Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Y26: Superconducting Qubits: Decoherence and Noise |
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Sponsoring Units: GQI Chair: Andrew Houck, Princeton University Room: D136 |
Friday, March 19, 2010 8:00AM - 8:12AM |
Y26.00001: Spins on Metals: Noise in SQUIDs and Spin Glasses Zhi Chen, Clare Yu Recent experiments at Stanford and Wisconsin have found evidence for magnetic defects on the surface of elemental metals like aluminum, niobium, and gold. Fluctuations of these impurities are the source of flux noise in SQUIDs. Flux noise is a major obstacle to the realization of using superconducting qubits to construct quantum computers. To see if flux noise can be described by spin glass noise, we have used Monte Carlo simulations of a 3D Ising spin glass to produce noise. We find that the noise is a maximum at the critical temperature. We compare our results to experimental measurements of the susceptibility, as well as the flux and inductance noise measured in SQUIDs. [Preview Abstract] |
Friday, March 19, 2010 8:12AM - 8:24AM |
Y26.00002: Flux Noise in SQUIDs due to Hyperfine Interactions of Spins on Metals Jiansheng Wu, Clare Yu Recent experiments at Stanford and Wisconsin have found evidence for magnetic defects on the surface of elemental metals like aluminum, niobium, and gold, but a much reduced signal on silicon. Fluctuations of these impurities are the source of flux noise in SQUIDs, and a major obstacle to the realization of using superconducting qubits to construct quantum computers. We discuss the possibility that the flux noise arises from hyperfine interactions and ways to test this hypothesis. [Preview Abstract] |
Friday, March 19, 2010 8:24AM - 8:36AM |
Y26.00003: Josephson Phase Qubit as a Flux-Noise Spectrum Analyzer Daniel Sank, Radoslaw Bialczak, Mike Lenander, Erik Lucero, Matteo Mariantoni, Matthew Neeley, Aaron O'Connell, Haohua Wang, Martin Weides, James Wenner, Tsuyoshi Yamamoto, Yi Yin, Andrew Cleland, John Martinis Increasing the phase coherence time of the Josephson phase qubit will require understanding and eliminating magnetic flux noise. Although this noise was first observed in SQUIDs thirty years ago, its origin has been elucidated only recently. We will show that a qubit can be used to measure the power spectrum of the flux noise from about 0.001 Hz to 100 MHz, a range of eleven orders of magnitude in frequency. At MHz frequencies we use the noise filtering property of Rabi oscillation decays. For low frequencies we introduce an improvement over a previous schemes, the ``Ramsey Tomography Oscilloscope,'' based on repeated Ramsey fringe decays, which should be easy to implement in other qubit architectures. The integrated noise over intermediate frequencies is also measured using a traditional decay envelope of Ramsey fringes. We believe these measurements of the noise spectrum will place constraints on the appropriate microscopic model of the flux noise. [Preview Abstract] |
Friday, March 19, 2010 8:36AM - 8:48AM |
Y26.00004: Investigations of Surface Magnetic Defects in Superconducting Thin Film Devices Steven Sendelbach, Umeshkumar Patel, David Hover, Robert McDermott Recent experiments indicate that there is a high density of unpaired spins residing on the surfaces of the superconducting thin films used to implement SQUIDs and superconducting qubits. Fluctuations of these spins give rise to low frequency flux noise and dephasing of the qubit state. Realization of phase and flux qubits with improved dephasing times will require a deeper understanding of the microscopic physics that governs fluctuations of the surface spins. Here we present data on the complex frequency-and temperature-dependent inductance of dc SQUIDs at millikelvin temperatures, which can be directly related to the complex susceptibility of the surface spin system. We observe low-frequency noise in the SQUID inductance, with a 1/f power spectrum. We investigate the statistics of the low-frequency flux and inductance fluctuations, and examine spatial correlations of the magnetic fluctuators. Finally, we describe experiments that probe the microscopic nature of the magnetic defect states. [Preview Abstract] |
Friday, March 19, 2010 8:48AM - 9:00AM |
Y26.00005: Localization of Metal-Induced Gap States at the Metal-Insulator Interface: Origin of Flux Noise in SQUIDs and Superconducting Qubits Sangkook Choi, Dung-Hai Lee, Steven G. Louie, John Clarke The origin of magnetic flux noise in dc Superconducting Quantum Interference Devices (SQUIDs) with a power spectrum scaling as 1/$f$ ($f$ is frequency) has been a puzzle for over 25 years. This noise limits both the low frequency performance of SQUIDs and the decoherence time of flux-sensitive superconducting qubits, making scaling-up for quantum computing problematic. Recent calculations and experiments indicate that the noise is generated by electrons that randomly reverse their spin directions. Their areal density of $\sim $ 5 $\times $ 10$^{17}$ m$^{-2}$ is relatively insensitive to the nature of the superconductor and substrate. Here, we propose that the local magnetic moments originate in metal-induced gap states (MIGSs) localized by potential disorder at the metal-insulator interface. MIGSs are particularly sensitive to such disorder, so that the localized states have a Coulomb repulsion sufficiently large to make them singly occupied. Our calculations demonstrate that a modest level of disorder generates the required areal density of localized moments. This result suggests that magnetic flux noise could be reduced by fabricating superconductor-insulator interfaces with less disorder. Support: NSF DMR07-05941, US DOE De-AC02-05CH11231, Samsung Foundation, Teragrid, NERSC. [Preview Abstract] |
Friday, March 19, 2010 9:00AM - 9:12AM |
Y26.00006: Noise-Induced Looping on the Bloch Sphere Robert Joynt, Dong Zhou For many implementations of quantum computing, 1/f and other types of broad-spectrum noise are an important source of decoherence. An important step forward would be the ability to back out the characteristics of this noise from qubit measurements and to see if it leads to new physical effects. For certain types of qubits, the working point of the qubit can be varied. Using a new mathematical method that is suited to treat all working points, we present theoretical results that show how this degree of freedom can be used to extract noise parameters and to predict a new effect: noise-induced looping on the Bloch sphere. We analyze data on superconducting qubits to that they are very near the parameter regime where this looping should be observed.~ In addition, we show that the number of noise sources in the experiments is small: of order about 30. ~ [Preview Abstract] |
Friday, March 19, 2010 9:12AM - 9:24AM |
Y26.00007: Characterizing and manipulating two level defect states in a superconducting phase circuit Nadav Katz, Yoni Shalibo, Yaara Rofe, David Shwa, Felix Zeides, Matthew Neeley, John M. Martinis Two level defect states (TLSs) in the Josephson junction of superconducting qubits are known to be a major source of decoherence. Alternatively, quantum memory using an individual TLS has been demonstrated by coherent and controlled coupling to a phase qubit. Characterization of TLS lifetimes is necessary in order to clarify TLS origins, how to eliminate them and/or possible usefulness. We measure, for an ensemble of TLSs, the strength of coupling to the qubit and relaxation and decoherence times. We find an anti-correlation between coupling strength and TLS lifetimes. We coherently transfer a photon into a long lived TLS (2.5 micro second relaxation lifetime) and by shifting the circuit frequencies dynamically and loading another photon we extract from the TLS to the second excited state of the macroscopic phase qutrit. [Preview Abstract] |
Friday, March 19, 2010 9:24AM - 9:36AM |
Y26.00008: A Microscopic Model for Interacting Two-Level-Systems in an Al$_2$O$_3$ Insulating Barrier Victor Galitski, Maxim Dzero We develop a generic theoretical model of interacting two-level systems (TLS) in Al$_2$O$_3$ dielectric and use it to analyze anomalous experimental data in superconducting qubits. Interactions between the TLS are mediated by local strains from neighboring atoms as well as electrons from the superconducting contacts. Our analysis is particularly focused on how the between the TLS affect the quality factor of superconducting circuits. This work is financially supported by IARPA. [Preview Abstract] |
Friday, March 19, 2010 9:36AM - 9:48AM |
Y26.00009: Radiative Losses in Superconducting Coplanar Resonators James Wenner, Radoslaw Bialczak, Michael Lenander, Erik Lucero, Matteo Mariantoni, Matthew Neeley, Aaron O'Connell, Daniel Sank, Haohua Wang, Martin Weides, Tsuyoshi Yamamoto, Yi Yin, Andrew Cleland, John Martinis Radiation is a potential loss mechanism in superconducting qubits. Radiation loss was studied in superconducting coplanar resonators, which are important both in coupling superconducting qubits and because they provide a simple system to quantitatively measure the resulting effects. We fabricated various designs of 8 GHz resonators and measured the resulting reduction of the high-power $Q$ due to radiation. We found that the radiation loss was suppressed by a factor of 15 as compared to a simple theoretical model. We attribute this to the effects of the device mount on the radiation pattern, and we conclude that radiation is an unlikely decoherence mechanism for the present generation of qubits and resonators. [Preview Abstract] |
Friday, March 19, 2010 9:48AM - 10:00AM |
Y26.00010: Energy Decay from Nonequilibrium Quasiparticles in Josephson Qubits M. Lenander, H. Wang, R. Bialczek, E. Lucero, M. Mariantoni, M. Neeley, A. O'Connell, D. Sank, M. Weides, J. Wenner, T. Yamamoto, Y. Yin, A. Cleland, J. Martinis Nonequilibrium quasiparticle excitations are thought to be an important source of decoherence in Josephson qubits. We present a theory for resonators and qubits that predicts energy decay proportional to the quasiparticle density. Our theory computes a non-thermal energy spectrum for a generic quasiparticle source at higher energy. Using prior experimental measurements of quasiparticle density, the theory predicts decay rates that are roughly consistent with phase qubit and resonator energy decay times. We will also present experimental data comparing the decay rate to the fractional shift in frequency when quasiparticles are directly injected into the system. With basic agreement between theory and experiment, we hope to develop methods for improving qubit design and to show whether careful measurements can bound the contribution to energy decay from quasiparticles. [Preview Abstract] |
Friday, March 19, 2010 10:00AM - 10:12AM |
Y26.00011: Superconducting microwave resonators in magnetic fields C. Song, M.P. DeFeo, K. Yu, B. Xiao, P. Bhupathi, B.L.T. Plourde Microwave resonators with high quality factors have enabled many recent breakthroughs with superconducting qubits and photon detectors. Vortices trapped in a superconducting resonator due to insufficient shielding or pulsed control fields constitute one potential loss mechanism that can lead to reduced quality factors. We have developed a straightforward method for enhancing the pinning, and thus reducing the excess loss from vortices trapped by field-cooling by over an order of magnitude, in Al resonators using nanofabricated surface pinning. We have also studied resonators in the absence of field-cooling, where magnetic fields applied below the transition temperature of the superconductor can still influence the behavior, by producing reversible shifts in the resonance frequency for small fields and by injecting vortices into the films at larger fields. [Preview Abstract] |
Friday, March 19, 2010 10:12AM - 10:24AM |
Y26.00012: Quality factor dependence on geometry and photon number in superconducting coplanar resonators Moe Khalil, Micah Stoutimore, Hanhee Paik, Frederick Wellstood, Kevin Osborn The loss in coplanar superconducting resonators at low-photon number is not yet understood, and has implications on the achievable coherence of superconducting qubits. We have fabricated and measured high quality factor superconducting aluminum coplanar resonators on sapphire, all with a fundamental resonance frequency of approximately 6GHz, to facilitate the study of loss mechanisms including loss caused by two-level systems and radiation.~We studied four resonator geometries that include a lumped-element resonator, a coplanar strip waveguide resonator, and two hybrid designs that contain a coplanar strip waveguide terminated by either a compact inductor or capacitor.~ The measurements are taken at 30mK with a probe frequency that is varied in amplitude and in some experiments a second drive frequency is also used to study the loss mechanism. We find that the internal quality factor of the resonators increases with photon number in a manner that is not merely explained by a surface distribution of independent two-level systems. [Preview Abstract] |
Friday, March 19, 2010 10:24AM - 10:36AM |
Y26.00013: Eliminating the Purcell Effect in Circuit QED Matthew Reed, Lev Bishop, Leonardo DiCarlo, Luigi Frunzio, Eran Ginossar, Andrew Houck, Blake Johnson, David Schuster, Steven Girvin, Robert Schoelkopf In circuit QED, it is desirable to have both a long qubit coherence time and a short microwave cavity lifetime in order to perform a high fidelity qubit measurement [1]. However, when a qubit is strongly coupled to a fast cavity, its lifetime is limited by spontaneous emission due to the multi-mode Purcell effect [2]. We present measurements of a new device in which the normal rate of spontaneous emission is reduced by more than an order of magnitude over a wide range of qubit frequencies. The single-shot readout fidelity of the transmon qubit in this device, which is strongly coupled (g/2$\pi \sim $300MHz) to a fast cavity ($\kappa $/2$\pi \sim $20MHz), will also be discussed. 1. Gambetta, Jay \textit{et al.} \textit{Phys. Rev. A }76, 012325 (2007). 2. Houck, A. A. \textit{et al}. \textit{Phys. Rev. Lett. }101, 080502 (2008). [Preview Abstract] |
Friday, March 19, 2010 10:36AM - 10:48AM |
Y26.00014: Novel Surface Treatments of Superconducting Thin Film Devices for Reduced Flux Noise U. Patel, S. Sendelbach, J. Kline, D.P. Pappas, M. Weides, J.M. Martinis, R. McDermott Low frequency magnetic flux noise is a dominant source of dephasing in Josephson phase and flux qubits. Recent experiments indicate that the noise is due to a high density of unpaired electron spins on the surfaces of the superconducting thin films and suggest that appropriate surface treatments could be used to reduce levels of flux noise. Here we describe two approaches to the realization of improved, low-noise devices: (1) surface passivation to reduce the density of unpaired spins at the metal-insulator interface and (2) treatment of the superconducting thin films with ferromagnetic or antiferromagnetic impurities in order to saturate dominant fluctuators. We discuss fabrication issues and present data on the flux and inductance noise of SQUID devices incorporating the novel surface treatments. [Preview Abstract] |
Friday, March 19, 2010 10:48AM - 11:00AM |
Y26.00015: Mesoscopic interface defects and subharmonic gap structure in Josephson tunnel junctions of superconducting phase qubits R.P. Erickson, D.P. Pappas We compare the measured I-V curves of several samples of superconducting phase qubits and fit them to a theoretical model that accounts for mesoscopic point-contact defects, or pinholes, at the Josephson junction interfaces. The pinholes are parametrized by the fraction of interface surface area they comprise. Our model incorporates diffuse inelastic scattering of quasiparticles from the junction interfaces via the Bogoliubov-de Gennes equation, and is an extension of an earlier theory of superconductive tunneling proposed by Shumeiko et al. [For a review, see V.S. Shumeiko, E. N. Bratus, and G Wendin, Low Temp. Phys. \textbf{23}, 181 (1997).] We fit our model to subharmonic-gap features observed in phase-qubit samples possessing a mix of single-crystal (c) Re and polycrystalline (p) Al electrodes, as well as single-crystal and amorphous (a) aluminum oxide junctions, including c-Re/c-Al2O3/c-Re and c-Re/a-AlOx/p-Al, with junction thicknesses not exceeding 20 ${\AA}$. We report on the I-V fits we obtain and the conclusions that can be drawn about the nature and extent of pinholes within these samples. [Preview Abstract] |
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