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 Two-Level SystemsLive
|
Hide Abstracts |
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 two-level 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 high-frequency TLS defects in superconducting qubits using a spin-locking 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 spin-locking pulse sequence has gained increasing attention as a tool for the characterization of low-frequency noise mechanisms. Here, we demonstrate that, in addition to the low-frequency noise, the spin-locking sequence can be used to identify high-frequency two-level-system (TLS) defects, both below and above the qubit frequency. Measurements were performed using a capacitively-shunted flux qubit embedded in a 3D cavity [1]. The amplitude of the spin-locking pulse was varied in the range 0-90 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 high-frequency TLS defect [2]. Thus, spin-locking noise spectroscopy can be used for the detection of off-resonant TLS defects which can be particularly useful for the case of fixed-frequency superconducting qubits. |
Tuesday, March 16, 2021 3:24PM - 3:36PM Live |
J28.00003: Modeling of qubit coherence variability due to two-level 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 two-level 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 device-scale 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 time-domain 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 coplanar-waveguide (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 finite-element 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 sub-nanometer feature size in the thin interface regions to micrometer and millimeter scales of the device structures, poses significant challenges on, e.g., finite-element meshing and numerical convergence. In this work, we discuss methods for measurement-based 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 low-loss 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 low-lying 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 charge-parity dependent fluctuation of Josephson junction properties as opposed to a resonant Two-Level System interaction. |
Tuesday, March 16, 2021 4:00PM - 4:12PM Live |
J28.00006: Positive- and negative-frequency noise from an ensemble of two-level fluctuators Xinyuan You, Aashish Clerk, Jens Koch The analysis of charge noise based on the Bloch–Redfield treatment of an ensemble of dissipative two-level fluctuators generally results in a violation of the fluctuation–dissipation theorem. The standard Markov approximation (when applied to the two-level 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 spectator-qubit method: by coupling an auxiliary qubit to the two-level 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/f2 crossover with a T3 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 Time-Correlated 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 large-scale quantum computing architectures. This task is especially challenging in the face of variability resulting from device-to-device differences and fluctuating device properties. In this work, we investigate the T1 statistics for set of 54 nominally identical, high-coherence transmon qubits (T1, T2 Ramsey, T2 Echo ~ 100 µs) and use this large data set to study the scaling of the error in the ensemble mean T1 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 magnetic-field resilience of 3D transmons with thin-film AlOx Josephson junctions Jonas Krause, Christian Dickel, Elmore Vaal, Michel Vielmetter, Junya Feng, Richard Bounds, Gianluigi Catelani, Johannes Fink, Yoichi Ando Magnetic-field-resilient 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 in-plane magnetic fields up to 1 T on the spectrum and coherence times of thin-film 3D aluminum transmons. Using a copper cavity, which is unaffected by strong magnetic fields, we can purely probe the magnetic-field response of the transmon. Our study includes single-junction and SQUID transmons. The latter allows for both careful alignment of the magnetic field and a flux-noise sensitivity analysis. As expected, qubit frequencies decrease with increasing fields, dominantly due to a suppression of the superconducting gap. Nevertheless, our thin-film transmons show enhanced magnetic-field resilience: Direct qubit operation is possible up to 650 mT, and SQUID oscillations remain visible in the cavity frequency up to 900 mT. Energy-relaxation times T1 remain at the micro-second level for the entire measurable range. Flux-sensitivity analysis of T2* and T2e shows a change but no clear freeze-out 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 self-assembled 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 two-level system (TLS) oxides and non-TLS quasiparticles that significantly decrease their quality factors. In this work, we show the characterization and fabrication of niobium CPW resonators with more than 106 internal quality factors at single-photon-excitation power, measured at 100 mK, for a particular surface treatment using self-assembled 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 two-level systems and quasiparticles simultaneously. At millikelvin temperatures, the sub-gap 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 single-electron tunneling in charge-parity-sensitive transmons Spencer Diamond, Valla Fatemi, Max Hays, Kyle Serniak, Luigi Frunzio, Robert J Schoelkopf, Leonid Glazman, Michel Devoret Single-electron tunneling across Josephson junctions in superconducting qubits contributes to decoherence and limits qubit performance. In the past, such decoherence was exclusively attributed to pre-existing non-equilibrium quasiparticles that tunnel across junctions and exchange energy with the qubit. However, it was recently predicted that high-frequency photons can be efficiently absorbed in transmon Josephson junctions and induce single-electron tunneling. This process requires no pre-existing quasiparticles; in fact it generates two quasiparticles and, in doing so, can change the qubit state. Past measurements of single-electron tunneling-induced excitation and relaxation in charge-parity-sensitive transmons were consistent with photon-assisted tunneling. Here, we will present theoretical and experimental results demonstrating that adding flux-tunability to a charge-parity-sensitive transmon can distinguish the contributions of different single-electron tunneling processes in our devices. |
Tuesday, March 16, 2021 5:12PM - 5:24PM Live |
J28.00012: Probing hundreds of individual two-level defects in polycrystalline and amorphous alumina. Chih-Chiao Hung, Neda Forouzani, Liuqi Yu, Stefan Fritz, Dagmar Gerthsen, Kevin Daniel Osborn Quantum two-level 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 multi-qubit devices. To better understand the spin dynamics of these fluctuating adsorbates we have extended our model for interacting adsorbed paramagnetic O2 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 Landau-Lifshitz-Gilbert equation simulations for the dynamics, both parametrized with vdW-corrected 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 high-fidelity 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 two-level systems in amorphous silicon Liuqi Yu, Yaniv J Rosen, Kevin Daniel Osborn We measure the material-based quantum two-level systems (TLSs) within amorphous silicon. They are probed within the dielectric of capacitors in high-Q superconducting LC resonators. In addition to a microwave probe field, the TLS's energy is swept by an applied electric-field to the capacitor plates. The non-equilibrium loss at various resonator microwave fields is thus studied by sweeps of many TLSs through resonance, where they may undergo Landau-Zener 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 well-established 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 Landau-Zener 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). |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700