Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P25: Superconductivity: Devices and Applications |
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Sponsoring Units: DCMP Chair: Norman Bergren, National Institute of Standards and Technology Room: D166 |
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P25.00001: Microwave amplifier based on an inline dc SQUID David Hover, Yung-Fu Chen, Leon Maher, Guilhem Ribeill, Shao-Jiang Zhu, Steve Sendelbach, Robert McDermott The dc SQUID can be used as a sensitive, low-noise microwave amplifier if the signal to be amplified is suitably coupled to the SQUID. We have designed and fabricated microwave amplifiers based on inline dc SQUIDs, where the SQUID loop is formed from a thin ($\sim$ 100 nm) dielectric layer separating the base and counterelectrode wiring traces. The SQUID is embedded in a microstrip transmission line resonator at a current anti-node, and signal current is injected directly into the SQUID loop. With this design we have achieved gain greater than 20 dB at a frequency of 8.5 GHz. We provide a theoretical analysis of amplifier noise temperature, bandwidth, and gain, and describe measurements of amplifier noise temperature. We discuss application of these devices to the readout of superconducting quantum circuits. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P25.00002: Spatial correlations of magnetic fluctuations in DC SQUIDS Steven Sendelbach, Umeshkumar Patel, Robert McDermott Recent experiments indicate that there is a high density of unpaired spins residing on the surfaces of 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 describe experiments to probe the spatial correlation of magnetic fluctuators in a SQUID circuit. The SQUID loop incorporates multiple current taps, enabling one to locally address magnetic fluctuators. Preliminary data reveal correlated fluctuations on a length scale of order 10 $\mu$m. We discuss implications for qubit dephasing. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P25.00003: Simulations of dependence of low frequency flux noise on SQUID geometry Steven Anton, Keenan Pepper, John Clarke, Ida Sognnaes It is generally accepted that the 1/f magnetic flux noise observed in dc SQUIDs and superconducting qubits originates in the random flipping of a uniform distribution of electron spins localized at the superconductor-insulator interface. Computer simulations and analytical calculations based on this model confirm the experimental result that the noise power at 1 Hz varies only slowly with SQUID dimensions. In particular, analytical calculations for a circular loop with radius R much greater than the loop linewidth W predict that the noise power scales as R/W. We present numerical computations that are valid for arbitrary geometry, including that of the square washer SQUID for which W approaches R. Making use of the reciprocity theorem, we solve the London equations numerically to find the current distribution in the superconductor, evaluate the Biot-Savart integral to find the corresponding magnetic field at any point and integrate over all spins, including those at the edges of the films, to find the total flux noise. We compare our results with our recent experimental measurements. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P25.00004: Dependence of low frequency flux noise on SQUID-washer dimensions J. Birenbaum, S.M. Anton, A.D. Fefferman, S.R. O'Kelley, J. Clarke, H-M Cho, G.C. Hilton, K.D. Irwin, F.C. Wellstood The 1/f spectral density of low frequency magnetic flux noise at 1 Hz in dc SQUIDs and qubits varies slowly with the dimensions of the superconducting loop, in reasonably good agreement with predictions. Previous measurements on SQUIDs fabricated from a variety of superconductors under different conditions and in a variety of geometries, however, showed that the slope of the power spectrum varied considerably. We report flux noise measurements on six resistively-shunted dc SQUIDs fabricated simultaneously on a single Si chip using a Nb-trilayer process. The noise spectra of all six devices were measured using a SQUID in a single cool-down of our dilution refrigerator. The linewidths of the SQUID loops were varied systematically by a factor of more than 30. The variation in noise power at 1 Hz was small compared with the variation in line width, while the slope varied significantly, from approximately -0.5 to -1. Furthermore, for a given SQUID, the slope depended on temperature. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P25.00005: A Novel System for Accurate Cryogenic S-Parameter Measurements Leonardo Ranzani, Lafe Spietz, Jose Aumentado In order to study microwave devices operating at cryogenic temperatures (4K and below), an accurate characterization of their full scattering parameters is needed. Simple response calibration using a single through standard is usually performed at cryogenic temperatures due to its simplicity, but it is inaccurate since it only determines 4 of the 10 unknowns present in a general two port network environment. In this talk we will discuss a fully automated through-reflect-line (TRL) calibration system suitable for accurately characterizing 2-port S parameters for devices such as SQUID amplifiers and other cryogenic microwave circuits. Data for some typical devices up to 8GHz will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P25.00006: Josephson Junction Circuits for Passive Non-Reciprocal Microwave Devices Lafe Spietz, Norm Bergren, Jose Aumentado We propose a method for using Josephson junction circuits to build non-reciprocal passive microwave components. We show that strong analogies can be made between the physics of ferrites used in traditional passive non-reciprocal microwave devices (that of the gyroscope) and certain classes of Josephson junction circuits. We describe a simple circuit which demonstrates these physical principles, and present both theoretical experimental results on this circuit. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P25.00007: Probing the origin of 1/f critical-current noise in nanoscale Al/AlOx/Al Josephson junctions Christopher Nugroho, Vladimir Orlyanchik, Allison Dove, Gustaf Olson, Zachary Yoscovits, James Eckstein, Dale Van Harlingen We present measurements of the low frequency noise in nanoscale Al/AlOx/Al Josephson junctions made by the shadow/angle evaporation technique. We investigate the differences in the nature of the charge trap fluctuations when the junction electrodes are in the normal state vs. in the superconducting state, as a test of some recent theoretical models. To do that, we compare the magnitude, temperature dependence, and magnetic field dependence of junction resistance fluctuations in the normal state above the Al transition temperature to that of the resistance and critical current fluctuations measured in the superconducting state. We also explore whether the observed fluctuators are thermally-activated or tunneling as a function of temperature. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P25.00008: Decoherence and energy shift in phase qubits due to nonequilibrium quasiparticles M. Lenander, R.C. Bialczak, E. Lucero, M. Mariantoni, A. O'Connell, M. Neeley, D. Sank, H. Wang, M. Weides, J. Wenner, T. Yamamoto, Y. Yin, J. Zhao, A.N. Cleland, J.M. Martinis Nonequilibrium quasiparticle excitations are thought to be an important source of decoherence in Josephson qubits. We present a model analogous to the Mattis-Bardeen theory wherein the effects of quasiparticles introduce a complex environmental impedance to the junction. The real part causes energy relaxation in the qubit while the imaginary part causes a frequency shift. We present experimental data comparing these effects while injecting nonequilibrium quasiparticles into the system. The theory is used to qualitatively check the injection process. Then by comparing the decay rate and frequency shift, we quantitatively verify the theory without the need to directly measure the quasiparticle density. Agreement between theory and experiment is observed to within experimental uncertainty, about 10\%. We examine infrared radiation as a source of nonequilibrium quasiparticles. Using these new tools, we hope to develop methods for improving qubit performance and to bound the contribution to energy decay from quasiparticles. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P25.00009: Effect of an Ohmic environment on an optimally controlled flux-biased phase qubit Amrit Poudel, Maxim Vavilov We analyze the effect of environment on the gate operation of flux-biased phase qubits. We employ the master equation for a reduced density matrix of the qubit system coupled to an Ohmic environment, described by the Caldeira-Leggett model. Numerically solving this equation, we evaluate the gate error as a function of gate time, temperature and environmental coupling strength for experimentally determined qubit parameters. Here we present the analysis for single-quadrature microwave (control) pulses as well as for two-quadrature pulses, which lower the gate error significantly for idealized systems in the absence of environment. Our results indicate that two-quadrature pulses with fixed and variable driving frequency have similar performance, which outweighs the performance of single-quadrature pulses, in the presence of environment. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P25.00010: Quasiparticle relaxation of superconducting qubits in the presence of flux Gianluigi Catelani, Jens Koch, Luigi Frunzio, Robert Schoelkopf, Michel Devoret, Leonid Glazman Quasiparticle tunneling across a Josephson junction sets a limit for the lifetime of a superconducting qubit state. We develop a general theory of the corresponding decay rate in a qubit controlled by a magnetic flux. The flux affects quasiparticles tunneling amplitudes, thus making the decay rate flux-dependent. The theory is applicable for an arbitrary quasiparticle distribution. It provides estimates for the rates in practically important quantum circuits and also offers a new way of measuring the phase-dependent admittance of a Josephson junction. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P25.00011: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P25.00012: High count-rate superconducting transition edge sensors for near-IR single photon detection Faustin Carter, Daniel Santavicca, Luigi Frunzio, Anthony Annunziata, Daniel Prober Detection of individual near-IR photons with GHz count rates, good timing resolution, and high quantum efficiency is important in a number of applications. These include quantum key distribution, single-photon classical communication, and CMOS imaging for defect analysis. We propose a nano-scale superconducting niobium transition edge sensor (TES). The extremely small detector volume allows for single-photon sensitivity at 4 K, with a much faster response time (nsec) than conventional TES detectors operating below 0.4 K. Efficient photon coupling is achieved with a resonant near-IR planar antenna. The proposed device is intrinsically photon number resolving, unlike a niobium-nitride nanowire detector or an avalanche photodiode. We present preliminary results for device performance. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P25.00013: Nano-structuring on the surface of high Tc-superconductors by STM\&AFM Kazuto Hirata, Tadashi Machida, Shuuichi Ooi, Minoru Tachiki, Takashi Mochiku We demonstrate local insulation on a cleaved surface of Bi- 2212 single crystals using a local anodic oxidation by a atomic force microscope (AFM) and a scanning tunneling microscope (STM) for the first time. We have investigated the electrical properties of the modified region by using an STM-assisted near- field microwave microscope. From the experimental observations, we conclude that the modified region becomes an insulator with an associated dielectric loss locally. Varying the applied bias- voltage and the scanning speed can control the protrusion height and the line width of the regions. This provides a potential technique for reproducibly fabricating high temperature superconducting devices with stable electronic characteristics. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P25.00014: An Integrated Balanced Superconductor-Insulator-Superconductor Heterodyne Mixer on a Silicon Membrane M.P. Westig, K. Jacobs, M. Schultz, M. Justen, J. Stutzki, C.E. Honingh We have designed and fabricated a 380-520~GHz integrated balanced Nb\textbackslash Al\textbackslash AlOx\textbackslash Nb superconductor-insulator-superconductor (SIS) heterodyne waveguide mixer for submillimeter astrophysics. The response of the mixer measured with a Fourier transform spectrometer shows excellent agreement with the design. The novelty of our device is that we deposit the complete superconducting mixer circuit (tapered slotline antennas, hybrid coupler, MIM capacitors, SIS junctions, tuning circuits and blocking filters) on top of a 9~$\mu$m silicon membrane. The membrane is held suspended in a waveguide by 2.5~$\mu$m thick gold plated beamleads. We will show that silicon membrane technology and a thorough device design render the integration of SIS devices with larger circuits feasible. This is an important step towards large arrays of mixers. When using an appropriate superconductor technology, these devices are scalable to higher frequencies. We will present the design, fabrication results and first results of heterodyne measurements. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P25.00015: c-Axis current flow arises in helically wound wire Ying Jia, U. Welp, G.W. Crabtree, W.K. Kwok, M. Rupich, S. Fleshler, A.P. Malozemoff c-Axis critical current density of second-generation YBCO wires in HTS cables due to the interaction of current flow with the induced magnetic field. However, the importance of c-axis critical current density ($J_{c}^{c})$ on the overall transport critical current is not clearly understood. We measured the temperature and field dependence of $J_{c}^{c}$ using a mesa structure patterned into the YBCO layer of 2$^{nd}$-generation HTS tapes. We found, $J_{c}^{c}$ --values of $\sim $ 4 kA/cm$^{2}$ at 77 K in self-field, corresponding to an unexpectedly high anisotropy of the critical current density $\gamma ={J_c^{ab} } \mathord{\left/ {\vphantom {{J_c^{ab} } {J_c^c }}} \right. \kern-\nulldelimiterspace} {J_c^c }=$ 500$\sim $600. We also investigated the effect of pinning microstructures on $J_{c}^{c}$ and $\gamma $. Our result shows a direct correlation of $J_{c}^{c}$ (77 K, sf) and $\gamma $ to the density of stacking faults. An estimation reveals that the fraction of tape width associated with $c$-axis current flow grows linearly from 5{\%} to 20{\%} with increasing $\gamma $ for a typical geometry and could affect the performance of power transmission in HTS cables. [Preview Abstract] |
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