Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session J12: Focus Session: Spin Control and Dynamics in Quantum Dots |
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Sponsoring Units: GMAG DMP FIAP Chair: Michael Flatte, University of Iowa Room: Colorado Convention Center Korbel 3C |
Tuesday, March 6, 2007 11:15AM - 11:51AM |
J12.00001: Single spins in diamond: polarization, readout, and coherent control Invited Speaker: The Nitrogen-Vacancy (N-V) color center in diamond is well suited for studying electronic and nuclear spin phenomena, since its spin state can be both initialized and read out optically. Moreover, N-V center spins may allow for quantum information processing, as measurements have shown long room- temperature electron spin coherence times well into the microsecond regime. Here, we report on recent experimental progress towards coherent control and coupling of single spins in diamond. Using magneto-photoluminescence imaging and electron spin resonance (ESR) measurements at room temperature, we have investigated single N-V center spins that are coupled to electron spins of nearby nitrogen (N) defects. These N spins are optically inactive (`dark'), but can be detected via the N-V center, as the N-V and the N spins are coupled via the magnetic dipolar interaction. Some of the N-V centers are strongly coupled to only one single N spin, allowing the controlled polarization and readout of this single `dark' N spin \footnote{R. Hanson, F. M. Mendoza, R. J. Epstein and D. D. Awschalom, Phys. Rev. Lett. {\bf 97}, 087601 (2006)}. From time-resolved pump-probe measurements we find the relaxation time of the single N electron spin to be 75 microseconds at room temperature. More recently, we have demonstrated the coherent control of the N-V center spin using optical detection of pulsed ESR and spin echo techniques \footnote{R. Hanson, O. Gywat and D.D. Awschalom, Phys. Rev. B {\bf 74}, 161203(R) (2006)}. Using these tools at different static magnetic fields, we have found that the main source of decoherence for the N-V center spins in our sample is the dipolar coupling to the surrounding bath of N spins. These results pave the way towards room-temperature coherent control of coupled spin states in diamond. [Preview Abstract] |
Tuesday, March 6, 2007 11:51AM - 12:03PM |
J12.00002: Valley Kondo Effect in Silicon Quantum Dots Shiueyuan Shiau, Sucismita Chutia, Robert Joynt Recent progress in the fabrication of quantum dots using silicon opens the prospect of observing the Kondo effect associated with the valley degree of freedom. We compute the dot density of states using an Anderson model with infinite Coulomb interaction U. The density of states is obtained as a function of temperature and applied magnetic field in the Kondo regime using an equation-of-motion approach to obtain the Green's functions of the electrons. We predict the appearance of a very complex peak structure near the Fermi energy, much richer than the one or two peaks of the usual spin Kondo effect. We also show that the valley index is typically not conserved when electrons tunnel into a silicon dot. Analysis of the conductance should enable experimenters to understand the interplay of Zeeman splitting and valley splitting, as well as the dependence of tunneling on the valley degree of freedom. [Preview Abstract] |
Tuesday, March 6, 2007 12:03PM - 12:15PM |
J12.00003: Tuning Hole g-Factors in Self-Assembled InAs/GaAs Quantum Dots with an Electric Field Joseph Pingenot, Craig E. Pryor, Michael E. Flatt\'e The g-factors of holes in quantum dots (QDs) determine the energy splittings of the spin states in a magnetic field, influencing spin precession, spin lifetimes, and photoluminescence polarization. Modulation of the g-factor by an electric field may permit spin manipulation for quantum information processing. Hole g-factors in quantum wells have a large anisotropy between the in-plane (g= 0) and growth (g=2.3) directions[1]. Calculations of hole g-factors for InAs/GaAs[2], CdTe[3], and Ge/Si QDs[4] have also indicated size dependency. Using 8-band k.p theory, we calculated electric field dependent hole g-factors on a variety of InAs/GaAs QDs. We find a large anisotropy: g=0.75 and 2.5 for B along (1-10) and (001) respectively for an elliptical dot with Eg=1.136, and g=0.059 and 2.8 for a round dot with Eg=1.133. A 100kV/cm field along (001) changes the (1-10) g-factor from 0.75 to 1.1 in the elliptical dot (0.059 to 0.058 for the round dot), and the (001) g-factor changes from 2.5 to 2.3 (2.8 to 2.9). [1] Sapega et al., PRB 45, 4320 (1992). [2] C. Pryor and M. E. Flatte, PRL 96, 026804 (2006). [3] Prado et al., PRB 69, 201310(R) (2004). [4] Nenashev et al., PRB 67, 205301 (2003). [Preview Abstract] |
Tuesday, March 6, 2007 12:15PM - 12:27PM |
J12.00004: Non-destructive Kerr rotation measurements of a single spin in a quantum dot J. Berezovsky, M.H. Mikkelsen, O. Gywat, N. Stoltz, L. Coldren, D.D. Awschalom A single electron spin in a quantum dot forms a natural two state system for use in quantum information processing. The ability to measure this spin without destroying the system is an important step towards observing various quantum measurement-related phenomena. In contrast to previous experiments, we have performed non-destructive Kerr rotation measurements on a single electron spin confined in a charge-tunable semiconductor quantum dot\footnote{J. Berezovsky \textit{et al., Science Express}, 9 November 2006, (10.1126/science.1133862).}. This measurement technique provides a means to directly probe the spin off-resonance, thus minimally disturbing the system. Energy-resolved Kerr rotation spectra demonstrate that we are probing a single electron, and also yield information about the optically-pumped spin polarization as a function of quantum dot charging. These results point the way towards quantum non-demolition measurements and optically-mediated entanglement of spins in the solid state. [Preview Abstract] |
Tuesday, March 6, 2007 12:27PM - 12:39PM |
J12.00005: Kerr rotation studies of single electron spin dynamics in a quantum dot M.H. Mikkelsen, J. Berezovsky, O. Gywat, N.G. Stoltz, L.A. Coldren, D.D. Awschalom Kerr rotation measurements are used to directly and non-destructively probe the dynamics of a single electron spin in a charge-tunable quantum dot \footnote{ J. Berezovsky, M. H. Mikkelsen, O. Gywat, N. G. Stoltz, L. A. Coldren, and D. D. Awschalom,{\em Science Express}, 9 November 2006, (10.1126/science.1133862)}. The dot is formed by interface fluctuations of a GaAs quantum well and embedded in a vertical optical cavity. Using Hanle techniques, we perform single electron Kerr rotation measurements at $T=10\mathrm{K}$ in order to monitor the depolarization of an optically pumped electron spin within an applied transverse magnetic field. This reveals information about the time averaged transverse spin lifetime, $T_2^*$. At gate voltages for which the charging rate of the dot is relatively low, the results yield a $T_2^*$ in agreement with values expected from the hyperfine interaction in these materials. In contrast, at larger charging rates, we find that $T_2^*$ is strongly reduced, indicating the importance of additional decoherence mechanisms in that regime. [Preview Abstract] |
Tuesday, March 6, 2007 12:39PM - 12:51PM |
J12.00006: Driven coherent oscillations of a single electron spin in a quantum dot Frank Koppens, Christo Buizert, Klaas-Jan Tielrooij, Ivo Vink, Katja Nowack, Tristan Meunier, Leo Kouwenhoven, Lieven Vandersypen The ability to control the quantum state of a single electron spin in a quantum dot is at the heart of recent developments towards a scalable spin-based quantum computer. In combination with the recently demonstrated controlled exchange gate between two neighbouring spins [1], driven coherent single spin rotations would permit universal quantum operations. In this talk, I will discuss the experimental realization of single electron spin rotations in a gate-defined GaAs double quantum dot. We coherently control the quantum state of the electron spin by applying short bursts of an on-chip generated oscillating magnetic field [2]. This allows us to observe up to eight Rabi oscillations of the electron spin in a microsecond burst. Via Ramsey-type pulse sequences we measure an apparent time-averaged coherence time which is limited by the hyperfine interaction with the nuclear spins. We erase these nuclear spin effects to a large extend via spin-echo pulse sequences and recover the intrinsic coherence time. [1] J.R. Petta et al., Science 309, 2180--2184 (2005). [2] F.H.L. Koppens et al., Nature 442, 766-771 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 12:51PM - 1:03PM |
J12.00007: Spin Coherence Modulated Trion Transitions and Probabilistic Initialization in Charged Semiconductor Quantum Dots Yanwen Wu, Erik Kim, Xiaodong Xu, Jun Cheng, Duncan Steel, Sophia Economou, Lu Sham, Dan Gammon, Alan Bracker The presence of symmetry breaking in a three-level $\Lambda $ system consisting of two spin ground states and a charged exciton (trion) state leads to new features, where the population excited to the trion state is modulated by the spin coherence. This phenomenon is due to the unique semiconductor environment of the quantum dot (QD) system, which allows for two simultaneously orthogonal spinor axes. In addition, the polarization dependent excitations due to the double spinor axes of the system can be utilized to create a net spin from a completely mixed spin state, which is impossible to achieve through unitary operation of the spin system. This result provides an important application to the practical implementation of ultrafast spin based quantum computation in the semiconductor QD system in terms of qubit initialization. [Preview Abstract] |
Tuesday, March 6, 2007 1:03PM - 1:15PM |
J12.00008: Radio frequency charge sensing and nuclear polarization of a two-electron double quantum dot David Reilly, Edward Laird, Jacob Taylor, Jason Petta, Charles Marcus, Micah Hanson, Art Gossard We report charge-sensing measurements of a two-electron double quantum dot using an electrometer based on a radio-frequency quantum point contact (rf-QPC). In analogy with the radio frequency single electron transistor (rf-SET) the rf-QPC makes use of a LC impedance transformer and radio frequency reflectometry to achieve high charge sensitivity over a bandwidth exceeding 20MHz. In addition, we use controlled nanosecond pulsing of the double-dot potential to drive singlet to triplet transitions that develop a partial polarization of the nuclear spins. Using the rf-QPC, the subsequent diffusion and dynamics of nuclear polarization is examined on fast timescales. [Preview Abstract] |
Tuesday, March 6, 2007 1:15PM - 1:27PM |
J12.00009: Fast spin state preparation in a single charged quantum dot Xiaodong Xu, Yanwen Wu, Bo Sun, Jun Cheng, Qiong Huang, Duncan Steel, Allan Bracker, Dan Gammon, Clive Emary, Lu Sham Electron spins trapped inside of semiconductor dots (QD) are promising candidates for quantum bits (qubits). Quantum computation requires both the initialization of qubits with a high fidelity and a continuous supply of fresh ancillary qubits. The latter is the key for quantum error correction (QEC). Here, we demonstrate fast spin state preparation (laser cooling of an electron spin) in a single charged InAs self-assembled QD by applying magnetic field in the Voigt geometry. The preparation efficiency of the spin state is 98.9{\%} at a magnetic field of 0.88T, which corresponds to the cooling of spin from 5 K (experimental temperature) to 0.06 K. To reach the same efficiency by thermal equilibration, a 69 T static magnetic field should be applied. The spin cooling rate of $3\cdot 8\times 10^9s^{-1}$ is achieved. This is three orders of magnitude faster than the spin decoherence rate, which is on the order of$1\times 10^6s^{-1}$. [Preview Abstract] |
Tuesday, March 6, 2007 1:27PM - 1:39PM |
J12.00010: Exchange-controlled single-spin rotations in quantum dots William Coish, Daniel Loss We show theoretically that arbitrary coherent rotations can be performed quickly (with a gating time $\sim 1\,\mathrm{ns}$) and with high fidelity on the spin of a single electron confined to a quantum dot using exchange. These rotations can be performed using experimentally proven techniques for rapid exchange control, without the need for spin-orbit interaction or ac electromagnetic fields. We expect that implementations of this scheme would achieve gate error rates on the order of $\eta \sim 10^{-3}$, within reach of several known error-correction schemes. [Preview Abstract] |
Tuesday, March 6, 2007 1:39PM - 1:51PM |
J12.00011: Mixing of trion spin states in single and coupled dots from electron-hole and electron-electron exchange S. C. Badescu, T. L. Reinecke Polarized light spectroscopy is of interest for initializing and reading the electron (e) spin state in quantum dot (QD) systems for quantum information applications. An additional electron-hole (e-h) pair is created in the QD giving rise to a transient trion [1]. The mechanisms behind the spin-nonconserving (asymmetric) e-e exchange and of the e-h exchange are important for understanding the spin dynamics of the trion. Here, first we show the importance of the long-range e-h exchange for the flip-flop mechanism in the lowest triplet of a single QD, particularly for highly-symmetric QDs. This adds to the strong e-e asymmetric exchange in a cylindrical QD [2]. Second, we consider a double-dot system, and we describe the combined effect of e-h and e-e asymmetric exchange in the lowest (delocalized) triplet by comparison to the first excited (localized) triplet [1] M. E. Ware et al., PRL 95(17), 177403 (2005) [2] S. C. Badescu and T. L. Reinecke, cond-mat/0610405 [Preview Abstract] |
Tuesday, March 6, 2007 1:51PM - 2:03PM |
J12.00012: Electric Dipole Spin Resonance for Heavy Holes in Quantum Dots Denis Bulaev, Daniel Loss We propose and analyze a new method for manipulation of a heavy hole spin in a quantum dot. Due to spin-orbit coupling between states with different orbital momenta and opposite spin orientations, an applied rf electric field induces transitions between spin-up and spin-down states. This scheme can be used for detection of heavy-hole spin resonance signals, for the control of the spin dynamics in two-dimensional systems, and for determining important parameters of heavy-holes such as the effective $g$-factor, mass, spin-orbit coupling constants, spin relaxation and decoherence times. [Preview Abstract] |
Tuesday, March 6, 2007 2:03PM - 2:15PM |
J12.00013: Quantum Hall Ferrimagnetism in lateral quantum dot molecules Ramin Abolfath, Pawel Hawrylak We demonstrate the existence of ferrimagnetic and ferromagnetic phases in a spin phase diagram of coupled lateral quantum dot molecules in the quantum Hall regime. The spin phase diagram is determined from Hartree-Fock Configuration Interaction method as a function of electron numbers N, and magnetic field B. The quantum Hall ferrimagnetic phase corresponds to spatially imbalanced spin droplets resulting from strong inter-dot coupling of identical dots. The quantum Hall ferromagnetic phases correspond to ferromagnetic coupling of spin polarization at filling factors between nu=2 and nu=1 [1]. [1] Ramin M. Abolfath, and Pawel Hawrylak, Phys. Rev. Lett 97, 186802 (2006). [Preview Abstract] |
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