Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session P1: Optoelectronic Manipulation and Control of Charges and Spins in Quantum Dots |
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Sponsoring Units: DCMP Chair: Alexander Efros, Naval Research Laboratory Room: Spirit of Pittsburgh Ballrom A |
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P1.00001: Optical control of spin coherence in singly charged quantum dots Invited Speaker: The most promising candidate for implementation of quantum information technologies in semiconductors is the spin of an electron confined in a quantum dot because of its good coherence properties. Our approach is based on using an electron spin ensemble for defining a robust macroscopic quantum bit. Typically such an ensemble suffers from inhomogeneities. Using tailored pulsed laser excitation protocols this ensemble can be homogenized, such that the involved electrons appear to be identical when precessing about an external magnetic field. In this contribution problems and perspectives related to this approach will be discussed. In particular collective initialization and manipulation of the electron spin ensemble will be addressed. The use of all-optical techniques ensures that the manipulation can be performed on picosecond time scales. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 9:12AM |
P1.00002: The quantum dot molecule from an optical point of view Invited Speaker: For over ten years the techniques of single quantum dot optical spectroscopy has enabled rapid progress in the fundamental understanding of quantum dots and in the application of quantum information concepts [1]. We now apply these ever improving optical techniques to two self-assembled InAs/GaAs quantum dots that are coherently coupled through tunneling – that is, a quantum dot molecule [2]. The optical spectrum of a quantum dot molecule is much richer than that of a single quantum dot. As one might expect, there is both new physics and enhanced opportunity for quantum information applications. We find that the optical spectrum of single QD molecules charged with 0, 1, or 2 electrons or holes show intriguing and unique patterns of anti-crossings and spin exchange splittings that are readily understood in terms of a few simple concepts. Closer inspection is revealing new information and opportunity, however. For example, on the fundamental side, we have recently discovered evidence that the ground state of the molecule can be an anti- bonding state when it is the hole that tunnels between the dots – a new effect not found in atoms. On the quantum information side, we have engineered a quantum dot molecule in which we can simultaneously control and nondestructively measure the spin of a single electron. This solves a serious limitation in the optical control of single quantum dots. These studies are laying the groundwork necessary to enable optically controlled entanglement of two spins. Here I give an overview of our current understanding of this system from an optical point of view. \\[4pt] [1] ``Optical Studies of Single Quantum Dots'' D. Gammon and D.G. Steel, Physics Today 55, 36 (2002). \\[0pt] [2] ``Optically Mapping the Electronic Structure of Coupled Quantum Dots,'' M. Scheibner et al. Nature Physics 4, 291 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:48AM |
P1.00003: Spectroscopy of Collective Modes in Few-electron Quantum Dots Invited Speaker: Quantum correlations among electrons confined in semiconductor quantum dots (QDs) are expected to lead to exotic states of matter, such as an electron molecule. In the limit of vanishing electron density, the distances between the confined electrons are rigidly fixed like those of nuclei in conventional molecules. The electronic excitations of such a molecule are quantized normal modes of roto-vibration, whose quanta have either a rigid-rotor or relative-motion character. Recent progress on the emergence of molecular roto-vibrational modes at experimentally attainable densities will be discussed. Signatures of the roto-vibrational spectrum are observed even if the localization in space of the electron wave functions is not yet fully achieved. I will present a joint experimental and theoretical investigation of the neutral electronic excitations of nanofabricated AlGaAs/GaAs QDs that contain four electrons. We use inelastic light scattering to probe electronic charge and spin excitations in an array of identical nanofabricated QDs. Spectra of low-lying excitations associated to changes of the relative-motion wave function -the analogues of the vibrational modes of a conventional molecule- do not depend on the rotational state represented by the angular momentum, which can be controlled by the application of a magnetic field. A theoretical model, based on full configuration-interaction method, offers an excellent quantitative agreement with the experimental findings. I will also demonstrate optical control of the number of electrons and lateral confining potential in our GaAs/AlGaAs QDs. This is achieved by illumination with a weak laser beam that is absorbed in the AlGaAs barrier. Precise tuning of the energy-level structure and number of electrons is manifested in the evolution of low-lying spin and charge excitations probed by inelastic light scattering. Our findings open a new venue towards the all-optical manipulation of single electrons in QDs. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:24AM |
P1.00004: Quantum coherence of electron spins in semiconductor quantum dots Invited Speaker: |
Wednesday, March 18, 2009 10:24AM - 11:00AM |
P1.00005: Ultrafast Coherent Control of a Single Electron Spin in a Quantum Dot Invited Speaker: Practical quantum information processing schemes require fast single-qubit operations. For spin-based qubits, this involves performing arbitrary coherent rotations of the spin state on timescales much faster than the spin coherence time. While we recently demonstrated the ability to initialize and monitor the evolution of single spins in quantum dots (QDs)\footnote{M. H. Mikkelsen, J. Berezovsky, N. G. Stoltz, L. A. Coldren, D. D. Awschalom, {\em Nature Physics} \textbf{3}, 770 (2007); J. Berezovsky, M. H. Mikkelsen, O. Gywat, N. G. Stoltz, L. A. Coldren, and D. D. Awschalom, {\em Science} \textbf{314}, 1916 (2006).}, here we present an all-optical scheme for ultrafast manipulation of these states through arbitrary angles. The GaAs QDs are embedded in a diode structure to allow controllable charging of the QDs and positioned within a vertical optical cavity to enhance the small single spin signal. By applying off-resonant optical pulses, we coherently rotate a single electron spin in a QD up to $\pi$ radians on picosecond timescales \footnote{J. Berezovsky, M. H. Mikkelsen, N. G. Stoltz, L. A. Coldren, D. D. Awschalom, {\em Science} \textbf{320}, 349 (2008).}.We directly observe this spin manipulation using time-resolved Kerr rotation spectroscopy at $T=10\mathrm{K}$. Measurements of the spin rotation as a function of laser detuning and intensity confirm that the optical Stark effect is the operative mechanism and the results are well-described by a model including the electron-nuclear spin interaction. Using short tipping pulses, this technique enables one to perform a large number of operations within the coherence time. This ability to perform arbitrary single-qubit operations enables sequential all-optical initialization, ultrafast control and detection of a single electron spin for quantum information purposes. [Preview Abstract] |
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