Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session J22: Focus Session: Optical Control and Electron-nuclear Effects in Quantum Dots |
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Sponsoring Units: GMAG DMP FIAP Chair: Roberto Myers, Ohio State University Room: 324 |
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J22.00001: Optical Spin Initialization and Non-Destructive Measurement in a Quantum Dot Molecule Invited Speaker: The spin of an electron is an ideal two level system for realizing a quantum bit. Spatial confinement in a self-assembled InAs quantum dot greatly extends its spin coherence times as well as making them optically addressable. Through the excited trion state the electron can be initialized, coherently manipulated, and read out: the essential operations for quantum information processing. For single quantum dots, initialization of the spin requires a transverse magnetic field in order to turn on the normally forbidden transitions, which breaks the symmetry of the system. A major drawback is that this precludes the use of sensitive 2-level cycling transitions. In a cycling transition measurement the system repeatedly returns to the same spin eigenstate because of selection rules, and in this sense is non-destructive. Cycling transition measurements are the established method of eigenstate readout as in the case of ion qubits. Spin initialization and non-destructive cycling transition read out are incompatible in single quantum dots. In this talk I show how we overcome this fundamental limitation by using a pair of quantum dots that are coupled through coherent tunneling. The electron is isolated in one quantum dot whose spin is initialized or read out by the optical creation of an electron-hole pair in the other quantum dot, forming a molecular trion. The unique energy level structure of a molecular trion eliminates the need for a transverse magnetic field. Instead a longitudinal magnetic field is used to tune two of the trion states into resonance such that an exchange interaction permits a spin-flip Raman process. At the same time other trion states maintain strict selection rules and are used for cycling transition measurements. Overall the singly charged quantum dot molecule forms a ``W'' energy level system which is comprised of a Lambda system and two two-level cycling transitions. Two-laser transmission spectroscopy is used demonstrate initialization and non-destructive measurement, simultaneously. \\[4pt] [1] D. Kim et al. Phys. Rev. Lett, \textbf{101} (2008) [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J22.00002: Dynamical Nuclear Polarization via Triplet States in Self-Assembled Quantum Dot Molecules S. C. Badescu, D. Kim, A. S. Bracker, D. Gammon, T. L. Reinecke Recent experiments on self-assembled quantum dot molecules used molecular trion states to initialize and measure optically the electron spin in one of the dots [1]. The key to this experiment is the anticrossing of two electron-triplet states in a magnetic field in Faraday configuration, which is due to spin-orbit coupling and electron-hole exchange. The experimental spin initialization and readout plots exhibit bifurcation and hysteresis features attributable to nuclear polarization. Here we present results for these effects from a model accounting for the feedback between the dynamical nuclear polarization and the electron spin states in the two dots, determined by the optical pumping and the asymmetric exchange. We explain the correlations between the nuclear polarizations in each of the dots and the asymmetry between the nuclear effects for the two measured electron states. [1] D. Kim et al, Phys. Rev. Lett. \textbf{101} (2008) [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J22.00003: Single frequency precession of inhomogeneous ensemble of electron spins. Alex Greilich, Stefan Spatzek, Irina Yugova, Ilja Akimov, Dmitri Yakovlev, Alexander Efros, Dirk Reuter, Andreas Wieck, Manfred Bayer We show that the spins of all electrons, each confined in a quantum dot of an (In,Ga)As/GaAs dot ensemble, can be driven into a single mode of precession about a magnetic field. This regime is achieved by allowing only a single mode within the electron spin precession spectrum of the ensemble to be synchronized with a train of periodic optical excitation pulses. Under this condition a nuclei induced frequency focusing leads to a shift of all spin precession frequencies into the synchronized mode. The macroscopic magnetic moment of the electron spins that is created in this regime precesses without dephasing. This ensemble can be used then as a macroscopic quantum bit. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J22.00004: Control of the direction and rate of nuclear spin flips in InAs quantum dots using detuned optical pulse trains Sam Carter, Sophia Economou, Andrew Shabaev, Thomas Kennedy, Allan Bracker, Thomas Reinecke Using two-color time-resolved Faraday rotation and ellipticity measurements, we show that control of the direction and rate of nuclear spin flips in InAs quantum dots can be achieved through optical manipulation of the electron spin. A circularly polarized pump pulse train excites an ensemble of dots with varying electron spin precession frequencies and pump detunings. Resonant excitation has been described in Ref. [1], in which the electron spin polarization is greatly enhanced when the precession is synchronized to a multiple of the pulse repetition rate. Nuclear spin flips occur rapidly when the electron spin is not synchronized, with equal probability to flip up or down, leading to random walks that eventually lead the system to stable synchronized modes. In detuned dots, rotations of the spin away from the plane of precession lead to asymmetry in the nuclear spin flip rates, giving a clear pathway for nuclear reconfiguration. For dot energies below (above) the pump, the nuclear reconfiguration pushes electron spins towards (away from) synchronization. This effect is observed through a spectral shift in the Faraday rotation/ellipticity amplitudes as a function of probe detuning. [1] A. Greilich, A. Shabaev \textit{et} al., Science \textbf{317}, 1896 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J22.00005: Theory of the effect of detuned optical pulse trains on the electron-nuclear hyperfine interaction in quantum dots Sophia Economou, Sam Carter, Andrew Shabaev, Tom Kennedy, Allan Bracker, Tom Reinecke A train of optical pulses detuned from resonance of the electron spin-trion transition in a quantum dot has the combined effect of generating and of rotating the spin polarization. The rotation is a direct consequence of the detuning and induces an electronic spin component parallel or antiparallel to the magnetic field, depending on the sign of the detuning. This electron spin component directs the nuclear spin to preferably flip in one direction. This pulse-assisted electron-nuclear flip-flop both affects the electron, because it changes the precession frequency and changes whether it is synchronized with the pulses, and it also opens up the opportunity for manipulating the nuclear polarization by using the detuning along with the pulse repetition rate as handles. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J22.00006: Many-body theory of spin bath dynamics for qubit decoherence Ren-Bao Liu, Wen Yang We have developed a cluster correlation expansion (CCE) theory for the many-body dynamics of a finite-size spin bath in a time scale relevant to decoherence of a center spin or a qubit embedded in the bath [1]. In terms of the linked cluster expansion, a cluster correlation term is the infinite summation of all the linked diagrams with all the spins in the cluster flip-flopped. The lowest order of the cluster correlation corresponds to the pair-correlation approximation developed previously [2]. In the thermodynamics limit, the CCE reduces to the standard cluster expansion. The CCE is especially useful for studying multi-spin coherence in small spin baths such as NV centers in diamonds and molecular magnets, where the cluster expansion fails to converge to the exact solution. \\[4pt] [1] W. Yang and R. B. Liu, Phys. Rev. B \textbf{78}, 085315 (2008).\\[0pt] [2] W. Yao, R. B. Liu and L. J. Sham, Phys. Rev. B \textbf{74}, 195301 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J22.00007: The Hyperfine-mediated and Nuclear-Dipolar-Induced Nuclear Spin Eddy Yusuf, Xuedong Hu We study nuclear spin diffusion in semiconductor quantum dots based on the density matrix approach. The nuclear spin interactions that we consider include both hyperfine-mediated and magnetic dipolar interactions. Furthermore, we take into account both the secular and the non-secular terms of the magnetic dipolar nuclear interactions. We discuss how the one- and two-electronic states in the quantum dots lead to quantitatively different nuclear spin polarization relaxation and nuclear spin diffusion time. We explore the behavior of the relaxation time and diffusion constant for various experimentally relevant parameters, and compare our results to the recently measured nuclear spin relaxation in GaAs double quantum dots [1]. \\[3pt] [1] D. J. Reilly, J. M. Taylor, J. R. Petta, C. M. Marcus, M. P. Hanson, and A. C. Gossard, arXiv:0803.3082 [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J22.00008: Theoretical study of the strain-induced nuclear spin depolarization in self-assembled quantum dots Chia-Wei Huang, Xuedong Hu We investigate how strain-induced quadrupole interaction is related to nuclear spin polarization in self-assembled quantum dots. Our calculation shows that the achievable nuclear spin polarization in In$_{x}$Ga$_{1- x}$As quantum dots is sensitively dependent on the strain distribution in the dots. There are two interesting regions of rapid changes in nuclear spin polarization when the Overhauser field is in the opposite direction to the external field. The first one occurs in the low field region (B $<$ 1T) where nuclear spin polarization of individual nuclear species is suppressed due to a degeneracy between different nuclear spin states. The second one is a peak in nuclear spin polarization showing up in the intermediate field region. This peak corresponds to a local maximum of the Overhauser field, which happens when electronic Zeeman energy vanishes. Our results are in qualitatively agreement with the measured nuclear spin polarization in the experimental work of various groups\footnote{B. Eble \textit{et al.}, Phys. Rev. B \textbf{74}, 081306 (2006).}$^,$ \footnote{A. S. Bracker \textit{et al.}, Phys. Rev. Lett. \textbf{94}, 047402 (2005).}$^,$ \footnote{P. Maletinsky \textit{et al.}, Phys. Rev. B \textbf {75}, 035409 (2007).} [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J22.00009: Preparation of Nuclear Spin States in Double Quantum Dots Jacob J. Krich, Michael Gullans, Jacob M. Taylor, Michael Stopa, Bertrand I. Halperin, Mikhail D. Lukin, Amir Yacoby Recent experiments on double quantum dot systems with two electrons have shown rich dynamics associated with the hyperfine coupling to nuclear spins. We examine how the cycles used to produce dynamic nuclear polarization in such double quantum dots can lead to interesting non-equilibrium configurations of the nuclear spins. We develop a master equation for the nuclear spins, which we solve using time-dependent mean field theory. We find a rich set of phenomena in the system, including tendencies of the system to approach two very different configurations, one with equal effective magnetic fields produced by the nuclei in the two dots and the other with a large difference between the magnetic fields produced by the nuclei in the two dots, both of which are seen in experiments. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J22.00010: Zamboni force in pumping of angular momentum from electron to nuclear spins via the Overhauser effect Michael Stopa, Amir Yacoby, Jacob Krich We identify a feedback mechanism between the electron states in a two-electron double quantum dot and the difference between the nuclear spin polarization in the two dots, which we term the ``Zamboni force.'' The Overhauser interaction is known to cause angular momentum transfer, spin flip-flops, between electrons and nuclei in GaAs-AlGaAs heterostructures. In double quantum dots, transport and pumping experiments have been performed to study the evolution of nuclear spin polarization in response to certain electronic transitions. We show that, in flipping from singlet (S) to triplet (T+), ``flopping'' of the nuclear spin can occur in the left dot, the right dot or in the barrier depending on the composition of the singlet state. Assuming a composite nuclear spin for each the left dot, the right dot and the barrier, we numerically integrate the Schr\"{o}dinger equation to study the gate voltage sweep through the S-T+ anti-crossing point. We show that the (nuclear) effective magnetic field gradient tends to produce spin flips in the dot with the weaker field and thereby constitutes a force toward nuclear spin equilibration. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J22.00011: Spin Fluctuations in Magnetic Quantum Dots A.G. Petukhov, R.M. Abolfath, Igor Zutic We present a theoretical description of magnetism in quantum dots (QDs) doped with magnetic ions. It has been recognized that the mean-field theory (MFT) is inadequate for small magnetic systems, such as bound magnetic polarons (BMPs), at finite temperatures [1]. Magnetic QDs are in many respects similar to BMPs, however the latter are one-electron systems while the former may contain many electrons. Our approach requires the minimization of the generalized ``free energy'' functional [2] for QDs, which leads to a set of self-consistent Kohn-Sham-type equations that coincide with MFT-equations [3] in the thermodynamic limit. We reveal that the well-known spurious MFT second order phase transition in magnetization is completely removed by thermodynamic spin fluctuations. \\[4pt] [1] T. Dietl and J. Spalek, Phys. Rev. Lett. {\bf 48}, 355 (1982).\\[0pt] [2] A. G. Petukhov, I. Zutic, and S. C. Erwin, Phys. Rev Lett. {\bf 99}, 257202 (2007)\\[0pt] [3] R. M. Abolfath, A. G. Petukhov, and I. Zutic, Phys. Rev. Lett. {\bf 101}, 207202 (2008); R. M. Abolfath, P. Hawrylak, and I. Zutic, Phys. Rev. Lett. {\bf 98}, 207203 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J22.00012: Carrier-mediated magnetism and bound magnetopolarons in quantum dots Rafal Oszwaldowski, Andre Petukhov, Igor Zutic While Mn-doped quantum dots (QDs) offer versatile control of magnetic order [1], important challenges remain in understanding of these systems beyond the mean-field approximation. Furthermore, to describe the carrier-mediated magnetism in arrays of magnetic QDs and their non-equilibrium properties, it is important to consider the presence of both electrons and holes in these systems. We develop a formalism that accounts for both equilibrium and light-controlled magnetopolaron effects [2]. We study QDs of different sizes and find that their magnetic and photo-induced properties are extremely size-sensitive. We compare our theory with recent experiments on circularly-polarized photoluminescence in magnetic QDs [3] where both the magnetopolaron energies and power dependence of the circular polarization were measured. We thank I. R. Sellers for valuable discussions. Supported by ONR and NSF-ECCS CAREER. [1] L. Besombes et al., Phys. Rev. Lett. 93, 207403 (2004); R. M. Abolfath, A. G. Petukhov, and I. Zutic, Phys. Rev. Lett. 101, 207202 (2008). [2] T. Dietl and J. Spalek, Phys. Rev. Lett. 48, 355 (1982). [3] I. R. Sellers et al., unpublished. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J22.00013: ABSTRACT WITHDRAWN |
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