Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session J28: Superconducting Qubits: Coherence and TwoLevel SystemsLive

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Sponsoring Units: DQI Chair: Robert McDermott, University of Wisconsin  Madison 
Tuesday, March 16, 2021 3:00PM  3:12PM Live 
J28.00001: Distinguishing and Mitigating Decoherence Mechanisms in Transmon Qubits Kyle Serniak, Greg Calusine, Alexander Melville, Wayne Woods, Thomas Hazard, Bethany Niedzielski, David K Kim, Jonilyn Yoder, William Oliver Extending the coherence times of superconducting qubits relies on iterative improvement of materials, fabrication processing, and design in order to distinguish and subsequently mitigate the numerous sources of decoherence. In this talk, we will outline the characterization tools that we use to assess many of the dominant contributions to energy relaxation in superconducting qubits, including dielectric twolevel systems and nonequilibrium quasiparticles, as a step towards developing a holistic account of transmon decoherence. 
Tuesday, March 16, 2021 3:12PM  3:24PM Live 
J28.00002: Spectroscopy of highfrequency TLS defects in superconducting qubits using a spinlocking pulse sequence Leonid Abdurakhimov, Imran Mahboob, Hiraku Toida, Kosuke Kakuyanagi, Yuichiro Matsuzaki, Shiro Saito Improving coherence times of superconducting qubits requires the identification of relevant noise sources. Noise spectroscopy using a spinlocking pulse sequence has gained increasing attention as a tool for the characterization of lowfrequency noise mechanisms. Here, we demonstrate that, in addition to the lowfrequency noise, the spinlocking sequence can be used to identify highfrequency twolevelsystem (TLS) defects, both below and above the qubit frequency. Measurements were performed using a capacitivelyshunted flux qubit embedded in a 3D cavity [1]. The amplitude of the spinlocking pulse was varied in the range 090 MHz in the units of the corresponding Rabi frequency. Spectral features were observed when the Rabi frequency was equal to the frequency detuning between the qubit and a highfrequency TLS defect [2]. Thus, spinlocking noise spectroscopy can be used for the detection of offresonant TLS defects which can be particularly useful for the case of fixedfrequency superconducting qubits. 
Tuesday, March 16, 2021 3:24PM  3:36PM Live 
J28.00003: Modeling of qubit coherence variability due to twolevel systems Jerry Tersoff, James B Hannon Superconducting transmon qubits exhibit considerable coherence variability. Both the decoherence and its variability are generally attributed in large part to coupling of the qubit to twolevel systems (TLSs) in the device materials. Much progress has already been made in understanding TLSs and their role in qubit decoherence. Building on this understanding, we implement a devicescale numerical model of a qubit interacting with the surrounding TLSs. TLSs are distributed according to the Standard Tunneling Model, and their contributions to qubit decoherence are summed incoherently. With judicious approximations based on existing literature, we are able to eliminate all but a few free parameters in the model, and even those are constrained by available data. In this way we generate ensembles of nominally identical qubits that differ only in the statistical selection of the millions of TLSs close enough to interact. The results explain the observed statistical variability of qubits, and quantify the relative importance of the few TLSs that interact most strongly, vs the background of many weak interactions. We also compare with qubit spectroscopy results. Finally, we address timedomain variability of the decoherence. 
Tuesday, March 16, 2021 3:36PM  3:48PM Live 
J28.00004: Dielectric loss at material interfaces in circuit quantum electrodynamics devices Valtteri Lahtinen, Mikko Möttönen Performance of circuit quantum electrodynamics (cQED) devices, such as superconducting qubits and coplanarwaveguide (CPW) resonators, is typically limited by dielectric losses that occur in the substrate and, in particular, in the thin dielectric regions at the material interfaces. These losses can be simulated utilizing finiteelement modeling, but reliable extraction of the relevant material parameters of these regions is challenging, introducing uncertainty to the simulations. Moreover, the wide span of length scales, ranging from subnanometer feature size in the thin interface regions to micrometer and millimeter scales of the device structures, poses significant challenges on, e.g., finiteelement meshing and numerical convergence. In this work, we discuss methods for measurementbased extraction of the material parameters and ways to overcome the challenges related to the wide range of length scales. The developed methods and obtained results guide the fabrication of lowloss cQED devices, such as CPW resonators and transmon qubits. 
Tuesday, March 16, 2021 3:48PM  4:00PM Live 
J28.00005: Investigation into Charge Noise in a Tantalum Transmon on Sapphire Substrate across Higher Energy Levels Daniel Tennant, Luis A. Martinez, Chris Wilen, Robert F McDermott, Yaniv J Rosen, Jonathan L DuBois Tantalum has recently emerged as a promising material for superconducting qubit fabrication. Transmon qubits constructed out of this metal can possess energy relaxation times greater than 300 µs and as such present an attractive platform for noise studies in higher energy levels. Here we present interwoven measurements of qubit spectroscopy, Ramsey decays, energy relaxation, and Echo decays across the lowlying one and two photon transitions. By observing the charge offset as a function of time, we validate the connection between charge dispersion present in qubit spectroscopy and Ramsey measurements, both in a single transition and across levels. In addition, we observe that discrete jumps in the charge offset dominate the long time charge dynamics, suggesting the charge environment fluctuates between a few metastable states. Finally, we observe occasional events of frequency instability that affect only one charge parity band but across multiple levels. We interpret these occurrences as a chargeparity dependent fluctuation of Josephson junction properties as opposed to a resonant TwoLevel System interaction. 
Tuesday, March 16, 2021 4:00PM  4:12PM Live 
J28.00006: Positive and negativefrequency noise from an ensemble of twolevel fluctuators Xinyuan You, Aashish Clerk, Jens Koch The analysis of charge noise based on the Bloch–Redfield treatment of an ensemble of dissipative twolevel fluctuators generally results in a violation of the fluctuation–dissipation theorem. The standard Markov approximation (when applied to the twolevel fluctuators coupled to a bath) can be identified as the main origin of this failure. The resulting decoherence rates only involve the bath response at the fluctuator frequency, and thus completely neglect the effects of frequency broadening. A systematic and computationally convenient way to overcome this issue is to employ the spectatorqubit method: by coupling an auxiliary qubit to the twolevel fluctuator ensemble, an analytical approximation for S(ω) fully consistent with the fluctuation–dissipation theorem can be obtained. We discuss the resulting characteristics of the noise which exhibits distinct behavior over several frequency ranges, including a 1/f to 1/f^{2} crossover with a T^{3} temperature dependence of the crossover frequency. 
Tuesday, March 16, 2021 4:12PM  4:24PM Live 
J28.00007: Evaluating Qubit Coherence Statistics in the Presence of TimeCorrelated Fluctuations Greg Calusine, Kyle Serniak, David K Kim, Bethany Niedzielski, Alexander Melville, Jonilyn Yoder, William Oliver Reducing loss in superconducting qubit circuits is critical for enabling the development of largescale quantum computing architectures. This task is especially challenging in the face of variability resulting from devicetodevice differences and fluctuating device properties. In this work, we investigate the T_{1} statistics for set of 54 nominally identical, highcoherence transmon qubits (T_{1}, T_{2} Ramsey, T_{2} Echo ~ 100 µs) and use this large data set to study the scaling of the error in the ensemble mean T_{1} estimate as a function of number of devices and sampling time. We then apply this knowledge to assess the statistical significance of weakly contributing loss mechanisms. 
Tuesday, March 16, 2021 4:24PM  4:36PM Live 
J28.00008: Studying magneticfield resilience of 3D transmons with thinfilm AlOx Josephson junctions Jonas Krause, Christian Dickel, Elmore Vaal, Michel Vielmetter, Junya Feng, Richard Bounds, Gianluigi Catelani, Johannes Fink, Yoichi Ando Magneticfieldresilient transmons enable sensing applications and hybrid architectures involving spin or topological qubits, as well as studying flux noise and quasiparticle loss. We investigate the effect of inplane magnetic fields up to 1 T on the spectrum and coherence times of thinfilm 3D aluminum transmons. Using a copper cavity, which is unaffected by strong magnetic fields, we can purely probe the magneticfield response of the transmon. Our study includes singlejunction and SQUID transmons. The latter allows for both careful alignment of the magnetic field and a fluxnoise sensitivity analysis. As expected, qubit frequencies decrease with increasing fields, dominantly due to a suppression of the superconducting gap. Nevertheless, our thinfilm transmons show enhanced magneticfield resilience: Direct qubit operation is possible up to 650 mT, and SQUID oscillations remain visible in the cavity frequency up to 900 mT. Energyrelaxation times T1 remain at the microsecond level for the entire measurable range. Fluxsensitivity analysis of T2* and T2e shows a change but no clear freezeout of flux noise at high fields. 
Tuesday, March 16, 2021 4:36PM  4:48PM Live 
J28.00009: Improving quality factors of superconducting coplanar wave guide resonators by surface passivation with selfassembled monolayer Mohammed Al Ghadeer, Ahmed Hajr, Archan Banerjee, Saleem Rao, John Mark Kreikebaum, D. Frank Ogletree, Virginia Altoe, Irfan Siddiqi Superconducting coplanar waveguide (CPW) microwave resonators are among the best to read and change the state of artificial atoms because of their excellent coupling to quantum systems. This coupling is the base of forming circuit quantum electrodynamics (cQED) architecture. CPW resonators are very sensitive to deleterious thin film amorphous defects in their surfaces mainly due the presence of twolevel system (TLS) oxides and nonTLS quasiparticles that significantly decrease their quality factors. In this work, we show the characterization and fabrication of niobium CPW resonators with more than 10^{6 }internal quality factors at singlephotonexcitation power, measured at 100 mK, for a particular surface treatment using selfassembled monolayers (SAM) molecules. We also compare our measurement results with finite element simulations (COMSOL and HFSS) and show analysis of resonator samples using structural characterization tools (XPS and TEM) to correlate the efficiency of surface treatment with resonators quality factors. 
Tuesday, March 16, 2021 4:48PM  5:00PM Live 
J28.00010: Power and temperature dependence of High Q superconducting resonators Ashish Alexander, Christopher Weddle, Christopher J K Richardson An integrated temperature and power dependent model of a resonator internal quality factor predicts the loss contribution from twolevel systems and quasiparticles simultaneously. At millikelvin temperatures, the subgap microwave photons generated by the resonator readout power drive the quasiparticle and phonon density far from its thermal equilibrium. 
Tuesday, March 16, 2021 5:00PM  5:12PM Live 
J28.00011: Investigating the mechanism of singleelectron tunneling in chargeparitysensitive transmons Spencer Diamond, Valla Fatemi, Max Hays, Kyle Serniak, Luigi Frunzio, Robert J Schoelkopf, Leonid Glazman, Michel Devoret Singleelectron tunneling across Josephson junctions in superconducting qubits contributes to decoherence and limits qubit performance. In the past, such decoherence was exclusively attributed to preexisting nonequilibrium quasiparticles that tunnel across junctions and exchange energy with the qubit. However, it was recently predicted that highfrequency photons can be efficiently absorbed in transmon Josephson junctions and induce singleelectron tunneling. This process requires no preexisting quasiparticles; in fact it generates two quasiparticles and, in doing so, can change the qubit state. Past measurements of singleelectron tunnelinginduced excitation and relaxation in chargeparitysensitive transmons were consistent with photonassisted tunneling. Here, we will present theoretical and experimental results demonstrating that adding fluxtunability to a chargeparitysensitive transmon can distinguish the contributions of different singleelectron tunneling processes in our devices. 
Tuesday, March 16, 2021 5:12PM  5:24PM Live 
J28.00012: Probing hundreds of individual twolevel defects in polycrystalline and amorphous alumina. ChihChiao Hung, Neda Forouzani, Liuqi Yu, Stefan Fritz, Dagmar Gerthsen, Kevin Daniel Osborn Quantum twolevel systems (TLSs) appear in the dielectrics of superconducting qubits and parasitically limit the qubit coherence time. The Josephson junction barrier, amorphous alumina, is known to host TLSs similar to other amorphous films, but TLSs are not microscopically understood in atomic composition. In this study we extend the quantitative data available on TLSs by using a superconducting resonator to characterize two alumina film types: polycrystalline and amorphous. We find a clear difference between the structure types when analyzing TLSs. A large sample of ~ 400 individual TLSs is analyzed from polycrystalline films. Their dipole moments directed vertically have a mean value of 2.6(0) Debye and a standard deviation of 1.6(5) Debye. They fit well to a single Gaussian distribution indicating a single defect type. In contrast, the amorphous film contains TLSs with the larger dipole moments but only few are observed due to larger frequency noise. 
Tuesday, March 16, 2021 5:24PM  5:36PM Live 
J28.00013: Flux Noise and Spin Dynamics of Multiple Interacting Adsorbates on Superconducting Qubits Keith Ray, Artur Tamm, Yaniv J Rosen, Jonathan L DuBois, Vincenzo Lordi Molecular oxygen, OH groups, and atomic hydrogen surface adsorbates have been identified as possible sources of magnetic flux noise in superconducting qubits. This noise causes decoherence and frequency jitter that can hinder tunable multiqubit devices. To better understand the spin dynamics of these fluctuating adsorbates we have extended our model for interacting adsorbed paramagnetic O_{2} molecules to include other species adsorbed from the device operation atmosphere, including water at different coverages. We calculate the effects of applied fields on the phases of the spin system, its dynamics, and the charge and flux noise generated. To do this we utilize a thermodynamic ensemble generated with Monte Carlo simulations along with LandauLifshitzGilbert equation simulations for the dynamics, both parametrized with vdWcorrected density functional theory calculations. 
Tuesday, March 16, 2021 5:36PM  5:48PM Live 
J28.00014: Ramsey Frequency Beating in the Higher Level States of a 3D Transmon Luis Martinez, Yaniv J Rosen, Keith Ray, Daniel Tennant, Jonathan L DuBois Utilizing higher levels of 3D transmons for computational operations (Qudits) holds promise for more efficient quantum computational architectures. In particular, they give access to an expanded Hilbert space while operating with a minimal number of control lines, and can operate with efficient highfidelity control gates via optimal control techniques. However, a limiting factor has been the reduced coherence times of the higher levels. Therefore, a full characterization and understanding of the fundamental sources of noise is a key step to mitigating these limitations. We present the observation of a beating phenomenon in the Ramsey fringes of higher qudit levels which is only visible in the overlay of several independent Ramsey measurements. In contrast to the understood deviation in Ramsey fringes caused by parity flips, the beating appears to be mostly coherent and does not correspond to the charge dispersion. We discuss several Ramsey measurements for multiple levels of two 3D transmons, including charge dispersion, phenomenological models, and potential sources for this anomalous noise. 
Tuesday, March 16, 2021 5:48PM  6:00PM On Demand 
J28.00015: Evidence for a gap in the density of states of twolevel systems in amorphous silicon Liuqi Yu, Yaniv J Rosen, Kevin Daniel Osborn We measure the materialbased quantum twolevel systems (TLSs) within amorphous silicon. They are probed within the dielectric of capacitors in highQ superconducting LC resonators. In addition to a microwave probe field, the TLS's energy is swept by an applied electricfield to the capacitor plates. The nonequilibrium loss at various resonator microwave fields is thus studied by sweeps of many TLSs through resonance, where they may undergo LandauZener transitions. The intrinsic material loss is measured in the single photon limit. However, at high bias rates, excess loss is measured far larger than the intrinsic loss, which contradicts to the general understanding of the wellestablished standard TLS model. The excess intrinsic loss is bias rate dependent, which leads to the discovery of a gap in the density of states of a second type of TLS. Remarkably, once the bias rate dependent density of states is taken into consideration, the excess loss is found to scale according to LandauZener theory, similar to the standard TLS, which in return validates its TLS origin. The second TLS type possesses a large dipole moment (~200 Debye) compared to the standard TLS (~7 Debye). 
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