Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session L39: Characterizing and Controlling Superconducting Circuits IIFocus

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Sponsoring Units: DQI Chair: Irfan Siddiqi, University of California Berkeley Room: LACC 501B 
Wednesday, March 7, 2018 11:15AM  11:51AM 
L39.00001: Measurement and Control of Superconducting Qubits Using Single Flux Quantum Digital Logic Invited Speaker: Matthew Beck While remarkable progress has been made in the realization of small scale superconducting quantum circuits comprised of order 10 qubits, it is unclear how to scale existing control and readout methods to the millions of qubits required for full implementation of a surface code that can outperform the best available classical computers. In this talk, I describe the development of highfidelity qubit control and measurement based on the superconducting Single Flux Quantum (SFQ) digital logic family. I present data on SFQdriven Rabi oscillations of a superconducting qubit and discuss limits to SFQbased gate fidelity. Additionally, I present a qubit measurement approach based on microwave photon counting. Qubit states are mapped to bright and dark pointer states of a linear resonator followed by subsequent photodetection. Combined, these approaches allow for qubit measurement and control at the millikelvin stage of the cryostat, without the need for microwave pulse shaping, heterodyning, or thresholding at room temperature. These experiments represent a first step toward the integration of a largescale superconducting quantum processor with proximal logic and control, with the potential for significant reductions in latency, wiring heat load and system footprint. 
Wednesday, March 7, 2018 11:51AM  12:03PM 
L39.00002: Qubit Feedback on a FiveQubit Transmon Device Amy Greene, Bharath Kannan, Morten Kjærgaard, Mollie Schwartz, Danna Rosenberg, Jonilyn Yoder, David Kim, Thorvald Larsen, Philip Krantz, Simon Gustavsson, William Oliver Feedbackbased dynamic control of quantum processors is a key component for many mediumtolargescale quantum information processing routines. Several groups have begun addressing this need, often using inhouse, custombuilt hardware. We have implemented realtime feedback for superconducting qubits using scalable, programmable and commercially available hardware from Keysight Technologies. We will use this hardware to implement a multiqubit reset protocol and to demonstrate a threequbit bitflip error detection code on a fivequbit transmon device. I will present our latest experimental progress towards this goal. 
Wednesday, March 7, 2018 12:03PM  12:15PM 
L39.00003: Mitigating Superconducting Qubit Crosstalk via Optimal Control Jérémy Béjanin, Matteo Mariantoni Current superconducting quantum computing experiments have reached a level of complexity where twodimensional lattices of quantum bits (qubits) comprising on the order of 50 qubits can be fabricated and operated. Such architectures are necessary to both implement quantum error correcting codes as well as to perform quantum simulations and machine learning experiments. However, a rectangular qubit arrangement necessarily requires each individual qubit to be coupled to 4 neighbors (qubits or resonators), leading to unwanted interactions or crosstalk. In fact, this frequency crowding often results in qubits idling within 200 MHz of each other, causing conventional single qubit gates to incur errors as large as 2%. By means of numerical simulations, we systematically analyze crosstalk in various physical configurations, and show that pulses generated via GRAPE can eliminate it. 
Wednesday, March 7, 2018 12:15PM  12:27PM 
L39.00004: Superconducting Qubit Control with Single Flux Quantum Pulses: Part I  Fabrication Edward Leonard, JJ Nelson, Matthew Beck, Kenneth Dodge, Caleb Howington, Jaseung Ku, Alex Kirichenko, Daniel Yohannes, Oleg Mukhanov, Britton Plourde, Robert McDermott A largescale superconducting quantum computer will require significant classical resources for operation, including elements for control, measurement, and feedback. A promising candidate technology to reduce both the wiring heat load and overall system footprint of a future control system is the Single Flux Quantum (SFQ) digital logic family. Here we describe the design and fabrication of an onchip SFQ pulse driver for control of a superconducting transmon qubit. We discuss optimization of the process flow to preserve the coherence properties of the qubit. Finally, we describe preliminary steps toward the development of a quantum/classical multichip module (MCM) involving a transmon qubit array on one chip and a classical SFQ pattern generator on a second chip, with the two chips coupled in a flipchip arrangement. 
Wednesday, March 7, 2018 12:27PM  12:39PM 
L39.00005: Coherent Control of a Superconducting Transmon with Single Flux Quantum Pulses: Part II  Experimental JJ Nelson, Edward Leonard, Matthew Beck, Kenneth Dodge, Caleb Howington, Jaseung Ku, Robert McDermott, Britton Plourde We demonstrate control of a superconducting qubit enacted by a Single Flux Quantum (SFQ) driver. An integrated onchip DCSFQ driver emits quantized pulses allowing subharmonic driving of qubit rotations. We compare gate operations between the DCSFQ driver and microwave pulses through the superconducting resonator. Operation of SFQ driven qubit as a function of control parameters will also be presented. 
Wednesday, March 7, 2018 12:39PM  12:51PM 
L39.00006: NonGaussian Noise Spectroscopy With Superconducting Qubits. Part 1: Theory. Felix Beaudoin, Leigh Norris, Youngkyu Sung, Fei Yan, Simon Gustavsson, William Oliver, Lorenza Viola NonGaussian noise can play an important role in qubit decoherence, for example in the presence of twolevel fluctuators or in environments far from thermal equilibrium. While there has been substantial experimental progress in characterizing Gaussian noise, nonGaussian noise spectroscopy has yet to be experimentally implemented. This task is essential for designing robust control and error correction for realistic noisy environments. To accomplish this, a nonGaussian noise characterization method based on dynamical decoupling was introduced in Ref. [1]. Before using these protocols on native noise, it is important to assess them with engineered nonGaussian noise. In this talk, we theoretically describe how to benchmark this method in realistic superconducting qubit systems like the transmon. We also report on progress toward characterizing native nonGaussian sources of noise in these systems. 
Wednesday, March 7, 2018 12:51PM  1:03PM 
L39.00007: NonGaussian Noise Spectroscopy With Superconducting Qubits. Part 2: Experiment Youngkyu Sung, Fei Yan, Felix Beaudoin, Leigh Norris, Lorenza Viola, Simon Gustavsson, William Oliver Most quantum control and quantum errorcorrection protocols assume that the noise causing decoherence is described by Gaussian statistics. However, the Gaussianity assumption breaks down when the system is strongly coupled to a sparse environment, such as those consisting of a few twostate fluctuators. Here, we implement openloop quantum control protocols to characterize both Gaussian and nonGaussian dephasing processes in superconducting qubits. 
Wednesday, March 7, 2018 1:03PM  1:15PM 
L39.00008: Gate set tomography on more than two qubits Erik Nielsen, Robin BlumeKohout, Kevin Young, Mohan Sarovar, Kenneth Rudinger, Timothy Proctor Quantum tomography is often deemed impractical for “large” quantum information processors of more than 2 qubits. Quantum process tomography and gate set tomography (GST) can be, and have been, used to probe the behavior of gates on 12 qubits, but are rarely applied to 3 or more qubits. This stems in large part from the sheer number of parameters that need to be probed in standard gate tomography: O(16^{N}) for N qubits. We present a framework for intelligently reducing this parameter explosion to poly(N) parameters. Using reduced models that capture physically plausible errors (e.g., fewqubit interactions and Pauli stochastic noise), we show how to perform gate set tomography effectively on larger systems. We present results from applying these methods to processors with 3+ qubits. 
Wednesday, March 7, 2018 1:15PM  1:27PM 
L39.00009: Spectroscopic Measurements on Error Protected Superconducting Qubit Design Based on Josephson Junction Arrays Andras Gyenis, Thomas Hazard, Andrei Vrajitoarea, Agustin Di Paolo, Peter Groszkowski, Alexandre Blais, Jens Koch, Andrew Houck Since the first demonstration of quantum coherence in superconducting qubits, the coherence times of the various designs have undergone a significant improvement. This remarkable evolution is mainly the result of the combination of new qubit designs with reduced sensitivity to external noise, engineered qubit environments, advances in materials and fabrication techniques. An alternative approach to further increase qubit coherence times is to harness potential intrinsic protections of certain superconducting circuits. This is the case in the recently proposed zeropi qubit [PRA 87, 052306 (2013)]. In this talk we present a qubit design which is based on the zeropi circuit in a parameter regime which is experimentally achievable by using long Josephson junction arrays. The circuit offers protection against both energy relaxation, due to disjoint support of the qubit subspace and dephasing, due to diminishing flux sensitivity at the sweet points. We describe the symmetric coupling scheme of the qubit used for dispersive readout and demonstrate the rich energy level spectrum of the device. 
Wednesday, March 7, 2018 1:27PM  1:39PM 
L39.00010: Strongly driven Josephson circuits for engineering parametric interactions Raphael Lescanne, Lucas Verney, Quentin Ficheux, Theau Peronnin, Michel Devoret, Benjamin Huard, Mazyar Mirrahimi, Zaki Leghtas An outstanding roadblock towards the emergence of large scale quantum devices is the finite lifetime of the underlying qubits. One possible solution is to use a subtle interplay between coherent drives and nonlinear dissipation to protect and manipulate quantum information. This nonlinear dissipation is engineered through parametric methods where a strong offresonant pump mediates a nonlinear interaction between the system of interest and its bath. The efficiency of this protection scales with the drive strength. In real Josephson circuits, this favorable scaling collapses for a critical drive strength. The purpose of this work is to understand this limitation, and design circuits which circumvent this breakdown. We experimentally investigate the behaviour of two circuit topologies in the presence of strong drives, and discuss the implications for strong qubit readout, and engineered parametric interactions. 
Wednesday, March 7, 2018 1:39PM  1:51PM 
L39.00011: Inductivelyshunted transmon for drivendissipative operations with superconducting circuits Jayameenakshi Venkatraman, Clarke Smith, Shantanu Mundhada, Nicholas Frattini, Angela Kou, Alexander Grimm, Mazyar Mirrahimi, Shyam Shankar, Michel Devoret The transmon, a remarkably versatile superconducting qubit, consists of a small Josephson junction shunted by a large capacitance. This capacitance reduces the susceptibility of the qubit to offset charge fluctuations by localizing the superconducting phase across the junction near zero. However, the finiteheight potential well of the transmon Hamiltonian renders higher excited states unstable due to their nonnegligible runningstate component. The effect associated with the nonfullyconfining nature of the transmon potential may include the experimentally observed limitations on the transmon ac Stark shift, which limits the efficacy of pumping schemes for drivendissipative operation such as error correction. We propose modifying the transmon by additionally shunting the junction with a linear inductance approximated by multiple larger junctions. The loop that is formed is maintained at zero flux. In this circuit, the Josephson energy is comparable to the inductive energy of the shunt inductance. Progress on circuit design and experimental results testing these ideas will be reported. 
Wednesday, March 7, 2018 1:51PM  2:03PM 
L39.00012: Coherent control of two qubits in coaxial circuit QED Andrew Patterson, Joseph Rahamim, Takahiro Tsunoda, Peter Spring, Tanja Behrle, Martina Esposito, Giovanna Tancredi, Peter Leek We demonstrate the extension of a coaxial circuit QED architecture [1] to two qubits, including tune up of twoqubit quantum logic gates based on the crossresonance interaction [2]. 
Wednesday, March 7, 2018 2:03PM  2:15PM 
L39.00013: Control of Superconducting Qubits: Merging Pulse Calibration and System Characterization using Optimal Control Shai Machnes, Frank Wilhelm The current methodology for designing control pulses for superconducting circuits often results in a somewhat absurd situation: pulses are designed using simplified models, resulting in initially poor fidelities. The pulses are then calibrated insitu, achieving highfidelities, but without a corresponding model. We are therefore left with a model we know is inaccurate, working pulses for which we do not have a matching model, and a calibration process from which we learned nothing about the system. We propose a novel procedure to rectify the situation, by merging pulse design, calibration and system characterization: Calibration is recast as a closedloop search for the bestfit model parameters, starting with a detailed, but only partially characterized model of the system. Fit is evaluated by fidelity of a complete set of gates, which are optimized to fit the current system characterization. The end result is a bestfit characterization of the system model, and a full set of highfidelity gates for that model. We believe the new approach will greatly improve both gate fidelities and our understanding of the systems they drive. 
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