the qubit frequency, while keeping maximal transmission at the readout frequency. 
Thursday, March 7, 2019 9:24AM  9:36AM 
R35.00006: Qubit Measurement by MultiChannel Driving Joni Ikonen, Jan Goetz, Jesper Ilves, Aarne Keränen, Andras Gunyho, Matti Partanen, Kuan Yen Tan, Leif Grönberg, Visa I Vesterinen, Slawomir Simbierowicz, Juha Hassel, Mikko Möttönen We theoretically propose and experimentally implement a method to measure a qubit by driving it close to the frequency of a dispersively coupled bosonic mode. The separation of the bosonic states corresponding to different qubit states begins essentially immediately at maximum rate, leading to a speedup in the measurement protocol. Also the bosonic mode can be simultaneously driven to optimize measurement speed and fidelity. 
Thursday, March 7, 2019 9:36AM  9:48AM 
R35.00007: Fast Measurement of a Tunable Superconducting Flux Qubit via a Driven Nonlinear Resonator with Applications in Quantum Annealing Daniel Tennant, Denis Melanson, Antonio Martinez, Muhammet Ali Yurtalan, Yongchao Tang, David K Kim, Alexander Melville, Bethany M Niedzielski, Jonilyn L Yoder, Steven Weber, Adrian Lupascu Development of readout schemes of flux qubits in the persistent current basis is important for quantum annealing applications. High fidelity detection, controlled backaction on the qubit, as well as rapid acquisition, are all relevant for flexible quantum annealing. In order to accommodate these requirements, we develop a readout based on the use of metastable oscillation states in a nonlinear oscillator detection circuit specifically tailored for capacitively shunted flux qubits. In this work, we discuss the design as well as preliminary results for a device specifically designed for tunable capacitively shunted flux qubits specifically designed for use in quantum annealers. 
Thursday, March 7, 2019 9:48AM  10:00AM 
R35.00008: Realizing a CatchDisperseRelease readout of a qubit Peronnin Theau, Danijela Markovic, Quentin Ficheux, Zaki Leghtas, Benjamin Huard Fast readout is an essential piece of measurement based error correction codes. The usual technique of driving a dispersively coupled resonator presents some limitations such as finite reset time. To overcome these limits Sete and al. [1] proposed a catch, disperse and release scheme that we recently realized. 

R35.00009: ABSTRACT WITHDRAWN

Thursday, March 7, 2019 10:12AM  10:24AM 
R35.00010: Tracking nonMarkovian quantum dynamics of a superconducting qubit with a recurrent neural network filter Noah Stevenson, Bradley Mitchell, Shiva Barzili, Razieh Mohseninia, Justin G. Dressel, Irfan Siddiqi Determining the timedependent Hamiltonian for control pulses of superconducting quantum circuits is critical for their use in reliable quantum information processing; however, interactions between coupled qubits and nearby resonators can cause transient dynamics to become nonMarkovian. We use quantum state tracking with continuous weak measurement to experimentally investigate nonMarkovianity in a transmon superconducting qubit coupled to a readout resonator. By weakly measuring the state of the transmon qubit undergoing tunable Rabi oscillations comparable to the cavity linewidth, we isolate dynamics that are difficult to describe with singlequbit trajectory theory. We address this difficulty by training a recurrent neural network to reconstruct the quantum trajectories, motivated by such a network's demonstrated ability to learn longtime correlations in sequential data. Here we detail the experimental protocol and present preliminary data. 
Thursday, March 7, 2019 10:24AM  10:36AM 
R35.00011: Quantum nondemolition detection of single itinerant microwave photons John Mark Kreikebaum, Kevin P O'Brien, Baptiste Royer, Arne Grimsmo, Alexandre Blais, Irfan Siddiqi The detection of microwave photons is an important capability for superconducting quantum information processing and microwave quantum optics but remains challenging due to the small energy of photons at this frequency. Our circuit quantum electrodynamics (cQED) based detector [1] exploits the superradiant ‘bright’ and subradiant ‘dark’ states that are formed when transmons are coupled an appropriate distance from each other on a waveguide [2]. Detuning each transmon inhomogeneously from the operating frequency leads to coupling of the bright and dark states which allows for absorbed photons to be trapped for longer than the inverse of the absorption bandwidth. We utilize this long interaction time to achieve highfidelity measurements of the photon number in the ensemble. Using a single photon source, we benchmark the performance of this protocol. 
Thursday, March 7, 2019 10:36AM  10:48AM 
R35.00012: Pulsed reset protocol for fixedfrequency superconducting qubits Daniel Egger, Max Werninghaus, Marc Ganzhorn, Gian Salis, Andreas Fuhrer, Peter Mueller, Stefan Filipp Improving coherence times of quantum bits is a fundamental challenge in the field of quantum computing. With longlived qubits it becomes, however, inefficient to wait until the qubits have relaxed to their ground state after completion of an experiment. Moreover, for errorcorrection schemes it is important to rapidly reinitialize syndrome qubits. We present a simple pulsed qubit reset protocol based on a twopulse sequence. A first pulse transfers the excited state population to a higher excited qubit state and a second pulse into a lossy environment provided by a lowQ transmission line resonator, which is also used for qubit readout. We show that the remaining excited state population can be suppressed to 1.7 + 0.1\% and that this figure may be reduced by further improving the pulse calibration. We also show that the reset protocol can be used for cooling by removing the thermal qubit population. 
Thursday, March 7, 2019 10:48AM  11:00AM 
R35.00013: Diagnostic SingleQubit Gate Monitoring with Continuous Measurements John Steinmetz, Andrew N Jordan The physical implementation of a quantum gate generally deviates from the desired qubit operation. In order to improve the gate fidelity, we use continuous weak measurements to track the quantum map as it develops in time, so as to identify the origin of any deviations from the desired evolution. This gives insight into how to modify the qubit design as well as the controls applied. We test this diagnostic method on single qubit gates with both correlated and uncorrelated noise. 
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