Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session B6: Spin-Based Quantum Computing |
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Sponsoring Units: DCMP Chair: Jason Petta, Harvard University Room: Baltimore Convention Center 310 |
Monday, March 13, 2006 11:15AM - 11:51AM |
B6.00001: Spin Readout and Hyperfine Interaction in Few-Electron Quantum Dots Invited Speaker: This talk presents recent experimental progress toward using few-electron quantum dots as spin qubits. First, we have developed a new spin readout technique that is based on the difference in tunnel rate between different spin states\footnote {R. Hanson, L.H. Willems van Beveren, I.T. Vink, J.M. Elzerman, W.J.M. Naber, F.H.L. Koppens, L.P. Kouwenhoven, and L.M.K. Vandersypen, Phys. Rev. Lett. 94, 196802 (2005).}. Unlike earlier techniques, it is robust against background charge fluctuations and high-frequency noise, and presently achieves single-shot fidelities up to 90\%. Using this readout technique, we have measured the two-electron spin relaxation time as a function of magnetic field strength and orientation, and studied the back-action of the charge detector. Second, we have investigated the effect of nuclear spins on the electron spin state\footnote{F.H.L. Koppens, J.A. Folk, J.M. Elzerman, R. Hanson, L.H. Willems van Beveren, I.T. Vink, H.P. Tranitz, W. Wegscheider, L.P. Kouwenhoven, and L.M.K. Vandersypen, Science 309, 1346 (2005).}. Transport measurements on a double quantum dot in the spin blockade regime reveal that the delocalized two-electron singlet and triplet states are mixed by an inhomogeneous hyperfine field of about 1 mT. We demonstrate that this mixing can be controlled by changing either the external magnetic field or the interdot tunnel coupling. The transitions between triplet and singlet states in turn lead to current-induced dynamical nuclear polarization. In certain regimes, marked current bistabilities appear as a function of both field and time. [Preview Abstract] |
Monday, March 13, 2006 11:51AM - 12:27PM |
B6.00002: Enhancement of Spin Coherence Using \textit{Q}-factor Engineering in Semiconductor Microdisk Lasers Invited Speaker: Semiconductor microcavities offer unique means of controlling light-matter interactions in confined geometries, resulting in a wide range of applications in optical communications and inspiring proposals for quantum information processing and computational schemes. Studies of spin dynamics in microcavities have revealed novel effects such as polarization beats, stimulated spin scattering, and giant Faraday rotation. Here, we study the electron spin dynamics in optically-pumped GaAs microdisk lasers with quantum wells (QWs) and interface-fluctuation quantum dots (QDs) in the active region. Using all-optical time-resolved measurement techniques, we examine how the electron spin dynamics are modified by the stimulated emission in the disks, and observe a surprising enhancement of the spin coherence time when the optical excitation is in resonance with a high quality ($Q\sim $ 5000) lasing mode\footnote{S. Ghosh, W. H. Wang, F. M. Mendoza, R. C. Myers, X. Li, N. Samarth, A. C. Gossard, and D. D. Awschalom, \textit{Nature Materials}, accepted for publication (2005). (cond-mat/0509500).}. This resonant enhancement, contrary to expectations from the observed trend in the carrier recombination time, is then manipulated by altering the cavity design and dimensions. In analogy to devices based on excitonic coherence, this ability to engineer coherent interactions between electron spins and photons may provide novel pathways towards spin dependent quantum optoelectronics. [Preview Abstract] |
Monday, March 13, 2006 12:27PM - 1:03PM |
B6.00003: Teleportation of electronic many-qubit states via single photons Invited Speaker: I will describe a proposed a teleportation scheme[1] that relies only on single-photon measurements and Faraday rotation, for teleportation of many-qubit entangled states stored in the electron spins of a quantum dot system. The interaction between a photon and the two electron spins, via Faraday rotation in microcavities, establishes Greenberger-Horne-Zeilinger entanglement in the spin-photon-spin system. The appropriate single-qubit measurements, and the communication of two classical bits, produce teleportation. This scheme provides the essential link between spintronic and photonic quantum information devices by permitting quantum information to be exchanged between them. Work supported by DARPA/ARO DAAD19-01-1- 0490. [1] M. N. Leuenberger, M. E. Flatt\'e, and D. D. Awschalom, Phys. Rev. Lett. 94, 107401 (2005). [Preview Abstract] |
Monday, March 13, 2006 1:03PM - 1:39PM |
B6.00004: Optically probing charge and spin states in quantum dots and molecules Invited Speaker: In this talk I will discuss recent experiments in which we electrically manipulate coupled excitonic states (neutral and negatively charged single excitons) in individual QD-molecules using static electric fields. The samples investigated consist of a single pair of vertically stacked, self assembled InGaAs QD-molecules embedded in an n-type GaAs Schottky photodiode. This device geometry enables us to control the coherent coupling between excitonic states in the upper and lower dots by tuning the electric field oriented along the axis of the QD-molecule by applying a bias voltage between the n-contact and the Schottky-gate. New information is obtained on the spin structure of negatively charged trions in coupled quantum dot nanostructures. At low excitation power densities, field dependent luminescence reveals a clear anticrossing of spatially direct (e,h in the same dot) and indirect (e,h in different dots) neutral excitons, with coupling energies in the range 2E$_{ee}$=1.2-3.2meV. Our experimental findings are shown to be in very good accord with realistic calculations of the single exciton spectrum, confirming that the tunnel coupling is mediated by hybridization of the electron component of the exciton wavefunction over the two dots. In contrast, the spectrum and controlled hybridization of \textit{negatively} charged excitons is shown to be much richer due to the complex spectrum of three particle states (X$^{-}$=2e+1h) that can exist in a QD-molecule. For example, the spin structure of the spatially localized and dissociated X$^{-}$ states is found to play a major role in the spectrum of controlled hybridization with distinct triplet and single states evolving very differently as the coupling is tuned. The demonstration of tunable coupling and manipulation of spin and exchange couplings in negatively charged systems may constitute an important step towards the development of optically gateable QD-molecules for applications in quantum information science. [Preview Abstract] |
Monday, March 13, 2006 1:39PM - 2:15PM |
B6.00005: Production and Detection of Spin-Entangled Electrons in Mesoscopic Conductors Invited Speaker: Electron spins are an extremely versatile form of quantum bits. When localized in quantum dots, they can form a register for quantum computation. Moreover, being attached to a charge in a mesoscopic conductor allows the electron spin to play the role of a mobile carrier of quantum information similarly to photons in optical quantum communication. Since entanglement is a basic resource in quantum communication, the production and detection of spin-entangled Einstein-Podolsky-Rosen (EPR) pairs of electrons are of great interest. Besides the practical importance, it is of fundamental interest to test quantum non-locality for electrons. I review the theoretical schemes for the entanglement production in superconductor-normal junctions [1] and other systems. The electron spin entanglement can be detected and quantified from measurements of the fluctuations (shot noise) of the charge current after the electrons have passed through an electronic beam splitter [2,3]. This two-particle interference effect is related to the Hanbury-Brown and Twiss experiment and leads to a doubling of the shot noise $S_I=\langle\delta I \delta I\rangle_{\omega=0}$ for spin-entangled states, allowing their differentiation from unentangled pairs. I report on the role of spin-orbit coupling (Rashba and Dresselhaus) in a complete characterization of the spin entanglement [4]. Finally, I address the effects of a discrete level spectrum in the mesoscopic leads and of backscattering and decoherence.\newline \newline [1] P. Recher, E. V. Sukhorukov, D. Loss, Phys.\ Rev.\ B {\bf 63}, 165314 (2001)\newline [2] G. Burkard, D. Loss, E. V. Sukhorukov, Phys.\ Rev.\ B {\bf 61}, R16303 (2000)\newline [3] G. Burkard and D. Loss, Phys.\ Rev.\ Lett.{\bf 91}, 087903 (2003)\newline [4] J. C. Egues, G. Burkard, D. Saraga, J. Schliemann, D. Loss, cond-mat/0509038, to appear in Phys.Rev.B (2005). [Preview Abstract] |
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