Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session H15: Focus Session: Superconducting Qubits I |
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Sponsoring Units: GQI Chair: Britton Plourde, Syracuse University Room: Morial Convention Center 207 |
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H15.00001: Designing quantum-information-processing superconducting qubit circuits that exhibit lasing and other atomic-physics-like phenomena on a chip Invited Speaker: Franco Nori Superconducting (SC) circuits can behave like atoms making transitions between a few energy levels. Such circuits can test quantum mechanics at macroscopic scales and be used to conduct atomic-physics experiments on a silicon chip. This talk overviews a few of our theoretical studies on SC circuits and quantum information processing (QIP) including: SC qubits for single photon generation and for lasing; controllable couplings among qubits; how to increase the coherence time of qubits using a capacitor in parallel to one of the qubit junctions; hybrid circuits involving both charge and flux qubits; testing Bell's inequality in SC circuits; generation of GHZ states; quantum tomography in SC circuits; preparation of macroscopic quantum superposition states of a cavity field via coupling to a SC qubit; generation of nonclassical photon states using a SC qubit in a microcavity; scalable quantum computing with SC qubits; and information processing with SC qubits in a microwave field. Controllable couplings between qubits can be achieved either directly or indirectly. This can be done with and without coupler circuits, and with and without data-buses like EM fields in cavities (e.g., we will describe both the variable-frequency magnetic flux approach and also a generalized double-resonance approach that we introduced). It is also possible to ``turn a quantum bug into a feature'' by using microscopic defects as qubits, and the macroscopic junction as a controller of it. We have also studied ways to implement radically different approaches to QIP by using ``cluster states'' in SC circuits. For a general overview of this field, see, J.Q. You and F. Nori, Phys. Today 58 (11), 42 (2005) [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H15.00002: A Tunable Coupling Architecture For Josephson Phase Qubits Radoslaw Bialczak, M. Ansmann, M. Hofheinz, E. Lucero, R. McDermott, M. Neeley, A.D. O'Connell, H. Wang, A. Cleland, J. Martinis Previous coupled-qubit experiments with Josephson phase qubits have used a fixed coupling scheme. However, in order to create high-fidelity multi-qubit gates, a tunable coupling scheme is needed. Fixed coupling schemes cannot be used because single-qubit operations on a coupled-qubit system cannot be performed with high fidelity due to the errors induced by always-on coupling. Fixed coupling also allows for crosstalk between coupled qubits during measurement. We show how to implement a tunable-coupling architecture for Josephson phase qubits using simple linear elements. This architecture can be used to vary the interaction strength from fully-off to fully-on allowing us to get around the problems inherent with the use of a fixed coupling scheme. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H15.00003: High-fidelity gates in Josephson phase qubits Erik Lucero, Markus Ansmann, Radoslaw Bialczak, Max Hofheinz, Nadav Katz, Mattthew Neeley, Aaron O'Connell, Haohua Wang, Andrew Cleland, John Martinis Complex algorithms for a quantum computer will require error correction, which calls for logic gates with fidelity below a fault tolerant threshold. We present significant progress towards this goal with our detailed measurements of gate fidelity. We carefully separate out gate and measurement error and construct a complete error budget to demonstrate single qubit gate fidelities of 0.98, limited by energy relaxation. We introduce a new metrology tool `a Ramsey interference error filter' that can measure the excited two-state population down to 10$^{-4}$, a magnitude near the fault tolerant threshold. This measurement demonstrates that our quantum system remains in the two-state qubit manifold during our single qubit operations. This precision and accuracy is made possible by custom control electronics that can create arbitrarily shaped microwave pulses. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H15.00004: Recent progress towards violating the CHSH Bell inequality in Josephson phase qubits Markus Ansmann, Radoslaw Bialczak, Max Hofheinz, Nadav Katz, Erik Lucero, Matthew Neeley, Aaron O'Connell, Haohua Wang, Andrew Cleland, John Martinis Improvements of gate fidelity and control electronics for Josephson phase qubits have provided the foundation for revisiting Bell's inequality. The violation of Bell's inequality is the primary argument against the possible existence of a hidden-variable-theory as an alternative to quantum mechanics. It also serves as a convincing demonstration that a given system behaves in a truly non-classical way. The most widely accepted form of Bell's inequality follows closely along a correlation measurement proposed by Clauser, Horne, Shimony and Holt (CHSH) in 1969. Here we present our latest attempt to implement the CHSH Bell test using Josephson phase qubits. The nature of this experiment places high demands -- compared to the current state of the art in solid state qubits -- on qubit performance measures such as the energy relaxation time T1, the decoherence time T2, single and two qubit gate fidelities, and measurement fidelities. We will examine these demands with respect to the number of fronts we have improved upon in our system. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H15.00005: Analysis of Bell inequality violation in superconducting phase qubits Abraham G. Kofman, Alexander N. Korotkov We analyze conditions for violation of the Bell inequalities, focusing on experiments with Josephson phase qubits. In the ideal case we discuss all possible situations of maximum violation, but mainly focus on two important types of optimal qubit-measurement directions in the pseudospin space, lying within either horizontal or vertical planes. Only the vertical type remains optimal in presence of local measurement errors, while in the case of local decoherence of qubits either the horizontal or vertical configuration is optimal. Besides local measurement errors and decoherence, we also discuss the effect of measurement crosstalk, which affects both the classical inequality and the quantum result. In particular, we propose a version of the Bell inequality which is insensitive to the crosstalk. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H15.00006: High Fidelity Universal Quantum Gates in Superconducting Qubit Systems Using Non-adiabatic Rapid Passage Frank Gaitan, Ran Li Recent theoretical work\footnote{R. Li, M. Hoover, and F. Gaitan, Quant. Info. Comp. \textbf{7}, 594 (2007).} has suggested that a class of non-adiabatic rapid passage sweeps first realized experimentally in NMR systems in 1991\footnote{J. W. Zwanziger, S. P. Rucker, and G. C. Chingas, Phys.\ Rev.\ A \textbf{43}, 3232 (1991).}, and which generate controllable quantum interference effects\footnote{F. Gaitan, Phys.\ Rev.\ A \textbf{68}, 052314 (2003).}$^{,}$\footnote{J. W. Zwanziger, U. Werner-Zwanziger, and F. Gaitan, Chem.\ Phys.\ Lett.\ \textbf{375}, 429 (2003).}, can be used to produce a high fidelity universal set of quantum gates. We show how this class of sweeps can be implemented in both superconducting charge and flux qubit systems. We discuss the current challenges facing the use of these sweeps to produce a universal set of high fidelity quantum gates in superconducting qubit systems. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H15.00007: Improving dc SQUID Phase Qubit Lifetimes through Increased Isolation from Bias Leads Anthony Przybysz, Tauno Palomaki, Sudeep Dutta, Fred Wellstood, Rupert Lewis, Hanhee Paik, Hyeokshin Kwon, Ben Cooper, Kaushik Mitra, Bob Anderson, Alex Dragt, Chris Lobb The dc SQUID phase qubit has been plagued by a relatively short coherence time, T$_{2}$, and relaxation time, T$_{1}$ (tens of ns). By using a sapphire substrate and small (15 $\mu $m$^{2}$ or less) Al/AlO$_{x}$/Al junctions the performance of the qubit has improved to the point where the impedance of the bias leads is the main source of dissipation and decoherence in the device. We identify the main circuit parameters that effect the isolation (junction capacitance, loop inductance, etc.), and present designs to improve the qubits isolation from the bias leads. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H15.00008: Anomalous Avoided Level Crossings in a Cooper-Pair Box Spectrum Zaeill Kim, V. Zaretskey, Y. Yoon, J.F. Schneiderman, M.D. Shaw, P.M. Echternach, F.C. Wellstood, B.S. Palmer We have used a radio-frequency superconducting single-electron transistor to measure the detailed spectrum of an Al/AlO$_{\mbox {x}}$/Al Cooper-pair box (CPB) qubit. The CPB had a charging energy $E_{C}/k_{B}$ = 0.58 K and a Josephson energy $E_{J}/k_{B} $, which can be tuned by an external magnetic flux, between 0.1 and 1 K. From 15 to 50 GHz we have found four anomalous avoided level crossings in the excited state spectrum of the CPB. We note the splitting size has a strong dependence on the Josephson energy and the location of the splitting depends on the gate voltage of the CPB, evidence that the CPB is coupled to other quantum systems that are charged. We compare our results to a model Hamiltonian that describes a charge fluctuator coupled to a CPB and extract fit parameters that provide microscopic information about the charge fluctuators. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H15.00009: Detailed Study of the Excited State Lifetime of a Cooper-Pair Box Vitaley Zaretskey, Z. Kim, Y. Yoon, J. F. Schneiderman, M. D. Shaw, P. M. Echternach, F. C. Wellstood, B. S. Palmer We have used a radio-frequency superconducting single-electron transistor (rf-SET) to measure the lifetime of the excited state ($T_{1}$) of an Al/AlO$_{\mbox{x}}$/Al Cooper-pair box (CPB) qubit with a charging energy $E_{C}/k_{B}$ = 0.58 K. We measured the lifetime by continuously measuring the decay of the qubit from a mixed state. By effectively decreasing $E_{J}/k_{B}$ from 1 K to 0.1 K we could increase $T_{1}$ from 50 ns to 5 $\mu$s which indicates that charge is the dominant noise source. Additionally we noted that the decay rate as a function of transition frequency had several narrow peaks in the range 15 to 50 GHz. These peaks correlated with the locations of anomalous avoided level crossings we observed in the excited state spectrum of the CPB\footnote{\textbf{Anomalous Avoided Level Crossings in a Cooper-Pair Box Spectrum}, ZAEILL KIM \emph{et al}., BAPS (March 2008)}, suggesting that interaction with microstates is a source of dissipation for these qubits. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H15.00010: Direct Observation of Quasiparticle Tunneling Rates in a Pair of Superconducting Charge Qubits M.D. Shaw, B. Palmer, P. Delsing, P.M. Echternach We directly measure quasiparticle tunneling rates in the time domain for a pair of superconducting charge qubits based on the single Cooper-pair box. We discuss the dependence of these rates on a variety of experimental parameters, such as RF power, microwave power, magnetic flux, sample temperature, and gate voltage. Measurements are performed using RF reflectometry to measure the quantum capacitance of each device. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H15.00011: Robust optimal quantum gates for Josephson charge qubits Simone Montangero, Tommaso Calarco, Rosario Fazio Quantum optimal control theory allows to design accurate quantum gates. We employ it to design high-fidelity two-bit gates for Josephson charge qubits in the presence of both leakage and noise. Our protocol considerably increases the fidelity of the gate and, more important, it is quite robust in the disruptive presence of 1/f noise. The improvement in the gate performances discussed in this work (errors $ \sim 10^{-3} \div 10^{-4}$ in realistic cases) allows to cross the fault tolerance threshold. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H15.00012: Suppression of decoherence due to classical noise by dynamical decoupling Lukasz Cywinski, Roman M. Lutchyn, Cody P. Nave, Sankar Das Sarma We consider a pure dephasing model in the context of superconducting qubits. We show that the coherence time $T_2$ can be significantly increased by the application of a series of $\pi$ pulses. The most well known example, spin echo, a one pulse sequence, removes inhomogeneous broadening. However, $T_2$ can be further increased by applying more pulses. We discuss the experimental implications of various pulse sequences in the context of classical $1/f^{\alpha}$ noise in a Cooper-pair box qubit. We show that a recently proposed coherence-restoring pulse sequence [1, 2], discovered in the context of the spin-boson model, is optimal in certain regimes of parameter space. [1] G. S. Uhrig Phys. Rev. Lett. 98, 100504 (2007) \newline [2] B. Lee, W. M. Witzel, S. Das Sarma, arXiv:0710.1416 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H15.00013: The effect of dynamical decoupling in the case of a single fluctuator coupled to a qubit Cody Nave, Roman Lutchyn, Lukasz Cywinski, Sankar Das Sarma We consider the role of dynamical decoupling in the case of a single classical fluctuator coupled to a qubit which is operated in a pure dephasing regime. We study the effect of various pulse sequences on the decoherence time for both weakly and strongly coupled fluctuators described by random telegraph noise (RTN). For a strongly coupled two-level system, the application of multiple pulses leads to a large enhancement of qubit coherence time. By theoretically comparing various dynamic decoupling schemes, we conclude that the Car-Purcell-Meiblum-Gill (CPMG) pulse sequence, well-known in NMR spectroscopy and recently discussed in the context of electron spin qubits in semiconductors [1], is the most optimal coherence-restoring scheme for the single fluctuator problem of relevance to superconducting qubits. We also find that for a large number of applied pulses the Gaussian approximation for the noise reproduces the exact results even in the strongly coupled regime. \newline [1] W. M. Witzel and S. Das Sarma, Phys. Rev. Lett. 98, 077601 (2007). [Preview Abstract] |
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