Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session W39: Superconductivity-Josephson Junctions and Qubits |
Hide Abstracts |
Sponsoring Units: DCMP Chair: J. Robert Anderson, University of Maryland Room: Baltimore Convention Center 342 |
Thursday, March 16, 2006 2:30PM - 2:42PM |
W39.00001: Finite size scaling analysis of the helicity modulus and the inverse dielectric constant in two capacitvely coupled Ultrasmall 2D Josephson Junction Arrays Guillermo Ramirez-Santiago, Jorge Jose We have carried out a finite size scaling analysis of the helicity modulus $\Upsilon_{i}$ and the inverse dielectric constant $\epsilon_{i}$, $(i=1,2)$ of two capacitively coupled Josephson junction arrays with charging energy, $E_c$, and Josephson coupling energy, $E_J$. The arrays are coupled via the capacitance, $C_{\rm inter}$, at each site of the lattices. The parameter that measures the importance of quantum fluctuations in the i-th array is, $\alpha_i\equiv \frac{E_{{c}_i}}{E_{J_i}}$. We have considered the interplay between vortex and charge dominated individual array phases by means of extensive path integral Monte Carlo simulations. It has been found that this system develops a {\it reentrant transition} in $\Upsilon(T,\alpha)$, at low temperatures, when one of the arrays is in the semiclassical limit (i.e. $\alpha_{1}=0.5 $) and the quantum array has $2.0 \leq\alpha_{2} \leq 2.5$, for $C_{{\rm inter}}= 0.26087, 0.52174, 0.78261, 1.04348$ and $1.30435$. Similar behavior was obtained for larger values of $\alpha_{2} =4.0$ with $C_{{\rm inter}}=1.04348$ and 1.30435. [Preview Abstract] |
Thursday, March 16, 2006 2:42PM - 2:54PM |
W39.00002: RF critical current of Josephson junction Vladimir Manucharyan, Etienne Boaknin, Michael Metcalfe, R. Vijay, Irfan Siddiqi, Andreas Wallraff, R.J. Schoelkopf, Michel Devoret The Josephson junction is the only radio-frequency electrical element which can be both non-dissipative and non-linear at low temperatures. While the stability of the junction dynamics in presence of a DC drive has been extensively studied, the microwave drive case is relatively poorly understood, at least experimentally. It is explored by driving an increasing AC current through a Josephson junction which is effectively biased by an AC voltage generator in series with a finite linear imbedding impedance $Z\left( \omega \right)$. For small signal amplitude, the junction behaves as a linear inductor. For higher signal amplitudes, we show that there exists a critical current $I_{c}^{RF}$ beyond which the dynamics of the junction changes qualitatively as a result of its non-linear characteristic. This AC critical current depends strongly on the biasing impedance. We provide a detailed stability diagram from experimental measurements and show that it obeys the simple theory of nonlinear resonance. [Preview Abstract] |
Thursday, March 16, 2006 2:54PM - 3:06PM |
W39.00003: Parametric amplification with the Cavity Josephson Amplifier Etienne Boaknin, R. Vijay, Michael Metcalfe, Vladimir Manucharyan, Irfan Siddiqi, Michel Devoret Several types of amplifiers are approaching the quantum limit, namely, the SQUID, the RF-SET (radio-frequency single electron transistor) and the QCP (quantum point contact). We investigate a new amplifier which harnesses the nonlinearity of a Josephson junction for parametric amplification. It consists of a Josephson junction placed in a high-quality on-chip superconducting cavity, pumped by microwave radiation. The high level of control over the environment provides a system which is well described by the simplest nonlinear oscillator formalism with no adjustable parameters. such that theoretical predictions can be compared with experimental results. The planar geometry of the device can accommodate operation over a wide range of frequencies, opening the possibility of a quantum limited amplifier for practical use. We present preliminary results on the performance of the amplifier and discuss the possibility of observing quantum noise squeezing. [Preview Abstract] |
Thursday, March 16, 2006 3:06PM - 3:18PM |
W39.00004: Josephson current through a molecular transistor in a dissipative environment Tomas Novotny, Alessandra Rossini, Karsten Flensberg We study the Josephson coupling between two superconductors through a single correlated molecular level, including Coulomb interaction on the level and coupling to a bosonic environment. All calculations are done to the lowest, i.e., the fourth, order in the tunneling coupling and we find a suppression of the supercurrent due to the combined effect of the Coulomb interaction and the coupling to environmental degrees of freedom. Both analytic and numerical results are presented. [Preview Abstract] |
Thursday, March 16, 2006 3:18PM - 3:30PM |
W39.00005: Vortex doping into finite-sized superconducting Pb networks. Takekazu Ishida, Hiroshi Noda, Masaru Kato, Osamu Sato, Masahiko Hayashi, Hiromichi Ebisawa, Kazuo Satoh, Tsutomu Yotsuya Superconducting finite-sized Pb square networks with 2x2, 3x3, 5x5 and 10x10 square holes have been fabricated by electron beam lithography of photoresist layer and a lift-off process after depositing Pb film on the resist patterns. The application of magnetic field corresponds to the particle (vortex) doping into networks. Vortex image observations were carried out by a SQUID microscope to compare with the theoretical predictions. We found the exactly reversed pattern between the vortex doping x and the anitivortex doping x into the fully occupied network (x=1/4). The Ginzburg-Landau calculations show that there are several vortex configurations with almost equivalent free energy. The complete coincidence of the two patterns might be due to residual randomness caused in the fabrication processes. [Preview Abstract] |
Thursday, March 16, 2006 3:30PM - 3:42PM |
W39.00006: Nano-mechanical-resonator induced synchronization in Josephson junction arrays Brad Trees, Stefan Natu, David Stroud We show that a serial array of N critical-current disordered, underdamped, Josephson junctions coupled piezoelectrically to a nanomechanical (NEM) oscillator results in phase locking (synchronization) of the junctions. We find a semi-classical solution to the coupled differential equations generated by Heisenberg operator equations, based on a Hamiltonian including the following effects: charging and Josephson energies of the junctions, junction dissipation, effect of a dc bias current, and an undamped simple harmonic oscillator representing the NEM. Synchronization of the array is signaled by a step in the current- voltage (I-V) curve. Stability analysis reveals that the phase-locked junctions are neutrally stable at the bottom and top of the step. We calculate an analytic expression for the location of the resonance step in the I-V curve. We also find it is possible to set a desired number $N_{a} \quad \le N$ of junctions on the resonance step, with $N_{a}$ --$N $junctions in zero-voltage state. [Preview Abstract] |
Thursday, March 16, 2006 3:42PM - 3:54PM |
W39.00007: Effect of Microwaves on the Current-Phase-Relation of diffusive SNS Junctions M. Fuchsle, J. Bentner, P. Tranitz, W. Wegscheider, C. Strunk We investigate the current-phase-relation (CPR) of long diffusive
superconductor - normal metal - superconductor (SNS) Josephson
junctions under microwave irradiation. The samples consist of
narrow Ag bridges with a length between 300 and 500 nm inserted
into a Nb loop by shadow evaporation on top of a mesoscopic Hall
cross. Our Hall-sensors are based on high mobility GaAs/AlGaAs-
heterostructures. They directly detect the magnetic response of
the loop to an external magnetic field, from which the full CPR
can be reconstructed. The measurements are done in
the high-temperature regime $E_{Th} |
Thursday, March 16, 2006 3:54PM - 4:06PM |
W39.00008: Generation of Microwave Single Photons and Homodyne Tomography on a Chip Matteo Mariantoni, Markus Storcz, Frank Wilhelm, William Oliver, Andreas Emmert, Achim Marx, Rudolf Gross, Henning Christ, Enrique Solano We show that flux-based qubits can be coupled to superconductive resonators by means of a quantum-optical Raman excitation scheme and utilized for the deterministic generation of propagating microwave single photons. We introduce also a microwave quantum homodyning technique that enables the detection of single photons and other weak signals, and full state reconstruction via quantum tomography, realizing linear optics on a chip. These generation and detection protocols are building blocks for the advent of quantum information processing in the field of circuit QED (M. Mariantoni {\em et al.} cond-mat/0509737) . We discuss further applications of these ideas to create multipartite nonclassical states of the electromagnetic field. [Preview Abstract] |
Thursday, March 16, 2006 4:06PM - 4:18PM |
W39.00009: Crossover from single electron counting to Cooper pair counting Tim Duty, Jonas Bylander, Per Delsing We present experimental studies of charge transport in a one-dimensional array of Josephson junctions using a single charge counting device based upon a radio-frequency single-electron transistor$^1$. We observe a crossover from time-correlated tunneling of single electrons to Cooper pairs as a function of an applied magnetic field. At relatively high magnetic field, single electron transport dominates and the frequency is given by f=I/e. As the magnetic field is lowered the frequency gradually shifts to f=I/2e, indicating tunneling of Cooper pairs. $^1$Jonas Bylander, Tim Duty and Per Delsing, Nature 434 (2005) 285. [Preview Abstract] |
Thursday, March 16, 2006 4:18PM - 4:30PM |
W39.00010: 1/f Noise in Josephson Junctions Magdalena Constantin, Clare C. Yu A major obstacle to the realization of Josephson junction qubits is decoherence due to noise. Our goal is to understand the microscopic mechanisms which lead to the 1/f critical current noise spectrum at low temperatures. One possible source of critical current fluctuations is the presence of defects such as two level systems in the insulating junction barrier. We present a model of the critical current noise spectrum and compare it with recent experiments. [Preview Abstract] |
Thursday, March 16, 2006 4:30PM - 4:42PM |
W39.00011: Decoherence in Josephson Vortex Quantum Bits Ju Kim, Ramesh Dhungana, Kee-Su Park We investigated decoherence of a Josephson vortex quantum bit (qubit) in dissipative and noisy environment. As the Josephson vortex qubit is fabricated by using a long Josephson junction (LJJ), we use the perturbed sine-Gordon equation to describe the phase dynamics representing a two-state system and estimate the effects of quasiparticle dissipation and weakly fluctuating critical and bias currents on the relaxation time $T_1$ and on the dephasing time $T_\phi$. We show that the critical current fluctuation does not contribute to dephasing of the qubit in the lowest order approximation. Modeling the weak current variation from magnetic field fluctuations in the LJJ by using the Gaussian colored noise with long correlation time, we show that the time $T_2$ is limited by the low frequency current noise at very low temperatures. Also, we show that a ultra-long coherence time may be obtained from the Josephson vortex qubit by using experimentally accessible value of physical parameters. [Preview Abstract] |
Thursday, March 16, 2006 4:42PM - 4:54PM |
W39.00012: Phase-space theory for nonlinear detectors of superconducting qubits Ioana Serban, Frank Wilhelm Superconducting circuits are envisioned as quantum bits and demonstrate quantum-coherent features i.e. Rabi oscillations and Ramsey fringes. The detector (e.g. a superconducting quantum interference device, SQUID) can itself be described by a Hamiltonian and treated quantum-mechanically.This allows more insights into the measurement process. Several experimental groups have recently realized good detectors with strong coupling to the measured system, where nonlinear dynamics plays a significant role. Motivated by the recent experiment [1], we study a nonlinear detector where the qubit couples to the square amplitude of a driven oscillator, which can be used for dispersive detection. We use a complex-environment approach treating the qubit and the oscillator exactly, expressing their full Floquet-state master equations in phase space. We investigate the backaction of the environment on the measured qubit and explore the resolution of measurement. We discuss the possibility for using the squeezing capability of the nonlinear interaction for beating the standard quantum limit and emphasize the resulting role of non-Gaussian and non-Markovian effects in the backaction including significant non-exponential shape of the coherence decay. \\ {[}1{]} A. Lupascu et al. PRL 93 177006 (2004) [Preview Abstract] |
Thursday, March 16, 2006 4:54PM - 5:06PM |
W39.00013: Quasiparticle Poisoning in a Cooper Pair Box caused by a measuring SET Carlos Sanchez, Benjamin Palmer, Marc Manheimer, Fred Wellstood We have developed a model to calculate the average charge on a Cooper pair box in the presence of quasiparticle poisoning. The model uses a master equation approach to find the probabilities for the box to be in the even or odd state. The transition rates between the two states are calculated assuming a fixed number of non-equilibrium quasiparticles in the leads and island of the box. We fabricated Al/AlOx/Al devices with a Cooper pair box capacitively coupled to an SET and measured the charge on the box for SET bias currents ranging from about 1 pA to 1 nA. We find good agreement between the theory and measurements in the temperature range from 60 mK to 300 mK. For large SET bias the poisoning in the Cooper pair box increases and the charge staircase develops additional features. Our model is capable of qualitatively reproducing the features induced by the measuring SET. [Preview Abstract] |
Thursday, March 16, 2006 5:06PM - 5:18PM |
W39.00014: Cavity Josephson Bifurcation Amplifier: a microwave readout for a superconducting qubit Michael Metcalfe, Etienne Boaknin, Vladimir Manucharyan, Simon Fissette, Irfan Siddiqi, Rajamani Vijayaraghavan, Chad Rigetti, Andreas Wallraff, Robert Schoelkopf, Michel Devoret A Josephson junction, embedded in a microwave circuit that displays a resonance, and driven near the resonance frequency by a sinusoidal signal with adequate amplitude, can adopt one of two dynamical metastable states. The transition between the two states can be triggered by a small variation in the environment of the junction. This switching phenomenon naturally lends itself to the readout of a superconducting quantum bit. We are approaching the problem of mapping the two states of a qubit onto the two dynamical states of the Josephson junction by placing it in an on-chip coplanar waveguide superconducting cavity. We present the characterization of the cavity Josephson bifurcation amplifier (CJBA) and show that it follows theoretical predictions over a wide range of operating frequencies and bandwidth. This architecture provides a calculable RF environment which can be readily optimized. We also discuss a multi-resonator chip geometry that would implement the multiplexed readout of more than 10 qubits. [Preview Abstract] |
Thursday, March 16, 2006 5:18PM - 5:30PM |
W39.00015: Quantum dynamics and leakage of the superconducting flux qubit Ezequiel Pozzo, Daniel Dominguez We study the quantum dynamics of the superconducting flux qubit of Mooij et al, which consists on a SQUID with 3 Josephson junctions. We simulate the corresponding time-dependent Schrodinger equation and also eigenfunctions and eigenvalues are computed. We calculate the dynamical evolution of the qubit device after a pulse in magnetic field is applied from the operational point at half flux quantum. The system is started in the lowest energy level and we calculate the leakage into the non-computational basis after the pulse is applied, computing the change in the average population of the two lowest energy levels. The leakage is analyzed for different pulse intensities. Two different regimes are found for weak and strong pulses. We discuss the relationship of the response to strong pulses with the quantum chaoticity of the spectrum of high energy levels outside the computational space. [Preview Abstract] |
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