Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session X10: Superconducting Devices and Applications |
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Sponsoring Units: DCMP Chair: Constantine Vlahacos, University of Maryland Room: Morial Convention Center RO8 |
Friday, March 14, 2008 8:00AM - 8:12AM |
X10.00001: A tunable parametric amplifier based on a SQUID array resonator with nearly quantum-limited sensitivity Manuel Castellanos-Beltran, Konrad Lehnert Recently, there has been an increasing number of compelling applications for quantum-limited amplifiers at microwaves frequencies. These include the readout of superconducting qubits and ultrasensitive measurements of the motion of nanomechanical beams. Although quantum-limited microwave amplifiers have already been demonstrated, they have suffered from both limited bandwidth and dynamic range. We create a Josephson parametric amplifier from a transmission line resonator whose inner conductor is made from a series SQUID array. By changing the magnetic flux through the SQUID loops, we are able to adjust the circuit's resonance frequency and, consequently, the center of the amplified band over an octave (4-8 GHz). This tunability circumvents some of the problems related to a limited bandwidth. We will discuss recent results that include demonstration of large gain (30 dB), nearly quantum-limited sensitivity and noise squeezing. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X10.00002: Parametric amplification in a DC SQUID amplifier at 1.7 GHz Jose Aumentado, Lafe Spietz, K.D. Irwin At NIST we have recently developed a DC SQUID-based microwave amplifier employing a resonant input circuit. In conventional operation this amplifier can be operated in a linear, phase-preserving mode. However, it can also be operated as a degenerate parametric amplifier with the SQUID functioning as a first stage amplifier. We will discuss the performance of this hybrid operation and the possibility of generating squeezed states in this system. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X10.00003: Progress at NIST on DC SQUID Microwave Amplifiers Lafe Spietz, K. D. Irwin, J. Aumentado We report on the development at NIST of microwave amplifiers using DC SQUIDs. Our design approach is to use small SQUIDs which can be modeled as lumped element circuits, thus separating the design process for the SQUID from that of the microwave impedance transformers. We present our model of the impedance, gain and noise of such a SQUID and measurements of the characteristics of our amplifiers. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X10.00004: Input Impedance of the Microstrip SQUID Amplifier Darin Kinion, John Clarke We present measurements of the complex scattering parameters of microstrip SQUID amplifiers (MSA) cooled to 4.2 K. The input of the MSA is a microstrip transmission line in the shape of a square spiral coil surrounding the hole in the SQUID washer that serves as the ground plane. The input impedance is found by measuring the reverse scattering parameter (S11) and is described well by a low-loss transmission line model. We map the low-loss transmission line model into an equivalent parallel RLC circuit in which a resistance R, inductance L, and capacitance C are calculated from the resonant frequency, characteristic impedance and attenuation factor. Using this equivalent RLC circuit, we model the MSA and input network with a lumped circuit model that accurately predicts the observed gain given by the forward scattering parameter (S21). We will summarize results for different coil geometries and terminations as well as SQUID bias conditions. A portion of this work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory in part under Contract W-7405-Eng-48 and in part under Contract DE-AC52-07NA27344 and by Lawrence Berkeley National Laboratory under Contract No. DE-AC02-05CH11231. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X10.00005: In vivo T$_{1}$ maps at fields from 0.001 to 150 mT with SQUID based MRI M. Hatridge, S. Busch, T. Wong, M. Moessle, A. Pines, J. Clarke High field magnetic resonance imaging (MRI) uses differences in the longitudinal relaxation times (T$_{1})$ of protons to differentiate tissue types. Using phantoms and \textit{ex vivo} human tissue, we have demonstrated that T$_{1}$ contrast at low fields can be much greater than that at high fields, suggesting that one can, for example, distinguish tumors from healthy tissue without the use of contrast agents. Our MRI system polarizes protons at fields up to 150 mT using a water-cooled electromagnet. Subsequently, we ramp down the polarizing field and measure the proton nuclear magnetic resonance at typically 0.132 mT in the presence of three-dimensional field gradients using a SQUID coupled to an untuned, second-derivative gradiometer. Our system is capable of imaging the human arm \textit{in vivo} with a resolution of 2x2x10 mm$^{3}$. By switching the field to an intermediate field value for a variable time, we can obtain T$_{1}$ maps at fields between 0.001 and 150 mT. We present \textit{in vivo} T$_{1}$ dispersion curves in the same range of magnetic fields for several types of tissue in the human arm. Work supported by USDOE. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X10.00006: Fast, single-photon detection with a superconducting Nb nanowire Anthony Annunziata, Andrew Mack, Joel Chudow, Daniel Santavicca, Aviad Frydman, Michael Rooks, Luigi Frunzio, Daniel Prober We investigate the performance of a superconducting nanowire detector made from an ultra-thin, pure Nb film. Single photon counting performance is shown with good quantum efficiency at 470 nm. We report the reset time, jitter, and dark count rate for single photon detection. We compare these results to reports for NbN detectors. The Nb detector has a faster reset time for the same size active area, with similar quantum efficiency. These detectors have a variety of potential applications ranging from VLSI circuit diagnostics to quantum communication and single molecule spectroscopy. This work is supported by NSF -- EPDT and IBM research. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X10.00007: Local superfluid densities probed via current-induced superconducting phase gradients Alexey Bezryadin, David Hopkins, David Pekker, Tzu-Chieh Wei, Paul Goldbart We have developed a superconducting phase gradiometer consisting of two parallel DNA-templated nanowires connecting two thin-film leads [1,2,3]. We have ramped the cross current flowing perpendicular to the nanowires, and observed oscillations in the lead-to-lead resistance due to cross-current-induced phase differences. By using this gradiometer we have measured the temperature- and magnetic-field dependence of the superfluid density, and observed an amplification of phase gradients caused by elastic vortex displacements. We examine our data in light of Miller-Bardeen theory of dirty superconductors and a microscale version of Campbell's model of field penetration. \newline [1] Hopkins et al., Science {\bf 308}, 1762 (2005). [2] Pekker et al., Phys. Rev. B {\bf 72}, 104517 (2005). [3] Hopkins et al., Phys. Rev. B Rapid Comm. (2007, in press), accepted for publication. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X10.00008: Entanglement of Two Josephson Vortex Quantum Bits in Resonant Cavity Isaac O'Bryant, Ramesh P. Dhungana, Ju H. Kim We investigate the entanglement between two Josephson vortex qubits (JVQ's) in a resonant cavity. A JVQ may be fabricated using two closely spaced microresistor sites in an insulator layer of a long Josephson junction. The phase dynamics of a Josephson vortex (or fluxon) may be described using the perturbed sine-Gordon equation. In a uniform electromagnetic field, it is found that the resonant cavity interacts with fluxons only when they are trapped on a microresistor site. The effect of a resonant cavity on the two JVQ's may be represented as a deformation of the two-qubit potential function. We examine the effects of resonant cavity and magnetic induction on the potential for two non-interacting JVQ's. The deformation of the potential due to the resonant cavity yields a significant increase in the two-fluxon tunneling compared to the single-fluxon tunneling, indicating that entanglement between the two JVQ's is significantly increased. We compute the concurrence to estimate how the entanglement is affected by the magnetic induction effect and the coupling between the fluxons and the resonant cavity. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X10.00009: Death of entanglement of two Josephson vortex qubit due to the dissipation effect Ramesh Dhungana, Isaac O'Bryant, Ju Kim We investigate the effects of the dissipation on the two entangled Josephson vortex qubits (i.e. JVQ) using spin-boson model. It has been suggested that the decoherence time for a JVQ can be long at ultra-low temperature because it couples only weakly to the sources of decoherence. The entanglement of two JVQs due to the magnetic induction effect between two long Josephson junctions and their coupling to a single mode resonant cavity may be destroyed due to the same source of decoherence, which are present in the environment. We consider the decoherence effect on the JVQ system by using a dissipative thermal bath. We estimate its effect on entanglement, which can be measured in terms of concurrence, to show that the entanglement may die down quickly due to this decoherence source. We compare the time scale for entanglement survival of two JVQs in the dissipative environment with the decoherence time for a single JVQ qubit and discuss its effect on the two qubit operation. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X10.00010: Probing Temperature Dependent Noise in Flux Qubits via Macroscopic Resonant Tunneling A.J. Berkley, R. Harris, M.W. Johnson, J. Johansson, P. Bunyk, S. Govorkov, M.C. Thom, S. Uchaikin, C.J.S. Truncik, M.H.S. Amin, S. Han, B. Bumble, A. Fung, A. Kaul, A. Kleinsasser, D.V. Averin Macroscopic resonant tunneling between the two lowest lying states of a bistable RF-SQUID is used to characterize flux noise in a potential qubit. Detailed measurements of incoherent decay rates as a function of flux bias revealed that the Gaussian shaped tunneling rate is not peaked at the resonance point, but is shifted to a flux bias at which the initial well is higher than the target well. This observation indicates that the dominant low frequency (1/f) flux noise in this device is quantum mechanical in nature. The r.m.s. amplitude of the noise, which is proportional to decoherence rate 1/T$^{*}_{2}$, was observed to be weakly dependent on temperature below 70 mK. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X10.00011: Simulation of a YBCO Superconducting Quantum Interference Filter Stephen M. Wu, Shane A. Cybart, John Clarke, R.C. Dynes A Superconducting Quantum Interference Filter (SQIF) consists of a serial or parallel combination of SQUIDs of varying area that at constant current bias produces a sharp voltage peak at zero magnetic field. We simulated a serial array of 300 SQUIDs to calculate the voltage response versus applied magnetic field. We chose representative values of the junction critical current I$_{C}$ and resistance R$_{N}$ for ion damaged YBCO Josephson junctions. We varied the areas to maximize the sharpness and height of the voltage peak. We used the results of the simulation to design a SQIF that we fabricated and tested. The measured voltage response of the device was smaller than the predictions of the model. The agreement was significantly improved by including the effects of the geometric inductances of the SQUID loops and the Fraunhofer diffraction pattern of the individual junctions, both of which reduced the predicted amplitude of the SQIF response. It is likely that the remaining discrepancies are due partly to random variations in I$_{C}$ and R$_{N}$ in the experimental device, which we shall include in future simulations, and partly to the effects of thermal noise. This work was supported by AFOSR. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X10.00012: Measurements of a YBCO superconducting quantum interference filter with planar ion-damaged Josephson junctions. Shane A. Cybart, S. Wu, I. Siddiqi, John Clarke, R.C. Dynes We have fabricated a superconducting quantum interference filter (SQIF) containing 300 SQUIDS connected in series. Loops in a YBCO film were patterned using photolithography and argon ion milling. The Josephson junction barriers were formed with ion bombardment through 30 nm wide slits that were patterned with electron beam lithography and reactive ion etching of a 25 nm germanium / 800 nm photoresist mask. The ion damage lowered the Tc of the 30 nm unmasked region resulting in SS'S junctions, where supercondconductor S has a Tc of 85 K, and S' of 75 K. The IcRn product for individual junctions was determined to be 0.02 mV from current-voltage characteristics measured at 73K. Voltage versus magnetic field curves were measured for different static bias currents. They show a dip at zero field which increases with increasing bias up to a saturation value of 2 mV. The maximum transfer coefficient was 17 mV/mT. Noise properties and linearity measurements will be presented. This work was supported by AFOSR, and by DOE through the LBNL Molecular Foundry. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X10.00013: Fabrication and evaluation of the superconducting d-dot device manufactured with the Y$_{0.9}$La$_{0.1}$Ba$_{1.9}$Cu$_{3}$O$_{y}$ thin film by a DC Sputtering method Masahide Nishiyama, Hiroaki Sato, Masuo Yamamoto, Seiji Adachi, Hironori Wakana, Keiichi Tanabe, Takakazu Ishida The d-dot device is composed of a square shaped d-wave superconductor buried into a s-wave superconductor thin film. The internal phase difference at neighboring square side junction causes a half-quantum-flux at each corner of square of d-wave superconductor. We have developed the method for preparing the d-dot with YBCO thin film by PLD method previously. In the present work, we employed a DC sputtered Y$_{0.9}$La$_{0.1}$Ba$_{1.9}$Cu$_{3}$O$_{y}$ thin film with ramp edge, which is a well-established standard process for fabricating high-T$_c$ single flux quantum (SFQ) device of SRL- ISTEC. Evaluations of this d-dot device are performed by I-V and R-T measurements. The magnetic flux structure has been investigated by a scanning SQUID microscope. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X10.00014: High frequency flux sampling SQUID microscope Constantine Vlahacos, John Matthews, Frederick Wellstood One important application of scanning SQUID microscopes is to locate electrical faults in integrated circuits and multi-chip modules. However, current computer microprocessors operate at over 1 GHz, well above the bandwidth of the present generation of SQUID microscopes. By removing the conventional flux-locked loop electronics we have overcome the bandwidth limitations of traditional scanning SQUID microscopes. Instead we use a pulsed sampling technique with a small Nb/AlO$_{x}$/Nb hysteretic dc SQUID. We present time-varying magnetic field images of room temperature samples obtained with the SQUID mounted on a 4.2 K pulse tube refrigerator in a scanning SQUID microscope, and discuss the advantages and limitations of this method. [Preview Abstract] |
Friday, March 14, 2008 10:48AM - 11:00AM |
X10.00015: Scanning Tunneling Spectroscopy Studies of AlN Tunnel Barriers Y. Li, J. Read, H. Tseng, R. Buhrman Typical Josephson junctions (JJ's) utilize alumina (AlO$_{x})$ tunnel barriers due primarily to the reliable thermal oxidation procedure that yield high quality Nb-Al-AlO$_{x}$-Nb JJs in the low and moderate ($\le $ 10$^{4}$ A/cm$^{2})$ critical current density (J$_{c})$ regime[1]. However, AlN provides the possibility of forming ultra-thin barriers with fewer defects, and hence lower sub-gap leakage currents, and thus could improve device performance in the high J$_{c}$ regime [2-4]. We present results from an X-ray photoelectron spectroscopy (XPS) and scanning tunneling spectroscopy (STS) study of thin AlN layers on Nb formed by reactive radio frequency (rf) sputtering from an AlN target in a mixture of Ar and N gases. The XPS spectra indicates that O is generally incorporated into the nitride layer during growth in high and near-ultra-high vacuum, thus forming AlO$_{x}$N$_{y}$. The STS measurements reveal that these AlO$_{x}$N$_{y}$ layers exhibit an increase in bandgap with increased N content in the process gas. Decreased band-tails and improved surface stability suggest the barrier defect density can be modified through moderate post-growth annealing. We will provide suggestions for optimization of rf sputtered AlO$_{x}$N$_{y}$ layers for use in high J$_{c}$ Nb and NbN based JJs. [1] Miller, APL 63, 1423 (1993) [2] Wang, APL 64, 2034 (1994) [3] Kleinsasser, IEEE TAS 5, 2318 (1995) [4] Kaul, JMRS 20, 3047 (2005) [Preview Abstract] |
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