Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session P40: Focus Session: Pathways to Practical Quantum Computing I |
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Sponsoring Units: DCOMP TGQI Chair: Gerardo Ortiz, Los Alamos National Laboratory Room: Baltimore Convention Center 343 |
Wednesday, March 15, 2006 11:15AM - 11:51AM |
P40.00001: Solid state technologies for quantum computers Invited Speaker: Impressive progress is now being made in realizing the rudiments of quantum computers in solid state devices. I will discuss two of them. First, single electron quantum dots now offer a higly coherent spin state for use as a qubit. Decoherence effects, arising from hyperfine interactions and the spin-orbit interaction, are well on their way to being understood and controlled. Second, Josephson junction devices, in many forms, are showing promise as qubits. The dynamics of these electric circuits can be designed to exhibit a wide variety of quantum effects; good two-level systems can be produced by careful design, and careful schemes for decoupling from the environment. Coupling to harmonic modes offer a wide variety of ``quantum optic'' realizations in the microwave regime. [Preview Abstract] |
Wednesday, March 15, 2006 11:51AM - 12:03PM |
P40.00002: CNOT logic for Josephson phase qubits Michael Geller, Emily Pritchett, Andrew Sornborger, Matthias Steffen, John Martinis Josephson junctions have demonstrated enormous potential as qubits for scalable quantum computing architectures. Here we study the speed and fidelity of four controlled-NOT gate implementations designed for capacitively coupled phase qubits. One gate applies to qubits fixed permanently in resonance, two require varying the dc current bias, and the fourth applies to permanently detuned qubits. Realistic simulations suggest that these implementations can be demonstrated with good fidelity using existing superconducting circuits. [Preview Abstract] |
Wednesday, March 15, 2006 12:03PM - 12:15PM |
P40.00003: Violation of Bell's Inequality using Josephson Phase Qubits Markus Ansmann, R. Bialczak, N. Katz, E. Lucero, R. McDermott, M. Neeley, M. Steffen, E.M. Weig, A.N. Cleland, J.M. Martinis Recent improvements of the measurement visibility and coherence times in Josephson Phase Qubits have enabled first tests of two- qubit quantum gates and examination of quantum phenomena using these devices. Here, we present an experiment in which we attempt to violate Bell’s Inequality, which would be further proof that the system at hand behaves in a truly quantum mechanical way. 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. This experiment illustrates the use of coherent control over capacitatively coupled qubits with always-on coupling, including the establishment of the system in eigenstates of the coupling, e.g. the singlet state. Single qubit rotations combined with a simultaneous, fast, high-visibility readout allow for state- tomography on the system. [Preview Abstract] |
Wednesday, March 15, 2006 12:15PM - 12:27PM |
P40.00004: Experimental State Tomography using Superconducting Quantum Bits M. Steffen, M. Ansmann, R. Bialczak, N. Katz, E. Lucero, R. McDermott, M. Neeley, E.M. Weig, A.N. Cleland, J.M. Martinis The superconducting approach to building a scalable quantum computer has enjoyed tremendous successes in the past several years with coherence times now sufficiently long to implement quantum gates on a system with coupled qubits. In order to quantify the performance or fidelity of the gates, quantum state tomography is required. Successful state tomography relies on high measurement fidelities and the ability to perform arbitrary rotations in the transverse plane of the Bloch sphere. Here, we have made significant progress towards overcoming these challenges and present, for the first time, experimental data on single and two-qubit state tomography. [Preview Abstract] |
Wednesday, March 15, 2006 12:27PM - 12:39PM |
P40.00005: Towards single shot read-out in circuit quantum electrodynamics (QED) Andreas Wallraff, David Schuster, Alexandre Blais, Jay Gambetta, Luigi Frunzio, Joe Schreier, Blake Johnson, Andrew Houck, Will Braff, Hannes Majer, Michel Devoret, Steve Girvin, Rob Schoelkopf In recent experiments we have demonstrated the resonant coherent coupling of individual photons to a single qubit implemented as a Cooper pair box in a high quality superconducting cavity [1]. In the non-resonant case, the dispersive coupling between the qubit and the cavity field is used to perform quantum non-demolition (QND) measurements of the qubit state [2]. Using this read-out technique we have performed high visibility measurements of Rabi oscillations and Ramsey fringes [3]. Here we present a detailed experimental and theoretical analysis of the cavity response for continuous and pulsed measurements in a wide range of cavity drive amplitudes. We also discuss an optimal read-out strategy for qubits in a continuous QND measurement and aim at demonstrating single shot read-out in the circuit QED architecture [4].\\ \\ $[1]$ A. Wallraff et al. Nature (London) 431, 162 (2004)\\ $[2]$ D. I. Schuster et al. Phys. Rev. Lett. 94, 123602 (2005)\\ $[3]$ A. Wallraff et al. Phys. Rev. Lett. 95, 060501 (2005)\\ $[4]$ A. Blais et al. Phys. Rev. A 69, 062320 (2004)\\ [Preview Abstract] |
Wednesday, March 15, 2006 12:39PM - 12:51PM |
P40.00006: Coherent control in circuit QED Alexandre Blais, Jay Gambetta, Andreas Wallraff, David Schuster, Luigi Frunzio, Johannes Majer, Steven M. Girvin, Robert J. Schoelkopf Superconducting charge qubits fabricated inside a transmission line resonator have been used to successfully demonstrate strong interaction of an artificial atom with a single photon [1]. This architecture has also been used to show high-visibility and long coherence time (T$_{1}\sim $7 $\mu $s, T$_{2}\sim $ 500 ns) Rabi oscillations [2] and in the detailed study of measurement-induced dephasing [3]. Here we will discuss protocols to realize one and two-qubit logical gates in circuit QED. These are based on resonant and off-resonant irradiation of the transmission line resonator. First experimental results towards the realization of these gates will be presented. Supported by NSA and ARDA under ARO Contract No. W911NF-05-1-0365 and the NSF under Grants No. ITR-0325580 and No. DMR-0342157. [1] A. Wallraff \textit{et al.}, Nature 431, 162 (2004). [2] A. Wallraff \textit{et al.}, Phys. Rev. Lett., 95, 060501 (2005). [3] D. Schuster \textit{et al.}, Phys. Rev. Lett., 94, 123602 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 12:51PM - 1:03PM |
P40.00007: Superconducting SET backaction on the Cooper-pair box Johannes Majer, Benjamin Turek, Aasish Clerk, Steven Girvin, Robert Schoelkopf, Kevin Bladh, David Gunnarsson, Per Delsing We report on measurements of the backaction of a superconducting single electron transistor (SSET) measuring a Cooper-pair box qubit. During the weak, continuous measurement made by the SSET, the charge noise acts on the Cooper-pair box. The quantum nature of that noise is able to dephase, relax and even excite the qubit. This noise depends strongly on the operating point of the SSET. We operate the SSET near the double Josephson quasiparticle (DJQP) feature, where the backaction of the SSET is well understood (A. Clerk, et al., Phys. Rev. Lett. 89, 176804 (2002)), and where there are no quasiparticle poisoning effects. Measurements of the relaxation time of the Cooper- pair box reveal the symmetric component of the quantum noise and measurements of the steady-state polarization reveal the anti-symmetric component. Both measurements vary as expected with SSET operating point and confirm this model of SSET backaction. [Preview Abstract] |
Wednesday, March 15, 2006 1:03PM - 1:15PM |
P40.00008: Information Flow in the Readout of a Superconducting Quantum Bit I. Siddiqi, R. Vijay, M. Metcalfe, E. Boaknin, C. Rigetti, L. Frunzio, R. Shoelkopf, M.H. Devoret Quantum computation requires efficient and well controlled coupling between qubits. Superconducting qubits can be strongly coupled using passive electrical circuit elements, but one of the major remaining challenges is to eliminate uncontrolled coupling to parasitic degrees of freedom. I will present experimental results on charge qubits integrated with a novel readout device -- the Josephson bifurcation amplifier (JBA). New experiments using the improved readout fidelity and speed of the JBA quantify parasitic losses and shed light on their mechanism. [Preview Abstract] |
Wednesday, March 15, 2006 1:15PM - 1:27PM |
P40.00009: Mach-Zehnder-type Interferometry in a Strongly Driven Persistent-Current Qubit William Oliver, Yang Yu, Janice Lee, Karl Berggren, Leonid Levitov, Terry Orlando We have demonstrated Mach-Zehnder-type interferometry with a niobium superconducting persistent-current qubit. The qubit’s ground and first-excited states exhibit an anti-crossing. Driving the qubit with a large-amplitude harmonic excitation sweeps it through this anti-crossing two times per period. The induced Landau-Zener (LZ) transitions act as coherent beamsplitters, and the accumulated phase between LZ transitions varies with the driving amplitude. We have observed quantum interference fringes as a function of the driving amplitude for 1 to 20 photon excitations. We present and discuss these results. [Preview Abstract] |
Wednesday, March 15, 2006 1:27PM - 1:39PM |
P40.00010: Double Quantum Dot Molecule Coupled with Single-Electron Transistors for Quantum Computation Applications Limin Cao, Phillip Wu, Fabio Altomare, A. M. Chang, M. R. Melloch We describe the fabrication of a series-coupled double quantum dot (DQD) with side-coupled single-electron transistors (SETs). The DQD are intended to work as qubits, and the SETs perform the quantum spin measurements. The device was fabricated on a GaAs/AlGaAs heterostructure using a one-step, two-angle, evaporation of aluminum. Our design is compatible with modern semiconductor techniques, and if proven successful, can readily be scaled into larger integrated qubit systems with spin manipulation and measurement circuitry. Our preliminary experimental results indicate that both the QDs and SETs have single-electron tunneling behaviors with good reproducibility. We will report on progress towards the in-situ detection of the spin and charge of a single electron trapped in the semiconductor quantum dots. [Preview Abstract] |
Wednesday, March 15, 2006 1:39PM - 1:51PM |
P40.00011: Charge fluctuation induced dephasing of exchange coupled spin qubits Xuedong Hu, S. Das Sarma Exchange coupled {\it spin} qubits in semiconductor nanostructures are shown to be vulnerable to dephasing caused by {\it charge noise} invariably present in the semiconductor environment. This decoherence of exchange gate by environmental charge fluctuations arises from the fundamental Coulombic nature of the Heisenberg coupling, and presents a serious challenge to the scalability of the widely studied exchange gate solid state spin quantum computer architectures. We explore the properties of the resulting exchange gate errors, and estimate dephasing times for coupled spin qubits in a wide range (from 1 nanosecond up to more than 1 microsecond) depending on the exchange coupling strength and its sensitivity to charge fluctuations in a particular nanostructure. [Preview Abstract] |
Wednesday, March 15, 2006 1:51PM - 2:03PM |
P40.00012: Solid-state quantum teleportation between nanomechanical modes L. Tian, S. M. Carr We study a quantum teleportation scheme between two nanomechanical modes without local interaction. The nanomechanical modes are connected by and linearly coupled to the continuous variable modes of a superconducting circuit made of transmission line and Josephson junctions. The phase sensitive measurement during the teleportation can be conducted by a superconducting single electron transistor operated as an rf mixer. Using a Wigner function approach, we calculate the fidelity of transferring coherent state under finite temperature and non-unit detector efficiency. We show that a fidelity above the classical limit of $1/2$ can be achieved for a large range of parameters. [Preview Abstract] |
Wednesday, March 15, 2006 2:03PM - 2:15PM |
P40.00013: Spin transport and quasi 2D architectures for donor-based quantum computing Austin Fowler, Lloyd Hollenberg, Andrew Greentree, Cameron Wellard The original Kane quantum computer architecture is based on a single line of $^{31}$P atoms spaced a few tens of nm apart in an isotopically pure $^{28}$Si lattice with electrodes above and between donor atoms. This architecture suffers from major technical issues including strong spatial oscillations in the nearest neighbour donor electron exchange coupling strengths at the scale of a single lattice site and an inability to limit the effect of a given electrode to its nearest donor or donor pair. Through the introduction of a new donor electron spin transport mechanism, a 2D donor electron spin quantum computer architecture is proposed. This new architecture addresses the exchange coupling and cross-talk issues, as well as a host of other physical barriers to implementation. [Preview Abstract] |
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