Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B17: Focus Session: Materials and Device Physics for Quantum Computing I |
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Sponsoring Units: DMP DCMP Chair: Henry Everitt, Army Research Office Room: LACC 404B |
Monday, March 21, 2005 11:15AM - 11:51AM |
B17.00001: Optically Controlled Quantum Dots for Quantum Computing Invited Speaker: An exciton, the elementary optical excitation of a semiconductor quantum dot, could act as a qubit of quantum information. It is relatively easy to address, measure, and control. In fact, great progress has been made in recent years in this area [1]. Yet there may be another even more exciting and powerful approach that, although involving similar quantum dot materials and optical techniques, relies on a spin. In this talk I will discuss the relationship between the exciton qubit and the spin qubit, and review some of our recent experiments that illuminate the physics underlying these two different physical embodiments of quantum information. [1] D. Gammon and D.G. Steel, \textit{Physics Today} \textbf{55(10)}, 36 (2002). [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:03PM |
B17.00002: Persistent optical charging of Electric-field tunable ensembles of Quantum Dots Kulvinder Gill, Nicholas Moskovitz, Mark Sherwin Controllably-charged semiconductor quantum dots have appeared in many proposals for quantum bits. Typical charging methods involve static electric fields that also distort the confining potential and the electronic energy states. We demonstrate a method of optically loading charges into InAs/GaAs quantum dot ensembles in a controllable way, while independently applying static electric fields to tune the quantum dot energy levels. We use a semiconductor N-I-N structure and a series of voltage and non-resonant inter-band light pulses. Charge storage times of 100's of ms to seconds have been achieved at 4K. This charging method promises to facilitate the use of quantum dots in quantum information processors. This work was supported by the DARPA QuIST program under Grant No. MDA972-01-1-0027 and Sun Microsystems. [Preview Abstract] |
Monday, March 21, 2005 12:03PM - 12:15PM |
B17.00003: Dynamic optical spin switch in a single quantum dot photodiode Jose M. Villas-Boas, Sergio Ulloa, Alexander Govorov The spin degree of freedom constitutes a naturally defined qubit; the basic element in quantum information processing that has been the subject of intense research. Optical pumping to generate a photocurrent in quantum dots that monitors the coherent state of the system has been used recently with great success [1,2]. In this paper we show that it is possible to switch the polarization of the photocurrent signal obtained from a {\em single} self-assembled quantum dot photodiode under the effect of elliptically-polarized light. This can be achieved by just increasing the light {\em intensity} without having to change the initial polarization of light. This response can be used as a dynamical switch to invert the spin-polarization of the extracted current. We use a multi-exciton density matrix formalism that includes the anisotropic e-h exchange in the quantum dot. The electron and hole tunneling is introduced by rates obtained from a microscopic description, and other parameters also describe realistic systems. We report on the role of the excitation detuning and mixed polarization for this optical source of spin-polarized electrons.Supported by the Indiana 21st Century Research and Technology Fund, and FAPESP- Brazil. [1] A. Zrenner \textit{et al.} Nature (London) \textbf {418}, 612 (2002); Q. Q. Wang \textit{et al.}, cond-mat/0404465. [2] J. M. Villas-B\^{o}as, Sergio E. Ulloa, and A. O. Govorov, cond-mat/0408570. [Preview Abstract] |
Monday, March 21, 2005 12:15PM - 12:27PM |
B17.00004: Coherent quantum states in functionalized semiconductor nanostructures Luis G.C. Rego, Sabas Abuabara, Victor S. Batista We investigate the feasibility of creating and manipulating coherent quantum states in the surface of functionalized semiconductor nanostructures. Functionalization of a semiconductor nanocrystal can be achieved by anchoring organic ligands to its surface, with the resulting surface complexes often introducing electronic states in the band gap. These states sensitize the host material for photo-absorption, leading to photoinduced electron-hole pair separation and interfacial electron transfer to the semiconductor conduction band. A method that combines {\it ab-initio} molecular dynamics (MD) simulations with semi-empirical modelling reveals super-exchange hole tunnelling between adjacent catechol molecules adsorbed on TiO$_2$-anatase nanostructures. It is shown that electronic coherences can persist for hundreds of picoseconds, despite the partial intrinsic decoherence induced by thermal ionic motion usually observed in photoexcited semiconductor nanostructures. Moreover, the observed relaxation dynamics can be coherently controlled by a sequence of ultrashort $2\pi$ pulses. The proposed studies of decoherence and coherent optical control intend to explore the basic constituents of a molecular electro-optic device, based on inexpensive and easily manufacturable semiconductor materials. [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 12:39PM |
B17.00005: Coherent Optical Control of Spin-Spin Interaction in Doped Semiconductors Carlo Piermarocchi, Guillermo Quinteiro We provide a theory of laser-induced interaction between spins localized by impurity centers in a semiconductor host. By solving exactly the problem of two localized spins interacting with one itinerant exciton, we study the light-induced spin-spin interaction as a function of the spin separation, laser energy, and intensity. We apply the theory to shallow neutral donors (Si) and deep rare-earth magnetic impurities (Yb) in III-V semiconductors. When the photon energy approaches a resonance related to excitons bound to the impurities, the coupling between the localized spins increases, and may change from ferromagnetic to anti-ferromagnetic. This light-controlled spin interaction provides a mechanism for the quantum control of spins in semiconductors for quantum information processing; it suggests the realization of spin systems whose magnetic properties can be controlled by changing the strength and the sign of the spin-spin interaction. [Preview Abstract] |
Monday, March 21, 2005 12:39PM - 12:51PM |
B17.00006: Tuning quantum entanglement in InGaAs/GaAs dot molecules with electric fields Gabriel Bester, Alex Zunger Self assembled quantum dots may provide a physical representation of a quantum bit (qubit) that supports a superposition of ``0'' and ``1''. In one possible realization, two qubits A and B are represented by a hole and an electron. The two different states of the qubits are given by their occupation probability, either occupying the top (T) or the bottom (B) dot of a self-assembled dot-molecule. The use of this system as a quantum register requires the ability to store {\em entangled} exciton states but entanglement was recently shown [1] to be small, unless a very specific interdot distance is chosen. Furthermore, this specific distance depends on detail of the dots geometry [1]. We present here an atomistic theory of a pair of vertically stacked InGaAs/GaAs dots and propose to tune the entanglment of the molecule using an electric field, applied in growth direction. We find that the entanglment can be maximized, using a field of -5.4 kV/cm in our case, and that at this field a specific spectroscopic signature is expected: the first 2 bright excitonic peaks merge. We suggest this feature as an identification of entangled states.\\[4pt] [1] G. Bester, J. Shumway and A. Zunger, Phys. Rev. Lett. {\bf 93}, 047401 (2004). [Preview Abstract] |
Monday, March 21, 2005 12:51PM - 1:03PM |
B17.00007: Correlation, Mott-transition, and singlet-triplet splitting for two electrons in self-assembled InAs/GaAs quantum dot molecules Lixin He, Gabriel Bester, Alex Zunger Quantum dot-molecules (QDM) occupied by two electronic spins have been proposed as a basis for quantum computation. Successful operations would require a large singlet-triplet splitting and a low probability of two electrons occupying the same dot. We investigated via atomistic pseudopotential single-particle theory and many-body screened configuration interaction calculations the degree of localization and the magnitude of the singlet-triplet splitting of two electrons in vertically stacked, strained InAs/GaAs self-assembled dots containing $\sim 10^5$ atoms. We find that the ground state is always singlet at all interdot distance and the singlet-triplet splittings are as large as 1 - 100 meV, much larger than these for electrostatically confined dots. The single-particle wavefunctions show symmetry breaking between the top and bottom dots due to the strain effect at inter-dot distance $<$ 4 nm. We find that while the two electrons of the triplet $^3\Sigma$ states show at all interdot distance a correlation-induced Mott-localization of each electron on a different dot, the ground state singlet $^1\Sigma_g$ shows a transition with increasing interdot distance from a molecular state where the two electrons are delocalized over both dots, to a Mott localized state in which each electron is on a different dot. [Preview Abstract] |
Monday, March 21, 2005 1:03PM - 1:15PM |
B17.00008: Encoding a qubit into multilevel subspaces Matthew Grace, Constantin Brif, Herschel Rabitz, Ian Walmsley, Robert Kosut, Daniel Lidar We present a formalism for encoding the logical states of a qubit into subspaces of multiple physical levels. The need for multilevel encoding arises naturally in situations where the speed of quantum operations exceeds the limits imposed by the addressability of individual energy levels of the qubit physical system. The basic feature of the multilevel encoding formalism is the logical equivalence of different physical states and, correspondingly, of different physical transformations. This logical equivalence is a source of a significant flexibility in designing unitary quantum-logic transformations. The multilevel structure inherently accommodates fast and intense broadband controls thereby facilitating faster quantum-gate operations. Another extremely important practical advantage of multilevel encoding is the ability to maintain full quantum-computational fidelity in the presence of mixing and decoherence within logical subspaces. The formalism is developed in detail for single-qubit operations and generalized for two-qubit and multiple-qubit gates. [Preview Abstract] |
Monday, March 21, 2005 1:15PM - 1:27PM |
B17.00009: Many-spin couplings in electron spin quantum computers Ryan Woodworth, Ari Mizel, Daniel Lidar We study the effective Hamiltonian governing the evolution of electron spins in coupled quantum dots. Modeling the dot confining potentials with successively more realistic forms, we predict that four-body interactions can be significant in real dots in physically relevant parameter regimes. We discuss the significance of these results for quantum computing. [Preview Abstract] |
Monday, March 21, 2005 1:27PM - 1:39PM |
B17.00010: Dynamical decoupling by shaped pulses with partial local control Leonid Pryadko, Pinaki Sengupta, Daniel Lidar, Mark Dykman We present an efficient scalable scheme for pulse-based coherent control in qubit systems with partial local control. The globally applied sequence of narrow-band NMR-like $X$ and $Y$ pulses used in combination with local $Z$ pulses (level splitting individually controlled at each qubit) is shown to allow for universal quantum computation. The scheme applies to a number of proposed quantum dot-based designs, both solid-state and based on the electrons on helium, the designs based on charge- or flux-dominated superconducting Josephson junction circuits, etc. Compared to control schemes based on tuning qubits in and out of resonance, this scheme allows for faster gates with narrower frequency band for controlling pulses. We demonstrate several two-qubit quantum gates with concurrent refocusing of both the inter-qubit and a low-frequency bath couplings. [Preview Abstract] |
Monday, March 21, 2005 1:39PM - 1:51PM |
B17.00011: Landau-Zener gates, errors, and refocusing Christian Hicke, Lea F. Santos, Mark Dykman We study single- and two-qubit Landau-Zener (LZ) gate operations and their robustness with respect to errors. In LZ operations, the qubit energies pass through the frequency of the external field (a single-qubit gate) or past each other (a two-qubit gate) \footnote{M.I. Dykman and P.M. Platzman, Quant. Inf. Comp. {\bf 1}, 102 (2001); V.G. Benza and G. Strini, Fortschr. Phys. {\bf 51}, 14 (2003); B. Golding and M.I. Dykman, cond-mat/0309147; A.V. Shitov, D.A. Ivanov, and M.V. Feigel'man, Eur. Phys. B {\bf 36}, 263 (2003); K. Saito and Y. Kayanuma, Phys. Rev. B {\bf 70}, 201304 (2004).}. Of central interest is stability against errors in the qubit energies. Such stability is particularly important for quantum computers with perpetually coupled qubits, where the energies of individual qubits depend on the state of neighboring qubits. We study refocusing based on a single-qubit LZ operation where an appropriate pulse of resonant radiation is applied concurrently with a pulse that controls the qubit energy. With such refocusing, arbitrary single-qubit LZ operations should become insensitive to small variations of the qubit energies. We show that a two-qubit LZ swap in computers with perpetually coupled qubits is much more robust when considered with respect to the basis of exact one-excitation states, which are not fully confined to individual qubits. We study the effect of errors in single-qubit energies on the LZ swap. [Preview Abstract] |
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