Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session P15: Focus Session: Superconducting Qubits II |
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Sponsoring Units: GQI Chair: Frank Wilhelm, University of Waterloo Room: Morial Convention Center 207 |
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P15.00001: Quantum non-demolition measurement of a superconducting two-level system Invited Speaker: In quantum mechanics, measurement can be understood as the interplay between extraction of information and disturbance of the state of the measured system. For projective measurements this disturbance is minimized: the post-measurement state is fully correlated to the indication of the detector. Quantum non-demolition (QND) detection is a strategy used to implement a projective measurement, which relies on a specific type of interaction between the measured system and the detector. In our experiments we apply these principles to the measurement of a superconducting flux qubit, which is an artificial two-level system built using mesoscopic Josephson junctions. Our detection method relies on probing the response of a hysteretic non-linear resonator coupled to the qubit. This setup allows for very efficient detection of the state of our system, with a measured contrast of 85{\%}. The large correlations between the results of two consecutive measurements demonstrate the QND nature of this method. This result establishes the validity of a QND strategy for projective measurement of superconducting qubits and has implications for quantum information processing. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P15.00002: Tunneling of a modulated oscillator: quantum interference in the classically forbidden region Michael Marthaler, Mark Dykman We describe a new coherent quantum effect in periodically modulated systems. It occurs in a modulated nonlinear oscillator and has no analog in two-level systems. The effect consists in oscillations and sign change, with the varying modulation frequency, of the tunnel splitting of the symmetric and antisymmetric time-periodic states. These states are formed by period-2 oscillator states, which classically have the same amplitudes and opposite phases. The effect is due to the wave function oscillations and the related interference in the classically forbidden region of the oscillator phase space. The tunnel splitting oscillations emerge already in the ``ground state'' of the oscillator Hamiltonian in the rotating frame. The WKB analysis in the rotating wave approximation is in excellent agreement with the numerical results. The tunnel splitting oscillations persist in the parameter range where the rotation wave approximation becomes inapplicable. The effect occurs in the parameter range accessible with currently available Josephson junction-based systems. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P15.00003: Dynamical tunneling in macroscopic systems Ioana Serban, Frank Wilhelm We investigate macroscopic dynamical quantum tunneling (MDQT) in the driven Duffing oscillator, characteristic for Josephson junction physics and nanomechanics. Under resonant conditions between stable coexisting states of such systems we calculate the tunneling rate. In macroscopic systems coupled to a heat bath, MDQT can be masked by driving-induced activation. We compare both processes, identify conditions under which tunneling can be detected with present day experimental means and suggest a protocol for its observation [1]. \\ {[1]} I. Serban and F. K. Wilhelm, Phys. Rev. Lett. 99, 137001 (2007) [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P15.00004: Measurement induced heat bath and decay rates in circuit QED. Maxime Boissonneault, Jay Gambetta, Alexandre Blais In circuit QED, a superconducting qubit is fabricated inside a high quality superconducting coplanar resonator. This system allows for strong interaction of the artificial atom with the photon field [1]. In the dispersive regime, where the detuning between the qubit and the resonator frequency is large with respect to their coupling, the physics of this system is understood in terms of Lamb and Stark shifts. However, as the coupling strength or the number of photons in the resonator increases, this description breaks down. In this talk, we will explain that, when taking into account higher order corrections to the dispersive approximation, measurement photons act as a heat bath inducing incoherent relaxation and excitation of the qubit. We will discuss how this can decrease achievable signal-to-noise ratio and may reduce the QND aspect of the measurement. [1] A. Wallraff, et al., Nature 431, 162 (2004) [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P15.00005: Single shot readout in a circuit QED system Andrew Houck, Alexandre Blais, Steven Girvin, Robert Schoelkopf In the dispersive limit of circuit QED, photon transmission can be used for quantum non-demolition measurements of the state of a superconducting qubit. Here, we present an optimization of the measurement of a transmon qubit, including a new understanding of how measurement affects the rate of demolition. Both cavity and qubit parameters were optimized to maximize signal to noise without introducing substantial new channels for decoherence. Single shot readout fidelities of over 70\% have been acheived, and greater than 90\% fidelity should be possible with presently acheivable coherence times. This opens up the possiblity of observing quantum jumps in the state of the qubit. Work done in collaboration with the Yale circuit QED team. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P15.00006: Dissipation and cooling of a nanomechanical oscillator coupled to a Cooper pair box Rakesh Tiwari, D. Stroud We calculate the dynamics of a nanomechanical oscillator (NMO) coupled capacitively to a Cooper pair box (CPB), by solving a stochastic Schrodinger equation with two Lindblad operators. Both the NMO and the CPB are assumed dissipative. We show numerically that, if the CPB decay time is smaller than the NMO decay time, the coupled NMO will lose energy faster, and the coupled CPB more slowly, than the uncoupled NMO and CPB. We find that both of these effects are largest if $\hbar$ times the NMO frequency equals the energy splitting of the CPB. Thus we show that an NMO can be cooled to low temperatures much more efficiently by coupling the NMO to a CPB. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P15.00007: Microwave Reflectometry Measurements of Flux States of a dc SQUID Phase Qubit B. K. Cooper, R. M. Lewis, S. K. Dutta, T. A. Palomaki, Anthony Przybysz, H. Kwon, Hanhee Paik, J. R. Anderson, C. J. Lobb, F. C. Wellstood We examine microwave reflectometry readout of a dc SQUID phase qubit. Our device is a Nb/AlOx/Nb SQUID fabricated by Hypres with loop inductance of 1.3 nH and symmetric junction critical currents of approximately 5 $\mu $A. The SQUID is current and flux biased, with one junction used as the qubit and the other used to provide isolation. The isolation junction is shunted by a large capacitor to depress its plasma frequency to about 1.5 GHz. This frequency can be shifted by flux-induced circulating current in the SQUID loop, allowing us to determine which flux state we are in by making reflectometry measurements of the resonant behavior of the isolation junction. The utility of this measurement for qubit state readout is discussed. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P15.00008: Measurements of a dc-SQUID phase qubit using rf-reflectometry R.M. Lewis, B.K. Cooper, S.K. Dutta, T. A. Palomaki, Hanhee Paik, A. Przybysz, H. Kwon, J. R. Anderson, C. J. Lobb, F. C. Wellstood We performed measurements of a $Nb/AlOx/Nb$ dc--SQUID phase qubit at 100 mK by monitoring the plasma frequency, $f_p$, of the readout/isolation junction. This qubit contains two Josephson junctions (JJ) separated by a 1.3 nH inductance; one JJ operates as a pure phase qubit, the second JJ isolates and reads out the qubit junction. When driving the isolation junction at 1.5 GHz, near $f_p$, current fluctuations in the SQUID loop cause fluctuations in $f_p$ which appear in the sidebands of the reflected microwave power. At 100 mK we find an effective flux noise, $S_{\Phi}^{0.5}$ of $50\ \mu \Phi_0 / Hz^{0.5}$ at 1 Hz. The measurement bandwidth is about 10 MHz, the upper limit being set by the $Q$ of the readout junction. We will discuss how the measurement performance depends on biasing parameters of each junction. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P15.00009: DC SQUID Phase Qubit with LC Filter Hyeokshin Kwon, A.J. Przybysz, Hanhee Paik, R.M. Lewis, T.A. Palomaki, S.K. Dutta, B.K. Cooper, J.R. Anderson, C.J. Lobb, F.C. Wellstood We investigate the use of an inductor-capacitor (LC) network to increase the isolation of a dc SQUID phase qubit from its current bias leads and thereby increase the dissipation time T$_{1}$ and coherence time T$_{2}$. One junction in the SQUID acts as an ideal phase qubit while the second junction and the SQUID loop inductance act as a broadband filter to isolate the first junction from the current bias leads. The LC-isolation network provides an additional isolation factor and allows flexibility in the choice of SQUID parameters. In addition to increasing the isolation from the leads, our design minimizes the effects of dielectric loss and two-level systems by using a relatively small Josephson junction, building the devices from Al/Al$_{2}$O$_{3}$/Al on sapphire, and only using insulating layers (SiN$_{x})$ in external capacitors for the phase qubit junction and LC network. *Funding provided by JQI, CNAM and the DOD. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P15.00010: Strong tunable coupling between a charge qubit and a phase qubit Aurelien Fay, Emile Hoskinson, Florent Lecocq, Laurent Levy, Frank Hekking, Wiebke Guichard, Olivier Buisson We have studied the quantum dynamics of a superconducting circuit based on a dc-SQUID coupled to a highly asymmetric Cooper pair transistor (ACPT). The dc-SQUID is a phase qubit controlled by a bias current and magnetic field. The ACPT is a charge qubit controlled by a bias current, magnetic flux and gate voltage. We have measured by microwave spectroscopy the lowest quantum levels of the coupled circuit as function of applied flux, bias current and gate voltage. Quantum state measurements of the phase and charge qubit are achieved by a nanosecond flux pulse applied to the dc-SQUID. Our circuit enables the independent manipulation of each qubit as well as the entanglement of the quantum states of the two circuits. We observe avoided level crossings between the two qubits when they are put in resonance. The coupling strength is measured over a large frequency range and varies from 100MHz to 1.3GHz. We succeed to realize a tunable coupling between the charge and the phase qubit. The measured tunable coupling strength is well explained by a combination of a capacitive and a Josephson coupling between the two qubits. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P15.00011: Superconducting phase qubit in a ``camel'' potential Emile Hoskinson, Florent Lecocq, Aur\'elien Fay, Nicolas Didier, Ralf Dolata, Alexander Zorin, Frank Hekking, Wiebke Guichard, Olivier Buisson A prototypical phase qubit consists of a single current biased Josephson junction, in which the dynamics of the phase across the junction is analogous to a quantum particle trapped in a quadratic-cubic potential. We demonstrate a phase qubit in a double barrier quadratic-quartic ``camel'' potential. This potential is formed by a 2-junction niobium circuit in a dc-SQUID configuration, with near zero current bias and flux bias close to half a flux quantum. Because of the symmetry of the potential, the qubit is predicted to be optimally insensitive to current fluctuations. We perform a nanosecond single shot measurement by applying a flux pulse which reduces the height of the two potential barriers, allowing the excited state of the qubit to escape by two independent paths to an adjacent flux state of the dc-SQUID. We find Rabi oscillation, Ramsey oscillation, and energy relaxation decay times on the order of 60 ns, 20 ns, and 100 ns, respectively. Via spectroscopy, we show that the effects of current noise are rendered negligible in this circuit. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P15.00012: Properties of a High-T$_c$ Intrinsic Phase Qubit X. Y. Jin, J. Lisenfeld, Y. Koval, A. V. Ustinov, P. M\"uller We discuss the properties of high-T$_c$ intrinsic phase qubits. An intrinsic phase qubit is a superconducting ring made of a Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystal, intercepted by two intrinsic Josephson junction stacks. As a stack consists of many intrinsic Josephson junctions, an intrinsic phase qubit can be regarded as a multi-junction system, i.e. a system of many degrees of freedom in phase space. However, I-V characteristics and switching current distributions of our samples show that an intrinsic phase qubit behaves like a system with only two degrees of freedom, independent of the number of junctions in the stacks, as long as the two stacks are uniform. Due to the large self-inductance, the potential of an intrinsic phase qubit has several minima. In order to perform quantum operations from a single well, a technique using low-frequency microwaves is presented. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P15.00013: ABSTRACT WITHDRAWN |
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