Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session W33: Josephson Junctions and SQUIDS II |
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Sponsoring Units: GQI Chair: Dietrich Leibfried, NIST Room: LACC 511C |
Thursday, March 24, 2005 2:30PM - 3:06PM |
W33.00001: Harnessing a Dynamical Bifurcation for a High Fidelity, Dispersive Qubit Readout Invited Speaker: A dynamical bifurcation, if sufficiently well controlled, can sensitively amplify quantum signals. We have successfully implemented this principle in a microwave driven anharmonic Josephson oscillator. I will present measurements on the Josephson bifurcation amplifier (JBA) as a controlled, projective readout for a superconducting Quantronium qubit. The JBA also lays the groundwork for a quantum ``erasure'' experiment with macroscopic tunnel junction circuits, and is thus a route for a novel test of the foundations of quantum mechanics. [Preview Abstract] |
Thursday, March 24, 2005 3:06PM - 3:18PM |
W33.00002: Multiphoton antiresonance Mark Dykman, Michael Fistul We show that nonlinear response of a quantum oscillator displays antiresonant dips or resonant peaks with varying frequency of the driving field. The effect accompanies resonant multiphoton mixing of oscillator states, which leads to anticrossing of the quasienergy levels in adiabatic passage of the field frequency through resonance. Usually one would expect anticrossing to be associated with switching from the response in one of the involved states to that in the other. The totally different behavior of the response of a nonlinear oscillator is a consequence of dynamical symmetry which shows that, in the neglect of multiphoton interstate transitions, (i) the response in the resonating states is the same, and (ii) many states are in resonance, pairwise, for the same field frequency. The width and height of the dips (peaks) of the response strongly depend on the field amplitude. We discuss the possibility to observe the antiresonance and the associated multiphoton Rabi oscillations in multilevel Josephson junctions used for quantum measurements. [Preview Abstract] |
Thursday, March 24, 2005 3:18PM - 3:30PM |
W33.00003: Characterizing the backaction of the Josephson Bifurcation Amplifier on the Quantronium qubit R. Vijay, I. Siddiqi, M. Metcalfe, E. Boaknin, L. Frunzio, M.H. Devoret We have constructed a novel amplifier - the Josephson Bifurcation Amplifier (JBA)- for reading out a superconducting quantum bit (Quantronium). The amplifier exploits the non-linearity of the Josephson junction to achieve high speed and single shot operation. The JBA is based on the switching between two dynamical states of an rf-driven Josephson junction biased near a dynamical bifurcation. I will present experimental results which characterize the backaction of the JBA on the qubit during the measurement process. The high speed readout (repetition rate of 5 - 10 MHz) allows us to make successive measurements of the quantum state and study the correlations between them. [Preview Abstract] |
Thursday, March 24, 2005 3:30PM - 3:42PM |
W33.00004: Low-bandwidth control scheme for an oscillator stabilized Josephson qubit R. H. Koch, J. R. Rozen, G. A. Keefe, F. M. Milliken, C. C. Tsuei, J. R. Kirtley, D. P. DiVincenzo We introduce a new flux-based Josephson junction circuit for which quantum operations are realized by low-bandwidth, nearly adiabatic magnetic-flux pulses. Coupling to the fundamental mode of a superconducting transmission line permits a stabilization of the rotation angle of the quantum operation against flux noise. A complete scheme for one-qubit rotations, and high-visibility Ramsey-fringe oscillations, is given. We show that high visibility depends on passing through a portal in the space of applied fluxes, where the width of the portal is proportional to the ramp-up rate of the flux pulse. Initial measurements of such a qubit show a measured visibility of 60 percent and a coherence time of greater than 35 ns. The fundamental mode frequency of the transmission line was 1.54 GHz. [Preview Abstract] |
Thursday, March 24, 2005 3:42PM - 3:54PM |
W33.00005: A Quantum Capacitance Qubit C.M. Wilson, T. Duty, F. Persson, M. Sandberg, K. Bladh, D. Gunnarsson, J. Bylander, P. Delsing We study a new type of superconducting quantum bit (qubit) based on the quantum capacitance (QC) of a cooper-pair box (CPB). Recent developments have shown the importance of manipulating qubits at optimal working points where they are insensitive to environmental noise. The next step is to develop readout schemes that also function at these optimal points. Our approach embeds a CPB in a resonant circuit and detects changes in the QC as changes in the phase of a reflected microwave signal. The QC is simply the curvature of the energy bands near the charge degeneracy point. The curvature arises from the avoided level crossing induced by the Josephson coupling of the CPB. The QC can be much larger than the geometric capacitance, and changes sign between the ground and excited states. Calculations show that the method should be able to achieve single-shot discrimination of the qubit state. We will also present preliminary measurements. Our approach is a lumped-circuit analogy of the recent circuit cQED experiment done at Yale, except our qubit and cavity our not resonant. [Preview Abstract] |
Thursday, March 24, 2005 3:54PM - 4:06PM |
W33.00006: Nanomechanical quantum memory for superconducting qubits Emily Pritchett, Michael Geller Many protocols for quantum computation require a quantum memory element to store qubits. We have tested the accuracy with which quantum states prepared in a Josephson junction qubit can be stored in a nanoelectromechanical resonator and then transfered back to the junction. We find that the fidelity of the memory operation depends on both the junction-resonator coupling strength and the location of the state on the Bloch sphere. Although we specifically focus on a large-area, current-biased Josesphson junction phase qubit coupled to the dilatational mode of a piezoelectric nanoelectromechanical disk resonator, many of our results will apply to other qubit-oscillator models. [Preview Abstract] |
Thursday, March 24, 2005 4:06PM - 4:18PM |
W33.00007: Energy relaxation time of an rf SQUID flux qubit Shaoxiong Li, Wei Qiu, Matthew Matheny, Zhongyuan Zhou, Siyuan Han Energy relaxation is one of the main decoherence mechanisms in superconducting flux qubits. In this work we measured the energy relaxation time in an rf SQUID qubit for which all of the important parameters were determined independently from thermal activation, MQT, and microwave spectroscopy measurements. The quasi-static flux biases were adjusted to have only three energy levels involved in the microwave pump- probe experiment, a situation most relevant to three-level quantum state manipulations. The result shows that the characteristic time of energy relaxation from the excited state to the ground state of the qubit is about 3.6 $\mu $s. This result indicates that the dominating sources of damping were the qubit's flux bias and state readout circuitry\textbf {.}. [Preview Abstract] |
Thursday, March 24, 2005 4:18PM - 4:30PM |
W33.00008: Tunneling Spectroscopy of Two-level Systems Inside Josephson Junction Ivar Martin, Lev Bulaevskii, Alexander Shnirman We consider a two-level (TL) system with energy level separation $\hbar\Omega_0$ inside the Josephson junction. The junction is shunted by a resistor $R$ and is current $I$ (or voltage $V=RI$) biased. If the TL system modulates the Josephson energy and/or is optically active, at the resonance condition $2eIR=\hbar\Omega_0 $ the resistor voltage oscillates with the Rabi frequency determined by the strength of the coupling between the TL system and the phase difference. This effect provides the option to fully characterize the TL systems and to find the TL's contribution to the decoherence when junction is used as a qubit for quantum computation. [Preview Abstract] |
Thursday, March 24, 2005 4:30PM - 4:42PM |
W33.00009: Current - Flux characteristics of a hysteretic asymmetric dc SQUID qubit Hanhee Paik, J. Matthews, S.K. Dutta, H. Xu, R.C. Ramos, T.A. Palomaki, R.M. Lewis, J.R. Anderson, C.J. Lobb, F.C. Wellstood We present data and simulations of the behavior of a superconducting qubit, the asymmetric hysteretic dc SQUID or inductively isolated Josephson jucntion. Measurements of the current-flux (I-$\Phi$) characteristics were taken on an Al/AlO$_x$/Al dc SQUID at 100 mK and compared to analytical and numerical simulations. The SQUID has one junction with twice the critical current of the other junction, and one arm of a relatively large inductance, compared to the other arm of the SQUID. With this configuration, which was first proposed by Martinis \textit{et al.}[1], the larger junction acts as an inductively isolated qubit, while the smaller junction acts as a detector. From the I-$\Phi$ curve, we extract the SQUID parameters and reveals how well isolated the qubit is from its leads. This work is supported by the Department of Defense, NSF and the Center for Superconductivity Research. [1] Martinis \textit{et al.} Phys. Rev Lett. \textbf{89} 117901 (2002) [Preview Abstract] |
Thursday, March 24, 2005 4:42PM - 4:54PM |
W33.00010: Selection of a Flux State in a Multi-well Asymmetric SQUID Tauno A. Palomaki, S.K. Dutta, Hanhee Paik, R.M. Lewis, R.C. Ramos, H. Xu, J.R. Anderson, C.J. Lobb, F.C. Wellstood We have investigated the behavior of an asymmetric hysteretic dc SQUID for quantum computing. One junction acts as the Qubit, the other a detector, and the loop inductance provides isolation from the leads. One complication of this approach is that many minima are present in the potential, so that different trapped flux states exist, the number determined by the critical currents and inductances. Isolating a state is essential for controlling the Qubit and obtaining data efficiently. By oscillating the applied flux, similar to Lefevre-Seguin \textit{et al}. [1], about a particular flux offset, a given flux state can be selected with high probability. We present data showing the effectiveness of this technique for flux selection at 20mK. This work is supported by the Department of Defense, the NSF and the Center for Superconductivity Research. [1] V. Lefervre-Seguin \textit{et al}. Phys. Rev. B \textbf{46}, 5507 (1992). [Preview Abstract] |
Thursday, March 24, 2005 4:54PM - 5:06PM |
W33.00011: 1/f Critical Current Noise in Nb-Al-AlOx-Nb Trilayer Junctions T.A. Crane, D.J. Van Harlingen Low frequency (1/f) fluctuations in the critical current of Josephson junctions introduce phase noise into the coherent oscillations of superconducting qubits incorporating them, leading to dephasing. In order to quantify the degree to which this noise will affect the dephasing of qubits, we have measured the 1/f critical current noise in Nb-Al-AlOx-Nb trilayer Josephson junctions using a SQUID potentiometer circuit mounted in a dilution refrigerator. We present measurements taken over a range of temperatures from 1K to 10mK on junctions of different sizes in order to verify the behavior and origin of the 1/f noise. Work supported by the National Science Foundation grant EIA01- 21568. [Preview Abstract] |
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