Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session S08: Advances in Qubit Measurement IIFocus

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Sponsoring Units: DQI Chair: Eric Rosenthal, University of Colorado, Boulder Room: 104 
Thursday, March 5, 2020 11:15AM  11:27AM 
S08.00001: Asymmetric Sensing Dot for Scaleable Baseband Readout of Spin Qubits Eugen Kammerloher, Matthias Künne, Inga Seidler, Arne Ludwig, Andreas Wieck, Lars Schreiber, Hendrik Bluhm High fidelity scalable readout is one of the key requirements for quantum computers with more than just a few qubits. Charge sensing dots are in this regard the most sensitive sensors for spin qubit readout. The most widespread readout technique is based on RF reflectometry, satisfying the requirement of high fidelity, but requires bulky, powerhungry components and is not well scalable. A more scalable alternative is to use transistors in close proximity to the qubit [1,2]. 
Thursday, March 5, 2020 11:27AM  11:39AM 
S08.00002: A Reservoir Computing Approach to Quantum State Measurement Gerasimos Angelatos, Hakan Tureci, Saeed Khan Quantum state measurement is an essential step in the probe or operation of any quantum system, and its optimization has accordingly been the focus of considerable ongoing research. Increases in the speed and fidelity of continuous measurements directly contribute to quantum information processing applications and the fundamental study of quantum systems. In this work, we propose a hardwarebased reservoir computing system for quantum state measurement and discuss its performance when compared to conventional approaches. We theoretically analyze the readout of a superconducting circuit via direct coupling to a minimal reservoir computer and demonstrate how such a system provides a low latency approach to state measurement. 
Thursday, March 5, 2020 11:39AM  11:51AM 
S08.00003: Neural Network assisted Superconducting Qubit Readout Benjamin Lienhard, Antti Vepsalainen, Luke Govia, Yanjie Qiu, Diego Ristè, Matthew Ware, David K Kim, Roni Winik, Alexander Melville, Bethany Niedzielski, Jonilyn Yoder, Guilhem Ribeill, Thomas A Ohki, Hari K Krovi, Terry Philip Orlando, Simon Gustavsson, William Oliver A significant error source in contemporary quantum processors is qubitstate readout. For a single qubit connected to a unique readout line, a linear matched filter is sufficient for highfidelity readout. However, it is more resource efficient to frequencymultiplex multiple qubits on the same readout line. In this case, the singlequbit matched filters are no longer optimal. Rather, readout discrimination becomes a computationally intensive, multistate classification problem. Here, we present a new approach to the readout problem based on neural networks. We discuss different types of neural network architectures and their readout discrimination performance compared to current readout methods when applied to experimental single and multiqubit readout data. 
Thursday, March 5, 2020 11:51AM  12:03PM 
S08.00004: SingleQubit Optimal Quantum Readout via Neural Networks Wei Tang, Zhaoqi Leng, Andrew Houck, Margaret Martonosi High fidelity readout is an essential part of quantum computing. Conventional approaches for superconducting circuit readout rely on linear models. We explore the benefits of employing alternative classification schemes based on neural networks to improve fidelity. Experimental results of qubit readout will be presented. 
Thursday, March 5, 2020 12:03PM  12:15PM 
S08.00005: Dispersive readout for Majorana qubits Thomas Smith, Stephen D Bartlett, Arne Grimsmo We analyze the use of dispersive readout for qubits encoded in topological superconducting nanowires. Two models are considered: the Majorana transmon qubit and the Majorana box qubit. We model the interaction with each qubit and a readout resonator, and calculate the size of the qubit statedependent dispersive shift of the resonator. We show that dispersive readoutu of Majorana qubits is protected against measurementinduced bit flips. Furthermore, we find that Majorana transmon qubits are wellsuited to dispersive readout, producing dispersive shifts comparable to that of conventional superconducting transmons. Majorana box qubits produce more modest, but still potentially viable, dispersive shifts. 
Thursday, March 5, 2020 12:15PM  12:27PM 
S08.00006: Tracking nonMarkovian quantum dynamics of a superconducting qubit with a recurrent neural network filter Noah Stevenson, Gerwin Koolstra, Bradley Mitchell, Akel Hashim, Shiva Barzili, Justin Dressel, Irfan Siddiqi Precise quantum control of superconducting qubits necessitates determining the timedependent Hamiltonian of control pulses with high fidelity. While continuous state tracking has proved effective for determining qubit timeevolution in regimes with Markovian dynamics, fast control pulses used for native quantum gates and entanglement generation can result in nonMarkovian transient dynamics. 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 qubit state during a Rabi oscillation sequence on a timescale comparable to the cavity decay rate, we isolate dynamics that are difficult to describe with singlequbit trajectory theory. We train a recurrent neural network to reconstruct the quantum trajectories, motivated by such a network's demonstrated ability to learn longtime correlations in sequential data, and estimate parameters of the stochastic master equation. 
Thursday, March 5, 2020 12:27PM  12:39PM 
S08.00007: Highfidelity quantum state estimation via autoencoder tomography Shiva Lotfallahzadeh Barzili, Noah Stevenson, Bradley Mitchell, Razieh Mohseninia, Irfan Siddiqi, Justin Dressel

Thursday, March 5, 2020 12:39PM  12:51PM 
S08.00008: Characterization and tomography of a hidden qubit Marek Pechal, Marc Ganzhorn, Max Werninghaus, Daniel Egger, Gian Salis, Stefan Filipp In circuitbased quantum computing it is typically assumed that the available gate set consists of single qubit gates acting on each individual qubit as well as an entangling gate between pairs of qubits. In some physical architectures, not all qubits may be addressable, but some may be hidden and only connected to another control qubit. In this case, no single qubit operations can be applied to the hidden qubit and its state cannot be measured directly, but entangling gates with the control qubit can be carried out. Tomography of the combined twoqubit system is still possible on the combined twoqubit system whenever an iSWAPtype interaction in combination with a controlledphase gate and single qubit operations on the control qubit is available. In our experiment we use transmontype superconducting qubits along with parametric tunablecoupler gates to realize both types of twoqubit interactions. We further discuss the tuneup process required to completely characterize the gate set used for tomography and we evaluate the resulting fidelities. 
Thursday, March 5, 2020 12:51PM  1:03PM 
S08.00009: Quantum Rifling: Protecting a Qubit from Measurement Backaction Daniel B Szombati, Alejandro Gomez Frieiro, Clemens Mueller, Tyler Jones, Markus Jerger, Arkady Fedorov Quantum mechanics postulates that measuring the qubit's wave function results in its collapse, with the recorded discrete outcome designating the particular eigenstate the qubit collapsed into. We show this picture breaks down when the qubit is strongly driven during measurement. More specifically, for a fast evolving qubit the measurement returns the timeaveraged expectation value of the measurement operator, erasing information about the initial state of the qubit, while completely suppressing the measurement backaction. We call this regime `quantum rifling', as the fast spinning of the Bloch vector protects it from deflection into either of its two eigenstates. We study this phenomenon with two superconducting qubits coupled to the same probe field and demonstrate that quantum rifling allows us to measure either one of the two qubits on demand while protecting the state of the other from measurement backaction. Our results allow for the implementation of selective read out multiplexing of several qubits, contributing to efficient scaling up of quantum processors for future quantum technologies. 
Thursday, March 5, 2020 1:03PM  1:39PM 
S08.00010: Rapid gatebased readout of spins in silicon using an onchip resonator Invited Speaker: guoji zheng Over the past decade tremendous progress has been made on spin qubits based on electron spins in silicon quantum dots. As with any qubit implementation, a critical requirement is the ability to read out the qubit rapidly, with high fidelity, and in a scalable manner. Much attention has been focused on improving singleelectron transistors embedded in radiofrequency reflectometry circuits as charge detectors to detect, in combination with a spintocharge conversion scheme, electron spin states. While these are the most sensitive detectors to date, they come with additional resources that take up valuable space near the quantum dots (gate electrodes, electron reservoirs), which makes scaling up to twodimensional spin qubit arrays difficult. More efficiently, readout can be performed utilizing gates that are already in place for defining quantum dots by connecting those gates to a resonant circuit. This promising method of gatebased sensing has been developed for quantum dots with offchip resonators, and only very recently achieved the sensitivity necessary for singleshot readout of spins in silicon [1]. In this talk, I will describe the use of an onchip superconducting microwave resonator to improve the sensitivity, aided by its high quality factor and impedance. Using Pauli Spin Blockade as the spintocharge conversion scheme, we demonstrate the gatebased readout of a twoelectron spin state in a single shot with an average fidelity of 98% in only 6 microseconds [2]. Furthermore, our latest work towards longdistance spinspin coupling will be presented. 
Thursday, March 5, 2020 1:39PM  1:51PM 
S08.00011: Improving Multilevel Qudit Readout Fidelity During Relaxation Events via Hidden Markov Models Luis Martinez, Yaniv J. Rosen, Jonathan L. DuBois Quantum state determination with high fidelity is a requirement for quantum computation. However, qubit relaxation imposes a limiting constraint on readout fidelity. Longer readout measurement times increase the distinguishability between the qudit states, however state decay during a long readout pulse can obfuscate the measurement and result in misclassification of qudit state. To overcome this constraint, we demonstrate highfidelity multistate readout by detecting qubit relaxation with Hidden Markov Models on LLNL’s Quantum Design and Integration Testbed (QuDIT). The ability of Hidden Markov Models to account for relaxation and thermal excitation processes allows for longer readout times, extraction of transition probabilities, and higher readout fidelity. 
Thursday, March 5, 2020 1:51PM  2:03PM 
S08.00012: Continuous Indirect detection of stabilizers for quantum error correction YiHsiang Chen, Todd Brun Measuring highweight operators is an important problem in quantum computation. The conventional procedure to measure a highweight operator involves multiple pairwise unitary operations, which may require a large number of quantum gates. We provide an alternative method to passively detect the value of an operator. This approach involves joint interactions between the system and continuouslymonitored ancillary qubits. The continuous measurement outcomes of the monitor qubits reveal information about the values of the stabilizer generators. This information can be retrieved using an estimator, which is driven by the measurement outcomes. We also show that there is a more efficient way to read out the outcomes directly from the time average of the signals. The interaction Hamiltonian can use only twolocal operators, based on techniques similar to perturbative gadgets. We apply this indirect detection scheme to the fourqubit BaconShor code, where the two stabilizers are indirectly monitored using four ancillary qubits. Since it is an errordetecting code, we show that nonMarkovian errors, e.g., the Hamiltonian 1/f noise, can be suppressed by the indirect detection. This detection scheme could be implemented in nearterm experimental systems and operate in real time. 
Thursday, March 5, 2020 2:03PM  2:15PM 
S08.00013: Diagnosing Errors in Quantum Gates Using Continuous Measurements John Steinmetz, Andrew N Jordan We use continuous weak measurements to track a quantum gate operation as it develops in time, which allows us to identify the full timedependent dynamics of any systematic errors. We account for measurement backaction such that it has no bearing on the error estimate. This diagnostic method is tested on imperfect single and twoqubit gates, and is shown to accurately extract known timedependent error pulses. This offers significant advantages over traditional quantum process tomography, as it provides the ability to identify the origin and nature of any deviations from the desired evolution, which can give insight into how to modify the gate pulse. 
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