Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session J43: Focus Session: Materials for Quantum Information Processing II |
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Sponsoring Units: DMP Chair: Bruce Kane, University of Maryland Room: Colorado Convention Center 506 |
Tuesday, March 6, 2007 11:15AM - 11:51AM |
J43.00001: Solid-state materials and devices for single-photon generation and more Invited Speaker: A single-photon device, which ideally emits exactly one photon on demand into a definite quantum state, can be constructed from a single atom or atom-like system excited by optical pulses and coupled to an optical micro-cavity. Solid-state single quantum systems are especially practical for this application because they do not require complicated trapping setups and can be integrated into monolithic micro-cavity structures. In the last several years single-photon generation has been demonstrated in a variety of solid-state systems including nitrogen-vacancy (NV) centers in diamond, epitaxial quantum dots in semiconductors such as InGaAs or AlGaN, and impurities in semiconductors. A variety of microcavity geometries have also been employed to improve photon extraction efficiency and to increase the spontaneous emission rate, including micro-pillars with distributed-Bragg-reflector mirrors, micro-disks and photonic crystal cavities. Results from various systems will be summarized and compared in terms of the suppression of the two-photon emission probability (compared with a Poisson distribution), efficiency, and quantum indistinguishability of the generated photon wave packets. A device that efficiently produces single photons with high spectral purity can also be used in other ways. For example, two photons incident onto such a device should in theory exhibit a strong optical nonlinearity. In addition, if the device uses a three-level Lambda-type system in which two lower long-lived levels are coupled by optical transitions to a common excited state, the possibility exists for efficient matter-photon quantum state inter-conversion, an important ingredient for quantum networks and other applications. It has recently been demonstrated that two solid-state systems, charged quantum dots and nitrogen-vacancy centers in diamond, have the required level structure for this scheme. Recent results demonstrating coherent population trapping in single NV centers will be described which are promising in terms of optical manipulation of single spins and eventually spin-photon inter-conversion. [Preview Abstract] |
Tuesday, March 6, 2007 11:51AM - 12:03PM |
J43.00002: Dynamical nuclear spin polarization and the Zamboni effect in gated double quantum dots Guy Ramon, Xuedong Hu The hyperfine interaction between electron spins confined in semiconductor quantum dots and the surrounding nuclear spins is one of the main sources for electron spin decoherence in low temperature GaAs quantum dots. We have investigated theoretically the dynamics of a system of two electrons and nuclear spin baths subject to the hyperfine interaction in a gated double dot system. It is shown that the hyperfine interaction can mediate a dynamical nuclear polarization by utilizing the degeneracy point between the two-electron singlet and polarized triplet states. Most importantly, we demonstrate that a small polarization ($~0.3\%$) is sufficient to enhance the singlet decay time by two orders of magnitude, in contrast with the single dot case, where nearly complete nuclear polarization is required to improve spin coherence time significantly. This enhancement is attributed to an equilibration process between the nuclear reservoirs in the two dots, mediated by the hyperfine interaction, an effect we have dubbed as the nuclear Zamboni effect. We explore other strategies to facilitate this effect and show that while equilibration of the two nuclear configurations is obtained, the singlet decay times are only modestly enhanced due to broadening of the nuclear spin distribution. [Preview Abstract] |
Tuesday, March 6, 2007 12:03PM - 12:15PM |
J43.00003: Dephasing of exchange coupled spin qubits by electron-phonon coupling Xuedong Hu Exchange coupled spin qubits in semiconductor nanostructures can be dephased by {\it charge fluctuations} in the semiconductor environment because of the fundamental Coulombic nature of the Heisenberg coupling. Even when charge fluctuations are suppressed through material improvement, such orbital-degree-of-freedom related fluctuations can still come from electron-phonon interaction in the semiconductor. Here we explore pure dephasing between the two-electron singlet and triplet states for two exchange-coupled spin qubits in a double quantum dot. [Preview Abstract] |
Tuesday, March 6, 2007 12:15PM - 12:27PM |
J43.00004: Two- and three- energy level mixing effects in vertically coupled quantum dots Chris Payette, David Austing, Guolin Yu, James Gupta, Selva Nair We investigate high bias single electron resonant tunneling through sub-micron gated AlGaAs/InGaAs/AlGaAs/InGaAs/AlGaAs triple barrier structures for which the tunnel coupling energy between the two quantum dots is very weak (less than 0.1meV). The two quantum dot ``disks'' in the vertical diatomic artificial molecule located in the circular device mesa can be almost circular or elliptically deformed. In a device where the constituent dots are elliptically deformed, the single particle states of each dot evolve almost ideally with magnetic field, except at several of the two- and three- energy level crossings. At these crossing points, we see pronounced two level anti- crossing behavior, with levels split by hundreds of micro-eV, and intriguing level crossing phenomena, like mixing of three resonances leading to resonance suppression. We analyze the observed quantum level mixing effects using a simple three level mixing model. [Preview Abstract] |
Tuesday, March 6, 2007 12:27PM - 12:39PM |
J43.00005: Magnetic field induced resonance and hysteresis effects in the current flowing through coupled vertical quantum dots at high source-drain bias David Austing, Chris Payette, Guolin Yu, James Gupta We report on the basic properties, including the temperature, range of magnetic field, sweep rate and voltage dependence, of recently observed magnetic field induced resonance and hysteresis effects in the current flowing through two weakly coupled vertical quantum dots at high source-drain bias. Similar looking effects, attributed to electron spin - nuclear spin (hyperfine) coupling, have been seen in the low bias two- electron spin-blockade regime (K. Ono and S. Tarucha Phys. Rev. Lett. 2004), when the magnetic field is applied perpendicular to the flowing current, but the regime we study here is at much higher bias (up to a few 10's of mV) and for a magnetic field (0- 6T) applied parallel to the current. ``Slow'' current oscillations/fluctuations are also observed on the timescale of seconds to tens of seconds for certain conditions. Can nuclear spin related effects occur outside the N=2 spin-blockade region? [Preview Abstract] |
Tuesday, March 6, 2007 12:39PM - 12:51PM |
J43.00006: Three-electron bonding and entanglement in single and molecular quantum dots Yuesong Li, Constantine Yannouleas, Uzi Landman The study of three-electron quantum dots (QDs) is interesting in several ways. First, it was demonstrated\footnote{C. Ellenberger, T. Ihn, C. Yannouleas, U. Landman, K. Ensslin, D. Driscoll, and A.C. Gossard, Phys. Rev. Lett. {\bf 96}, 126806 (2006).} recently that detailed ground-state and excited spectra of few-electron elliptic QDs can be measured as a function of the externally applied magnetic field. Second, three-qubit electron spin devices are expected to exhibit enhanced efficiency for quantum computing purposes compared to single-qubit and two-qubit gates. We carry out exact diagonalization (EXD) studies for a three-electron single QD and for a wide range of anisotropies. We analyze the properties of the EXD many-body wave functions with respect to electron localization in a linear geometry, as well as to generation of model quantum entangled states that are often employed in the theory of quantum computing. We further examine three-electron bonding and entanglement in the case of a double quantum dot. [Preview Abstract] |
Tuesday, March 6, 2007 12:51PM - 1:03PM |
J43.00007: Long range spin qubit interaction mediated by microcavity polaritons Carlo Piermarocchi, Guillermo F. Quinteiro, Joaquin Fernandez-Rossier Planar microcavities are semiconductor devices that confine the electromagnetic field by means of two parallel semiconductor mirrors. When a quantum well (QW) is placed inside a planar microcavity, the excitons in the QW couple to confined electromagnetic modes. In the strong-coupling regime, excitons and cavity photons give rise to new states, cavity polaritons, which appear in two branches separated by a vacuum Rabi splitting. We study theoretically the dynamics of localized spins in the QW interacting with cavity polaritons. Our calculations consider localized electron spins of shallow neutral donors in GaAs (e.g., Si), but the theory is valid for other impurities and host semiconductors, as well as to charged quantum dots. In the strong-coupling regime, the vacuum Rabi splitting introduces anisotropies in the spin coupling. Moreover, due to their photon-like mass, polaritons provide an extremely long spin coupling range. This suggests the realization of two-qubit all-optical quantum operations within tens of picoseconds with spins localized as far as hundreds of nanometers apart. [G. F. Quinteiro et al., Phys. Rev. Lett. 97 097401, (2006)]. [Preview Abstract] |
Tuesday, March 6, 2007 1:03PM - 1:15PM |
J43.00008: Optically detected quantum dynamics of hydrogenic donor qubits Dan Allen, Sangwoo Kim, Mark Sherwin Orbital states of electrons bound to shallow donors in GaAs provide many of the advantages of trapped atoms for quantum information studies, including optical readout and long lived excited levels. Shallow donors (e.g. S, Si) have a scaled hydrogenic potential with a bound electron 1S-2P transition at 1 THz (4 meV). In a 5 T magnetic field the 1S state and lowest 2P state (2P$^-$) serve as qubit levels. A cycling transition exists for detecting neutral donors in the ground state via the donor bound exciton resonance; excited bound states are dark. Using this optical quantum nondemolition measurement, the relaxation (T$_1$) of donors after THz excitation of the 1S- 2P$^-$ transition is observed to be $>1\mu$s. High resolution spectroscopy indicates dephasing (T$_2^*$) of an ensemble of neutral donors is limited by inhomogeneous broadening to 50 ps. In order to measure the decoherence time (T$_2$), which is expected to be much longer, a rephasing technique is required. For Hahn echo measurements of T$_2$ a 0-24ns, diffraction- compensating free space THz delay line has been constructed. [Preview Abstract] |
Tuesday, March 6, 2007 1:15PM - 1:27PM |
J43.00009: Characterization of (In,Ga)As Quantum Posts for Terahertz Quantum Information Processing C. M. Morris, D. G. Allen, J. He, C. Pryor, P. M. Petroff, M. S. Sherwin Quantum posts (QPs) are a new kind of self-assembled semiconductor nanostructure which may be suitable for quantum information processing using terahertz frequencies.$^{1}$ A QP is a roughly cylindrical In-rich region embedded in a GaAs matrix whose height can be controlled with monolayer resolution. For a single electron trapped in a 40 nm high QP, the orbital transition between the ground and first excited state is predicted to occur near 1 THz. Since this is well below the optical phonon frequency (9 THz), decoherence is expected to arise primarily from very weak interactions with acoustic phonons. QPs grown in the insulating region of a metal-insulator-semiconductor structure allow voltage-controlled charging, which is measured by capacitance-voltage spectroscopy. Terahertz absorption spectra are also measured by Fourier-transform infrared spectroscopy. $^{1}$ M. S. Sherwin, A. Imamoglu and C. Montroy, PRA 60, 3508 (1999) Work supported by the NSF NIRT grant No. CCF 0507295 [Preview Abstract] |
Tuesday, March 6, 2007 1:27PM - 1:39PM |
J43.00010: Enhancement of electron spin coherence by optical preparation of nuclear spins Dimitrije Stepanenko, Guido Burkard, Geza Giedke, Atac Imamoglu We study a large ensemble of nuclear spins interacting with a single electron spin in a quantum dot under optical excitation and photon detection. When a pair of applied laser fields satisfy two-photon resonance between the two ground electronic spin states, detection of light scattering from the intermediate exciton state acts as a weak quantum measurement of the effective magnetic (Overhauser) field due to the nuclear spins. If the spin were driven into a coherent population trapping state where no light scattering takes place, then the nuclear state would be projected into an eigenstate of the Overhauser field operator and electron decoherence due to nuclear spins would be suppressed: we show that this limit can be approached by adapting the laser frequencies when a photon is detected. We use a Lindblad equation to describe the time evolution of the driven system under photon emission and detection. Numerically, we find an increase of the electron coherence time from $5\,{\rm ns}$ to $500\,{\rm ns}$ after a preparation time of 10 microseconds. [Preview Abstract] |
Tuesday, March 6, 2007 1:39PM - 1:51PM |
J43.00011: Exchange energy in vertically coupled double quantum dots Tetsuo Kodera, Yousuke Kitamura, Keiji Ono, Shinichi Amaha, Yasuhiro Tokura, Seigo Tarucha The exchange separation between spin singlet and triplet states was studied for vertically coupled double quantum dots in the Pauli spin blockade regime with the inter-dot level detuning as a parameter. Pauli blockade is established by the formation of an excited but long-lived triplet state in the double dot, and is lifted by a spin flip transition to the singlet state, generating a leakage current. The leakage current shows a step when the Zeeman energy equals the exchange energy thus turning on the flip-flop interaction with the nuclei. The threshold magnetic field increases on approaching the anti-crossing of the two triplets reflecting the increased exchange energy. We present a quantitative comparison of the exchange energy derived experimentally with exact theory. [Preview Abstract] |
Tuesday, March 6, 2007 1:51PM - 2:03PM |
J43.00012: Electronic structure, entanglement and double occupancy in asymmetric dot molecule quantum gate Lixin He, Alex Zunger First, we describe the energy levels, degree of entanglement and double occupancy in fully symmetric (homopolar) quantum dot molecules (QDM) made of InGaAs dots in a GaAs barrier containing $\sim$ 3 x10$^6$ atoms. We describe the single-particle part by atomistic pseudopotential theory including strain and alloy effects, and the many body part via configuration interaction. Second, we note that in a realistic vertically coupled QDM the two dots often have different geometries, sizes, alloy compositions, (heteropolar QDM) and therefore, deviates from ideal homopolar QDM model used previously. We show that the electronic properties of such heteropolar QDMs are greatly modified by the asymmetry of the QDMs, showing larger two-electron double occupation rate, lower two-electron entanglement, and therefore reduced quantum gate quality. By symmetrizing the QDM via application of electric field, one can overcome these difficulties. [Preview Abstract] |
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