Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q17: Superconducting Qubits: Coherence and New Implementations |
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Sponsoring Units: DPOLY Chair: Matthias Steffen, IBM Room: 318 |
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q17.00001: Experimental progress toward single phonon creation in a mechanical resonator Aaron O'Connell, M. Ansmann, R. C. Bialczak, M. Hofheinz, E. Lucero, M. Neeley, D. Sank, H. Wang, J. Wenner, J. M. Martinis, A. N. Cleland Coupling a high frequency ($\sim $6 GHz) mechanical resonator to a Josephson phase qubit may enable the creation and manipulation of single phonons. Previously, we have shown the creation of arbitrary photon states in a superconducting coplanar waveguide (CPW) resonator coupled to a phase qubit (Max Hofheinz et al., Nature 454, 310-314, 2008). That experiment illustrated the capability of the phase qubit to controllably create and measure quantum states in a capacitively coupled resonator. By replacing the CPW resonator with a film bulk acoustic resonator (FBAR) we can potentially transfer the quantum state of the phase qubit to a vibrational mode of the FBAR. This talk will focus on our experimental progress to date in realizing this aim. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q17.00002: Lower limit on the achievable temperature in resonator-based sideband cooling M. Grajcar, S. Ashhab, J.R. Johansson, F. Nori A resonator with eigenfrequency $\omega_r$ can be effectively used as a cooler for another linear oscillator with a much smaller frequency $\omega_m \ll \omega_r$. A huge cooling effect, which could be used to cool a mechanical oscillator below the energy of quantum fluctuations, has been predicted by several authors. However, here we show that there is a lower limit $T^*$ on the achievable temperature, given by $T^* = T_{m} \; \omega_m / \omega_r$, that was not considered in previous work and can be higher than the quantum limit in realistic experimental realizations. We also point out that the decay rate of the resonator, which previous studies stress should be small, must be larger than the decay rate of the cooled oscillator for effective cooling. M. Grajcar, S. Ashhab, J.R. Johansson, F. Nori, Lower limit on the achievable temperature in resonator-based sideband cooling, Phys. Rev. B 78, 035406 (2008). URL: http://link.aps.org/abstract/PRB/v78/e035406 [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q17.00003: Strong single-qubit lasing and cooling at the symmetry point? Carsten Hutter Recent theoretical and experimental work discussed the possibility to achieve single-qubit lasing and cooling in systems of driven superconducting qubits coupled to an oscillator. The considered system and Hamiltonian in Refs. [1,2] were such that a first-order coupling term vanishes at the symmetry point. While Ref. [2] operated close to the symmetry point for using both first and second order coupling terms, another interesting regime would be the one with the opposing conditions of lowest dephasing (at the symmetry point) and strongest coupling (far away from the symmetry point).\\ Here, I address the question whether it is possible by different design to achieve single-qubit lasing or cooling with both strongest coupling and lowest dephasing, without compromising between the two. Starting from a more general model Hamiltonian than Refs. [1,2], I find the optimal conditions for a strong first order coupling at the symmetry point, realized by a different Hamiltonian than the one used in Refs. [1,2]. I also suggest first designs, which could realize this alternative model Hamiltonian, and discuss their practical limitations.\\ {[1]} J. Hauss, A. Fedorov, C. Hutter, A. Shnirman, G. Sch{\"o}n, Phys. Rev. Lett. 100, 037003 (2008)\\ {[2]} M. Grajcar et al, Nature Physics 4, 612 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q17.00004: Two-level systems driven by large-amplitude fields F. Nori, S. Ashhab, J.R. Johansson, A.M. Zagoskin We analyze the dynamics of a two-level system subject to driving by large-amplitude external fields, focusing on the resonance properties in the case of driving around the region of avoided level crossing. In particular, we consider three main questions that characterize resonance dynamics: (1) the resonance condition, (2) the frequency of the resulting oscillations on resonance, and (3) the width of the resonance. We identify the regions of validity of different approximations. In a large region of the parameter space, we use a geometric picture in order to obtain both a simple understanding of the dynamics and quantitative results. The geometric approach is obtained by dividing the evolution into discrete time steps, with each time step described by either a phase shift on the basis states or a coherent mixing process corresponding to a Landau-Zener crossing. We compare the results of the geometric picture with those of a rotating wave approximation. We also comment briefly on the prospects of employing strong driving as a useful tool to manipulate two-level systems. \\ \\ S. Ashhab, J.R. Johansson, A.M. Zagoskin, F. Nori, Two-level systems driven by large-amplitude fields, Phys. Rev. A 75, 063414 (2007). S. Ashhab et al, unpublished. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q17.00005: Quantum coherence in a Josephson junction array circuit. Vladimir Manucharyan, Jens Koch, Leonid Glazman, Robert Schoelkopf, Steven Girvin, Michel Devoret We introduce a novel superconducting quantum electrical circuit where a small capacitance Josephson tunnel junction is shunted by an array of larger junctions to form a loop. The loop is capacitively coupled to a microwave transmission line resonator in order to perform a dispersive readout of the qubit state. The low-lying energy states of such circuit belong to the microwave band and tune with magnetic flux threading the loop. Our circuit differs significantly from the well-established charge, flux and phase qubit circuits. Namely, while staying highly anharmonic, the energy spectrum is neither sensitive to the offset charges nor it is exponentially sensitive to the junction parameters or flux bias. We demonstrate experimentally strong coupling to the readout resonator, map the spectrum over wide range of bias fluxes and frequencies and observe coherence times in excess of one microsecond. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q17.00006: Quantum information processing with a Josephson ring modulator Nicolas Bergeal, Flavius Schackert, Michael Metcalfe, R. Vijay, Vladimir Manucharyan, Luigi Frunzio, Robert Schoelkopf, Steven Girvin, Michel Devoret We have developed and operated a new type of phase preserving parametric amplifier, the Josephson Parametric Converter, which approaches the quantum limit. Our device consists of two microwave resonators coupled to each other through a Josephson Ring Modulator. This latter element resembles a DC-SQUID, but has four junctions, and four active current modes instead of two. A pump line is non-resonantly coupled to one of the modes of the ring while the signal and idler are serviced by two others and are tuned in the band of the resonators. The fourth mode, which is the dc superconducting circulating current in the ring, is biased with half a flux quantum. Our design ensures that the non-linearity presented by the Ring Modulator is pure and involves the minimal number of modes, thus placing the JPC very close to the ideal non-degenerate parametric amplifier. This is supported by recent results on the amplification and frequency conversion operations. Furthermore, measurements of the noise temperature with an auto-calibrated source based on a nanowire in the hot electron regime will be presented. In combination with correlation measurements of the noise at the signal and idler ports, these results show that the JPC can perform two-mode squeezing of quantum noise. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q17.00007: 1/f Flux Noise in SQUIDs and Josephson Junction Qubits David Cardamone, Clare Yu 1/f flux noise represents an important, universal source of noise and decoherence in Josephson junction devices, one which must be overcome if their promise of a scalable, reliable mesoscopic qubit is to be achieved. Recent experiments at millikelvin temperatures (S. Sendelbach et al., Phys. Rev. Lett. 100, 227006(2008)) have suggested that this noise may be due to magnetic impurities residing at the surface of the superconductor. We examine this possibility, considering various models for the spacing and interactions of the impurities, and comparing the results of our numerical Monte Carlo simulations with experiment. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q17.00008: Josephson junction array protected from local noises. Sergey Gladchenko, David Olaya, Eva Dupont-Ferrier, Benoit Doucot, Lev Ioffe, Michael Gershenson We have developed small arrays of Josephson junctions (JJs) that can be viewed as prototypes of superconducting qubits protected from local noises [1]. The array consists of twelve superconducting loops interrupted by four sub-micron JJs. The protected state is realized when each loop is threaded by half of the magnetic flux quantum. It has been observed that the array with the optimized amplitude of quantum fluctuations is protected against magnetic flux variations well beyond linear order, in agreement with theoretical predictions [2]. 1. S. Gladchenko et al., ``Superconducting Nanocircuits for Topologically Protected Qubits'', arXiv:cond-mat/0802.2295, to be published in \textit{Nature Physics.} 2. L.B. Ioffe and M.V. Feigelman, \textit{Phys.~Rev.}~B~\textbf{66}, 224503 (2002); B. Doucot \textit{et al}.,\textit{ Phys. Rev}. B \textbf{71}, 024505 (2005); B. Doucot and L.B.~Ioffe,\textit{ Phys. Rev}. B \textbf{76}, 214507 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q17.00009: Optimization and characterization of protected Josephson circuits Eva Dupont-Ferrier, Sergey Gladchenko, Lev Ioffe, Michael Gershenson Recently, it was proposed that small Josephson arrays can operate as superconducting qubits protected from local noises [1,2]. Here we present measurements of several optimized array designs. The read-out circuit for these arrays consists of an inductively-coupled DC SQUID, which helps to minimize perturbations of the system during measurement. We will discuss the current-phase characteristics of these arrays and their response to microwave radiation. Our results indicate that the scattering of Josephson junction parameters can be made small enough to implement the symmetry-protected superconducting qubits; our theoretical model [1] captures all essential features of real devices. 1. see e.g., B. Doucot and L.B.~Ioffe,\textit{ Phys. Rev}. B \textbf{76}, 214507 (2007) and references therein. 2. S. Gladchenko, D. Olaya, E. Dupont-Ferrier, B. Dou\c{c}ot, L.B.~Ioffe, and M.E. Gershenson, ``Superconducting Nanocircuits for Topologically Protected Qubits'', arXiv:cond-mat/0802.2295, to be published in \textit{Nature Physics.} [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q17.00010: Controllable scattering of photons in a one-dimensional resonator waveguide C.P. Sun, L. Zhou, Z.R. Gong, Y.X. Liu, F. Nori We analyze the coherent transport of a single photon, which propagates in a one-dimensional coupled-resonator waveguide and is scattered by a controllable two-level system located inside one of the resonators of this waveguide. Our approach, which uses discrete coordinates, unifies low and high energy effective theories for single-photon scattering. We show that the controllable two-level system can behave as a quantum switch for the coherent transport of a single photon. This study may inspire new electro-optical single-photon quantum devices. We also suggest an experimental setup based on superconducting transmission line resonators and qubits. \\[4pt] L. Zhou, Z.R. Gong, Y.X. Liu, C.P. Sun, F. Nori, Controllable scattering of photons in a 1D resonator waveguide, Phys. Rev. Lett. 101, 100501 (2008). URL: http://link.aps.org/abstract/PRL/v101/e100501 [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q17.00011: Quantum two-level systems in Josephson junctions as naturally formed qubits Alexander Zagoskin, Sahel Ashhab, Robert Johansson, Franco Nori The two-level systems (TLSs) naturally occurring in Josephson junctions constitute a major obstacle for the operation of superconducting phase qubits. Since these TLSs can possess remarkably long decoherence times, we show that such TLSs can themselves be used as qubits, allowing for a well controlled initialization, universal sets of quantum gates, and readout. Thus, a single current-biased Josephson junction (CBJJ) can be considered as a multiqubit register. It can be coupled to other CBJJs to allow the application of quantum gates to an arbitrary pair of qubits in the system. We also show that using the dynamics of a driven qubit, it could be possible to characterize the nature of the two-level systems and their coupling to the phase qubit. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q17.00012: Topological states and braiding statistics using quantum circuits Xiao-Feng Shi, Jianqiang You, Franco Nori Using superconducting quantum circuits, we propose an approach to construct a Kitaev lattice, i.e., an anisotropic spin model on a honeycomb lattice with three types of nearest-neighbor interactions. We study two particular cases to demonstrate topological states (i.e., the vortex and bond states) and show how the braiding statistics can be revealed. Our approach provides an experimentally realizable many-body system for demonstrating exotic properties of topological phases. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q17.00013: Topological Transition in a Non-Hermitian Quantum Walk: a new test for quantumness in driven artificial atoms and Josephson arrays Mark Rudner, Leonid Levitov We analyze a quantum walk on a bipartite one-dimensional lattice, in which the particle can decay whenever it visits one of the two sublattices. The corresponding non-Hermitian tight-binding problem with complex potential for the decaying sites exhibits two distinct phases, distinguished by a winding number defined in terms of the Bloch eigenstates in the Brillouin zone [1]. We find that the mean displacement of a particle initially localized on one of the non-decaying sites is quantized as an integer, changing from zero to one at the critical point. By mapping this problem onto a Jaynes-Cummings-type model with decay, we find that the topological transition is relevant for a variety of experimental settings, in particular for superconducting qubits coupled to high quality resonators [2]. The quantized behavior stands in contrast with the smooth dependence expected for a classical random walk, and can serve as a hallmark of coherent quantum dynamics in ladder-like multilevel systems. A real-space implementation of the quantum walk may help to verify quantum coherence in vortex transport in Josephson arrays [3]. [1] M. S. Rudner, L. S. Levitov, arXiv:0807.2048. [2] A. Wallraff et al., Nature 431, 162-167 (2004). [3] A. van Oudenaarden, S. J. K. Vardy, and J. E. Mooij, Phys. Rev. Lett. 77, 4257 (1996). [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q17.00014: Two Types of Loss Expected for Josephson Qubit Circuits Arthur Davidson The energy of a coulomb blockade capacitor, or of a current biased Josephson junction, is known to depend on the difference between a continuous charge term and a discrete one. For example the energy in the coulomb blockade is the square of (ne-k), where n is an integer, e is the electron charge, and k is a continuous charge value. This suggests that there may be two types of quantum losses in the Schroedinger dynamics of these systems. One type would reduce k to ne to arrive at the ground state energy and would be the quantum analog of external classical resistance. The other type of loss would couple energy bands at constant k, minimizing n, and corresponding to tunnel losses. Historically, two quantum loss terms have been proposed: one due to Mortin Kostin in 1972; and another from the present author in 1990. The Kostin type of loss affects the continuous charge term, while the Davidson type affects the discrete charge term. These properties are linked to the boundary conditions of these systems. The loss mechanisms are likely to be important for understanding coherence times in Josephson qubit circuits. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q17.00015: Entanglement of Two Josephson Vortex Qubits in Resonant Cavity Ramesh Dhungana, Isaac O'Bryant, Ju Kim We discuss the entanglement of two Josephson vortex qubits (JVQs) interacting via the magnetic induction effect. A JVQ may be fabricated by implanting two closely spaced microresistors in the insulating layer of a long Josephson junction (LJJ). These two microresistors generate a double-well potential which traps a Josephson vortex (i.e., fluxon). The macroscopic quantum tunneling (MQT) of the fluxon from one well to another gives rise to a two-state system. The magnetic induction effect in a stack of LJJs introduces an asymmetry in the double-well potential and leads to the interaction between the qubits. We compute the MQT of the fluxons between the minima of the symmetric and asymmetric double-well potentials by using the instanton and the valley-instanton approaches, respectively. We compute the concurrence to estimate the level of entanglement of two JVQs. Our result indicates that the entanglement between the two JVQs is significant. Also, we show that the concurrence of the JVQs, placed in a resonant cavity, is enhanced since the cavity acts as a mediator for the interactions between the JVQs. This suggests that the degree of entanglement may be controlled by varying either the resonant frequency or the strength of coupling between the LJJ and cavity. [Preview Abstract] |
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