Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session X08: Qubit Coherence and Noise Characterization 
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Sponsoring Units: DQI Chair: Kevin Young, Sandia National Laboratories Room: 104 
Friday, March 6, 2020 11:15AM  11:27AM 
X08.00001: Anomalous Charge Noise in Superconducting Qubits Chris Wilen, Bradley Christensen, Alexander M Opremcak, Ameya Riswadkar, Lara Faoro, Lev B Ioffe, Robert F McDermott We describe experiments to probe fluctuations in offset charge in weakly chargesensitive superconducting qubits that depart from the transmon regime. We explore a range of device geometries to elucidate the surprising scale dependence of charge noise in these qubits, and we describe experiments to investigate possible spatial correlation in charge fluctuations in neighboring devices. Since quantum error correction schemes such as the twodimensional surface code rely on the assumption of uncorrelated noise, these studies could have implications for scaleup towards faulttolerant superconducting qubit arrays. 
Friday, March 6, 2020 11:27AM  11:39AM 
X08.00002: Phonon traps reduce the quasiparticle density in superconducting circuits Francesco Valenti, Fabio Henriques, Thibault Charpentier, Marc Lagoin, Clement Gouriou, Maria Martínez, Laura Cardani, Marco Vignati, Lukas Gruenhaupt, Daria Gusenkova, Julian Ferrero, Sebastian T. Skacel, Wolfgang Wernsdorfer, Alexey V. Ustinov, Gianluigi Catelani, Oliver Sander, IoanMihai Pop Out of equilibrium quasiparticles (QPs) are a main source of decoherence in high impedance superconducting quantum circuits. Despite significant progress in the understanding of QP dynamics, pinpointing their origin and decreasing their density remain outstanding tasks. The cyclic process of recombination and generation of QPs implies the exchange of phonons between the superconducting film and the underlying substrate. Reducing the number of substrate phonons with frequencies above the spectral gap of the superconductor should result in a reduction of QPs [1]. Indeed, we demonstrate that surrounding high impedance resonators made of granular aluminum (grAl) with lower gapped thin film aluminum islands increases the internal quality factors of the resonators in the single photon regime, suppresses the noise, and reduces the rate of observed QP bursts [2]. The aluminum islands are positioned far enough from the resonators to be electromagnetically decoupled, thus not changing the resonator frequency, nor the loading. We therefore attribute the improvements observed in grAl resonators to phonon trapping at frequencies close to the spectral gap of aluminum, well below the grAl gap. 
Friday, March 6, 2020 11:39AM  11:51AM 
X08.00003: Photonassisted chargeparity switches in superconducting qubits Kyle Serniak, Manuel Houzet, Gianluigi Catelani, Spencer Diamond, Max Hays, Valla Fatemi, Michel H. Devoret, Leonid Glazman Here we identify a mechanism by which stray photons, with energy large enough to break Cooper pairs, can cause decoherence in superconducting qubits. Similar to decoherence induced by steadystate nonequilibrium quasiparticles, these photonassisted tunneling (PAT) processes are associated with a change in the charge parity of the qubit. This enables the separation of these quasiparticlerelated processes from other contributions to decoherence. Our theory predicts similar rates of PATinduced relaxation and excitation of the qubit, in agreement with recent experiments. This talk will detail the similarities and differences between PAT processes and decoherence induced by nonequilibrium quasiparticle tunneling. 
Friday, March 6, 2020 11:51AM  12:03PM 
X08.00004: Microscopic charging and ingap states in superconducting granular aluminum Fang Yang, Tim Storbeck, Thomas Gozlinski, Lukas Gruenhaupt, IoanMihai Pop, Wulf Wulfhekel Granular aluminum (grAl) is a viable material for highimpedance quantum circuits [1], thanks to its high kinetic inductance, low microwave frequency losses [2], and fabrication by deposition of pure aluminum in an oxygen atmosphere. The material’s microstructure – pure aluminum grains in non stoichiometric aluminumoxide – can be modeled as a Josephson junction array [3], in which the oxide thickness is controlled by the oxygen partial pressure during deposition. Here we present scanning tunneling microscope measurements of the local electronic structure of superconducting grAl and we confirm an increased superconducting gap in the grains of films with ρ ≈ 300 μΩcm and ρ ≈ 2000 μΩcm. In the high resistivity film, we find Coulomb charging effects, a first indication for grain decoupling, and ingap states on individual grains, which could contribute to flux noise and dielectric loss not only in devices employing grAl, but also in pure aluminum circuits, where such states could form in Josephson junctions or in oxidized aluminum surfaces. 
Friday, March 6, 2020 12:03PM  12:15PM 
X08.00005: Probing nonequilibrium quasiparticle populations in superconducting quantum circuits. James Farmer, Darian Hartsell, Haimeng Zhang, Evangelos Vlachos, Eli M LevensonFalk Nonequilibrium quasiparticle (QP) populations in superconducting quantum circuits are a limiting source of decoherence. When properly phase biased, nanobridge Josephson junctions provide subgap bound states for QPs and so function as QP traps [1]. The occupation of a trap state by a QP alters the inductive contribution of the junction to its host circuit. Thus, the population of QPs trapped in the junction can be directly measured by probing the resonance of a capacitively shunted nanobridge with standard superconducting qubit hardware. By measuring QP trapping in real time and performing statistical analyses, one can infer information about the QP generation, annihilation, and trapping mechanisms. We discuss these singleshot fidelity measurements of QPs trapping in the junctions of a nanobridge SQUID embedded in a superconducting resonator. We further discuss the use of our device as a tool for measuring QP properties and for testing methods of QP mitigation. [1] E. M. LevensonFalk, F. Kos, R. Vijay, L. Glazman, and I. Siddiqi, Phys. Rev. Lett. 112, 047002 (2014). 
Friday, March 6, 2020 12:15PM  12:27PM 
X08.00006: Improved material loss measurement using cryogenic resonator testbed Haozhi Wang, Corey Rae McRae, Keegan Mullins, Josh Mutus, David Fork, David Pappas Loss measurements are critical for the superconducting qubit community as the coherence of quantum systems are limited by loss through energy relaxation. The standard procedure for measuring loss is to design and fabricate resonators that incorporate the materialundertest and measure the temperature, power, and geometry dependence. By analyzing the transmission S_{21} data on complex plane, it is possible to extract the internal quality factor, Q_{i}, i.e. the inverse of the loss tangent of the material. However, there are several variations of experimental setup, measurement and fitting algorithms used in the field. This results in uncertainty when comparing Q_{i }values between research groups. In this talk, we will compare the results of different measurement configurations, especially for input and output line filtering and isolation, and quantify the accuracy of loss measurement in order to identify best practices for resonator measurement and setup procedures. 
Friday, March 6, 2020 12:27PM  12:39PM 
X08.00007: Detection of nonequilibrium quasiparticles in charge sensitive transmons Cihan Kurter, Martin O Sandberg, Vivekananda Adiga, Zlatko Minev, Aaron Finck, Andrew Eddins, Robert M Shelby, Markus Brink, Jerry M. Chow Nonequilibrium quasiparticles are considered possible sources causing decoherence in superconducting qubits. Thus, exploring the dynamics of quasiparticles is crucial to understand their mechanism and how to minimize their presence in superconductors. Here, we study charge parity fluctuations in 2D transmon qubits with sufficiently strong coupling and low E_{J}/E_{C} ratio. We will show measurements of density of quasiparticles on various qubits to discuss impact on coherence. 
Friday, March 6, 2020 12:39PM  12:51PM 
X08.00008: Experimental study of flux noise in nanowire transmons subject to an applied magnetic field Thijs Stavenga, Florian Luthi, Elmore Vaal, Oluwatumininu Aluko, Peter Krogstrup, Leonardo DiCarlo Flux noise is generally the dominant dephasing mechanism in transmon qubits using SQUID loops as a tunable inductive element. Continuing experimental research ambitions to elucidate the microscopic origin and mitigation of flux noise. In this work, we experimentally investigate flux noise using an unconventional knob for circuit QED: an applied magnetic field. We first present various changes introduced to our circuit QED system to further extend the field compatibility of resonators and nanowire transmons beyond 70 mT. Next, we use standard coherence measurements and sensitivity analysis on and off flux sweetspots to investigate the dependence of flux noise on field applied along the axis of the two constituent nanowire junctions. 
Friday, March 6, 2020 12:51PM  1:03PM 
X08.00009: Surface spin induced 1/f flux noise dependent on SQUID geometry (1) Leon Ding, Jochen Braumueller, Antti Vepsalainen, Youngkyu Sung, Morten Kjaergaard, Tim Menke, Roni Winik, David K Kim, Bethany Niedzielski, Alexander Melville, Jonilyn Yoder, Cyrus F. Hirjibehedin, Terry Philip Orlando, Simon Gustavsson, William Oliver Magnetic flux noise is the dominant source of dephasing in tunable superconducting qubits. While the mechanism behind this 1/f flux noise is poorly understood, it has been proposed that it originates from random fluctuations of spin impurities located on the surface of SQUIDs. A previously proposed microscopic model predicts that flux noise increases with the perimeter and decreases with the wire width of the SQUID loop. Here, we demonstrate the validity of this model by measuring the flux noise amplitudes of about 50 capacitively shunted flux qubits over a wide range of geometric SQUID parameters. Our results show good agreement with the proposed model assuming independent spins, and may therefore serve as a guide on how to improve SQUID designs to minimize 1/f flux noise. 
Friday, March 6, 2020 1:03PM  1:15PM 
X08.00010: Surface spin induced 1/f flux noise dependent on SQUID geometry (2) Jochen Braumueller, Leon Ding, Antti Vepsalainen, Youngkyu Sung, Morten Kjaergaard, Tim Menke, Roni Winik, David K Kim, Bethany Niedzielski, Alexander Melville, Jonilyn Yoder, Cyrus F. Hirjibehedin, Terry Philip Orlando, Simon Gustavsson, William Oliver The dominant source of decoherence in frequency tunable superconducting qubits is 1/f flux noise, presumably originating from local magnetic spin impurities located at the surface of their SQUID loops. Here, we measure the flux noise amplitudes of capacitively shunted flux qubits and study their dependence on geometric parameters of their SQUID loops. Our data show good agreement with a previously presented microscopic model for independent spin impurities which has so far eluded experimental verification. We discuss limitations of the proposed model for superconducting films of finite thickness and provide numerical simulations of the current distribution in our SQUIDs, which can refine the considered model. Finally, we apply and confirm our results by observing consistently low flux noise amplitudes in samples with optimized SQUID parameters – small perimeters and large wire widths. 
Friday, March 6, 2020 1:15PM  1:27PM 
X08.00011: Characterizing nonclassical nonGaussian noise: photon shot noise fluctuations Yuxin Wang, Aashish Clerk Understanding how to accurately describe environmental noise is of crucial importance for designing control protocols that enable highfidelity quantum operations. While standard approaches often assume classical Gaussian noise, going beyond these assumptions is important in many settings. Here, we consider both the nonclassical and nonGaussian nature of photon number fluctuations in a driven dissipative quantum resonator; such fluctuations can be a significant limitation on the performance of superconducting qubits. We extend the Keldysh approach for characterizing this quantum noise [1,2] to calculate its frequencyresolved third cumulant, the socalled bispectrum. We discuss how the quantum noise bispectrum directly reveals features of nonclassicality and nonequilibrium bath dynamics. We also show how this quantum bispectrum could be measured using standard experimental tools in circuit QED. 
Friday, March 6, 2020 1:27PM  1:39PM 
X08.00012: Identifying Noise Sources via Multilevel Quantum Noise Spectroscopy Youngkyu Sung, Antti Vepsalainen, Jochen Braumüller, Fei Yan, Joel Wang, Morten Kjaergaard, Roni Winik, Philip Krantz, Andreas Bengtsson, David K Kim, Alexander Melville, Bethany Niedzielski, Mollie Schwartz, Jonilyn Yoder, Terry Philip Orlando, Simon Gustavsson, William Oliver Identifying dominant sources of noise and mitigating or eliminating them is crucial for engineering robust quantum devices. Although existing quantum noise spectroscopy (QNS) protocols provide insight into noise sources by measuring the noise power spectral density, they do not directly identify the physical origin of the noise process. Here, we develop and experimentally validate a multilevel QNS protocol using a superconducting transmon qubit as a spectrometer. This technique has two notable implications: First, it expands the applicability and spectral range of a qubit spectrometer beyond the twolevel approximation. Second, it can be applied to higher excited levels, which enables us to extract and identify the noise contributions from multiple noise sources. 
Friday, March 6, 2020 1:39PM  1:51PM 
X08.00013: Measuring effective temperatures of qubits using correlations Anatoly Kulikov, Rohit Navarathna, Arkady Fedorov Initialization of a qubit in a pure state is a prerequisite for quantum computer operation. Qubits are commonly initialized by cooling to their ground states through passive thermalization or by using active reset protocols [1, 2]. To accurately quantify quality of the initialization and to shed light on origin of spurious excitation one requires an accurate tool to measure the excited state population [3]. We propose a new technique of finding the excited state population of a qubit using measurement correlations in time. We experimentally implement the proposed method using a circuit QED platform and compare its performance with previously developed techniques [4]. The method does not rely on having highfidelity measurement or the higher energy levels. We measure the spurious qubit population with accuracy of up to ≈0.01% and discuss its possible sources. 
Friday, March 6, 2020 1:51PM  2:03PM 
X08.00014: Photon Shot Noise Limited Charge Sensitivity in a Hybrid Quantum System Sisira Kanhirathingal, Miles Blencowe, Benjamin Brock, Alexander J Rimberg Hybrid mesoscopic quantum systems serve as essential building blocks for quantum information and metrology devices. One such system that displays a rich variety of behavior in the quantumclassical regime is a Cooper pair transistor embedded in a quarter wave cavity (cCPT). As one application, this device can act as a highly sensitive charge detector, and form the basis for an optomechanical system in the single photon strong coupling regime. We present a theoretical analysis of the photon shot noise limited charge sensitivity of the device, assuming a low average photon number drive. This lower bound is difficult to achieve using conventional detection approaches, owing mainly to the low frequency noise caused by the coupling of thermally activated twolevelfluctuators to the cCPT. The effects of this interaction are observed as 1/f noise in the microwave cavity fundamental resonance frequency. Using careful calibration of a feedback mechanism called PoundDreverHall (PDH) locking, we should be able to observe this noise near the low photon limit. Moreover, a modified PDH scheme may possibly suppress this noise, and achieve charge sensitivity close to our theoretical, minimum shot noise prediction. 

X08.00015: Characterization of noise correlations in superconducting quantum circuits Tenghui Wang, Feng Wu, Jingwei Zhou, Hao Deng, Gengyan Zhang, Xun Jiang, Wenlong Yu, HsiangSheng Ku, Chunqing Deng Superconducting quantum circuits have been an important platform to realize a scalable quantum computing system. Qubit decoherence, which is caused by the fluctuations of its environment, remains a main obstacle to realize faulttolerant quantum computing. To reduce decoherence, it is important to identify the source of fluctuations by precisely characterizing its correlations. Prevalent methods only consider the second order correlation under the Gaussian approximation. However, environment with higher order correlations is not uncommon in mesoscopic quantum systems. Based on a weak measurement scheme ^{[1]}, we present an experiment to detect highorder correlations in a superconducting circuit. We verified the scheme by reconstructing the noise power spectral density and compared the results with that obtained from dynamical decoupling. We also demonstrated the characterization of higherorder correlation of the environment. 
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