Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session D4: Probing Spin and Charge States in Semiconductor Quantum Dots and “Molecules” |
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Sponsoring Units: DCMP Chair: David Awschalom, University of California, Santa Barbara Room: Morial Convention Center 206 |
Monday, March 10, 2008 2:30PM - 3:06PM |
D4.00001: Spin tunneling in optically excited quantum dot molecules: Controlling g-factors with electric field Invited Speaker: We have recently demonstrated coherent tunneling of electron and hole spin between two quantum dots using optical spectroscopy [1,2]. In the case of a hole spin, a very large and resonant enhancement or reduction of g-factor is controlled with an applied electric field [3]. This effect arises because of the corresponding enhancement or suppression of the hole wavefunction in the tunnel barrier for the bonding (symmetric) and anti-bonding (anti-symmetric) states, respectively. This effect was discovered for single holes, but also occurs for two-particle states (two holes or 1 hole and 1 electron). Using this effect to identify the symmetry of the wavefunction, we have now found that the energetic order of the bonding and anti-bonding molecular states goes through a reversal as a function of tunnel barrier thickness. That is, the bonding state is the low energy state for a 2nm barrier thickness (as expected in the simple particle-in-a-box model, or the one-band effective mass theory). But for thicknesses larger than 3nm, a transition occurs such that the anti-bonding state becomes the low energy state. This dramatic and non-intuitive effect arises from the spin-orbit interaction. \newline \newline [1] ``Optical Signatures of Coupled Quantum Dots,'' E. A. Stinaff \textit{et al}, \textit{Science }\textbf{311}, 636 (2006). \newline [2] ``Spin Exchange in Optically Excited Quantum Dot Molecules,'' M. Scheibner, M. F. Doty, I. V. Ponomarev, \textit{et al}., \textit{PRB} \textbf{75}, 245318 (2007). \newline [3] ``Electrically Tuneable g Factors in Quantum dot Molecular Spin States'' M.F. Doty \textit{et al., Phys. Rev. Lett.} \textbf{97}, 197202 (2006). [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:42PM |
D4.00002: Universal Quantum Gates for Two- and Three-Spin Qubits in Coupled Quantum Dots Invited Speaker: The ability to control the exchange coupling between coupled quantum dots allows for quantum gate operations on quantum dot spin qubits. Supplemented with single-spin rotations, the exchange coupling is universal for quantum computation on qubits that are formed by the spin 1/2 of single electrons. If qubits are formed by two spins, the requirement for single-spin rotations is reduced to the presence of a fixed inhomogeneous magnetic field, while for three spins, the exchange coupling is universal on its own. In this talk, we discuss the implementation of universal gate operations for two- and three-spin qubits in coupled quantum dots. In the case of the two-spin singlet-triplet qubit on a double quantum dot, we propose a set of universal gates that can be generated by controlling the electrostatic potential between the two dots without time-dependent control of the tunnel coupling between the dots [1]. This simplification should facilitate the implementation of quantum gates in the systems that are presently studied experimentally. We present explicit gate sequences for single-qubit rotations about two orthogonal axes, and a CNOT gate sequence, completing the universal gate set. Finally, the trade-off between leakage errors and simple operations will be briefly discussed. \newline [1] R. Hanson and G. Burkard, Phys.\ Rev.\ Lett.\ {\bf 98}, 050502 (2007). [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 4:18PM |
D4.00003: Nondestructive optical probe of coherent single spin dynamics in a quantum dot Invited Speaker: Understanding the coherent dynamics of a single electron spin in a quantum dot (QD) is important for potential applications in solid-state, spin-based quantum information processing. Here, results will be presented focusing on optical detection of a single spin and observation of the temporal evolution of the spin state. First, we demonstrate the detection of a single electron spin in a QD using a continuously averaged magneto-optical Kerr rotation (KR) measurement \footnote{J. Berezovsky, M. H. Mikkelsen, {\it et al.}, {\it Science} {\bf 314}, 1916 (2006).}. In contrast to many other single spin detection schemes, the KR measurement minimally disturbs the system, making it potentially useful for exploring quantum measurementphenomena or spin-photon entanglement. This continuous single QD KR technique is then extended into the time domain using pulsed pump and probe lasers, allowing the observation of the coherent evolution of an electron spin state with nanosecond temporal resolution \footnote{M. H. Mikkelsen, J. Berezovsky, {\it et al.}, {\it Nature Physics} {\bf 3}, 770 (2007).}. This provides a direct measurement of the electron g-factor and spin lifetime, and additionally serves as a sensitive probe of the local nuclear spin environment. Finally, we perform ultrafast coherent optical manipulation of the electron spin state in the QD using the optical Stark effect \footnote{J. Berezovsky, M. H. Mikkelsen, {\it et al.}, {\it submitted} (2007).}, where an off-resonant optical pulse induces rotations of the spin state through angles up to $\pi$ radians on picosecond timescales. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:54PM |
D4.00004: Spin Decoherence and Maxwell Angels Invited Speaker: The advantages of a quantum machine are rooted in the coherent superposition of its states. In the paradigmatic quantum system of a single-electron in the environment of a quantum dot of interacting nuclear spins, how does its spin coherence decay? Is the coherence doomed to dissipate (the H-theorem)? I shall present a theory of decoherence with a simple quantum explanation with no a priori stochastic assumption, based on two solutions [1,3] of the many-body dynamics of the single electron spin and a mesoscopic number of nuclear spins. The theory is followed by a description of the principle of how the coherence lost can be restored by controlling only the electron spin [2,4]. Work done in collaboration with Wang Yao, Renbao Liu, and Semion Saikin. \newline \newline [1] Wang Yao, Ren-Bao Liu, and L. J. Sham, Phys. Rev. B \textbf{74}, 195301 (2006). \newline [2] Wang Yao, Ren-Bao Liu, and L. J. Sham, Phys. Rev. Lett. \textbf{98}, 077602 (2007). \newline [3] S. K. Saikin, Wang Yao, and L. J. Sham, Phys. Rev. B \textbf{75}, 125314 (2007). \newline [4] Ren-Bao Liu, Wang Yao, and L. J. Sham, New J. Phys. \textbf{9}, 226 (2007). [Preview Abstract] |
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