arXiv subjects

L. J. Sham

Publications and source records attributed to L. J. Sham.

At least 73 records · Page 4Linked to original sources

arXiv2004

Ultrafast initialization and QND-readout of a spin qubit via control of nanodot-vacuum coupling

Ultrafast initialization enables fault-tolerant processing of quantum information while QND readout enables scalable quantum computation. By spatially assembling photon resonators and wave-guides around an n-doped nanodot and by temporally designing optical pump pulses, an efficient quantum pathway can be established from an electron spin to a charged exciton to a cavity photon and finally to a flying photon in the waveguide. Such control of vacuum-nanodot coupling can be exploited for ultrafast initialization and QND readout of the spin, which are particularly compatible with the optically driven spin quantum computers.

cond-mat.mes-hall↗

arXiv2004

Theory of control of spin/photon interface for quantum networks

A cavity coupling a charged nanodot and a fiber can act as a quantum interface, through which a stationary spin qubit and a flying photon qubit can be inter-converted via cavity-assisted Raman process. This Raman process can be controlled to generate or annihilate an arbitrarily shaped single-photon wavepacket by pulse-shaping the controlling laser field. This quantum interface forms the basis for many essential functions of a quantum network, including sending, receiving, transferring, swapping, and entangling qubits at distributed quantum nodes as well as a deterministic source and an efficient detector of a single photon wavepacket with arbitrarily specified shape and average photon number. Numerical study of noise effects on the operations shows high fidelity.

quant-ph↗

arXiv2004

Spin accumulation in forward-biased MnAs/GaAs Schottky diodes

We describe a new means for electrically creating spin polarization in semiconductors. In contrast to spin injection of electrons by tunneling through a reverse-biased Schottky barrier, we observe spin accumulation at the metal/semiconductor interface of forward-biased ferromagnetic Schottky diodes, which is consistent with a theory of spin-dependent reflection off the interface. Spatiotemporal Kerr microscopy is used to image the electron spin and the resulting dynamic nuclear polarization that arises from the non equilibrium carrier polarization.

cond-mat.mtrl-sci↗

arXiv2003

Coherently photo-induced ferromagnetism in diluted magnetic semiconductors

Ferromagnetism is predicted in undoped diluted magnetic semiconductors illuminated by intense sub-bandgap laser radiation . The mechanism for photo-induced ferromagnetism is coherence between conduction and valence bands induced by the light which leads to an optical exchange interaction. The ferromagnetic critical temperature T_C depends both on the properties of the material and on the frequency and intensity of the laser and could be above 1 K.

cond-mat.mes-hall↗

arXiv2003

Nanodot-Cavity Electrodynamics and Photon Entanglement

Quantum electrodynamics of excitons in a cavity is shown to be relevant to quantum operations. We present a theory of an integrable solid-state quantum controlled-phase gate for generating entanglement of two photons using a coupled nanodot-microcavity-fiber structure. A conditional phase shift of $O(π/10)$ is calculated to be the consequence of the giant optical nonlinearity keyed by the excitons in the cavities. Structural design and active control, such as electromagnetic induced transparency and pulse shaping, optimize the quantum efficiency of the gate operation.

cond-mat.mes-hall↗

arXiv2003

The Silicon Inversion Layer With A Ferromagnetic Gate: A Novel Spin Source

Novel spin transport behavior is theoretically shown to result from replacing the usual metal (or poly-silicon) gate in a silicon field-effect transistor with a ferromagnet, separated from the semiconductor by an ultra-thin oxide. The spin-dependent interplay between the drift current (due to a source-drain bias) and the diffusion current (due to carrier leakage into the ferromagnetic gate) results in a rich variety of spin dependence in the current that flows through such a device. We examine two cases of particular interest: (1) creating a 100% spin-polarized electrical current and (2) creating a pure spin current without a net electrical current. A spin-valve consisting of two sequential ferromagnetic gates is shown to exhibit magnetoresistance dependent upon the relative orientations of the magnetization of the two ferromagnets. The magnetoresistance ratio grows to arbitrarily large values in the regime of low source-drain bias, and is limited only by the spin-flip time in the channel.

cond-mat.mes-hall↗

arXiv2003

Theory of Quantum Optical Control of Single Spin in a Quantum Dot

We present a theory of quantum optical control of an electron spin in a single semiconductor quantum dot via spin-flip Raman transitions. We show how an arbitrary spin rotation may be achieved by virtual excitation of discrete or continuum trion states. The basic physics issues of the appropriate adiabatic optical pulses in a static magnetic field to perform the single qubit operation are addressed.

cond-mat↗

arXiv2003

Thermoelectric properties of junctions between metal and models of strongly correlated semiconductors

We study the thermopower of a junction between a metal and a strongly correlated semiconductor. Both in the electronic ferroelectric regime and in the Kondo insulator regime the thermoelectric figures of merit, ZT, of these junctions are compared with that of the ordinary semiconductor. By inserting at the interface one or two monolayers of atoms different from the bulk, with a suitable choice of rare-earth elements very high values of ZT can be reached at low temperatures. The potential of the junction as a thermoelectric device is discussed.

cond-mat.str-el↗

arXiv2003

Theory of Spin Transport Induced by Ferromagnetic Proximity On a Two-Dimensional Electron Gas

A theory of the proximity effects of the exchange splitting in a ferromagnetic metal on a two dimensional electron gas (2DEG) in a semiconductor is presented. The resulting spin-dependent energy and lifetime in the 2DEG create a marked spin-splitting in the driven in-plane current. The theory of the planar transport allows for current leakage into the ferromagnetic layer through the interface, which leads to a competition between drift and diffusion. The spin-dependent in-plane conductivity of the 2DEG may be exploited to provide a new paradigm for spintronics devices based on planar devices in a field-effect transistor configuration. An illustrative example is provided through the transport theory of a proposed spin-valve which consists of a field-effect transistor configuration with two ferromagnetic gates. Results are provided for two experimentally accessible systems: the silicon inversion layer and the naturally-formed InAs accumulation layer.

cond-mat.mes-hall↗

arXiv2003

Theory of Spin Orientation of Semiconductor Carriers at a Ferromagnetic Interface

A quantum theory of the spin-dependent scattering of semiconductor electrons by a Schottky barrier at an interface with a ferromagnet is presented. The reflection of unpolarized non-equilibrium carriers produces spontaneous spin-polarization in the semiconductor. If a net spin-polarization pre-exists in the semiconductor, the combination of the ferromagnet magnetization and the incident carrier polarization combine to tilt the reflected polarization in the semiconductor. The spin reflection properties are investigated as functions of the system characteristics: the Schottky barrier height, semiconductor doping and applied bias. The effect on reflection due to the variation of the barrier width with electron energy is contrasted for two means of excitation: optical or electrical. Optically excited electrons have a wider energy spread than the near-equilibrium excitation from non-magnetic ohmic contacts.

cond-mat.mes-hall↗

arXiv2002

Spin-dependent properties of a two-dimensional electron gas with ferromagnetic gates

A theoretical prediction of the spin-dependent electron self-energy and in-plane transport of a two-dimensional electron gas in proximity with a ferromagnetic gate is presented. The application of the predicted spin-dependent properties is illustrated by the proposal of a device configuration with two neighboring ferromagnetic gates which produces a magnetoresistance effect on the channel current generated by nonmagnetic source and drain contacts. Specific results are shown for a silicon inversion layer with iron gates. The gate leakage current is found to be beneficial to the spin effects.

cond-mat.mes-hall↗

arXiv2002

Optical RKKY Interaction between Charged Semiconductor Quantum Dots

We show how a spin interaction between electrons localized in neighboring quantum dots can be induced and controlled optically. The coupling is generated via virtual excitation of delocalized excitons and provides an efficient coherent control of the spins. This quantum manipulation can be realized in the adiabatic limit and is robust against decoherence by spontaneous emission. Applications to the realization of quantum gates, scalable quantum computers, and to the control of magnetization in an array of charged dots are proposed.

cond-mat↗

arXiv2002

Ferromagnetic imprinting of spin polarization in a semiconductor

We present a theory of the imprinting of the electron spin coherence and population in an n-doped semiconductor which forms a junction with a ferromagnet. The reflection of non-equilibrium semiconductor electrons at the interface provides a mechanism to manipulate the spin polarization vector. In the case of unpolarized excitation, this ballistic effect produces spontaneous electron spin coherence and nuclear polarization in the semiconductor, as recently observed by time-resolved Faraday rotation experiments. We investigate the dependence of the spin reflection on the Schottky barrier height and the doping concentration in the semiconductor and suggest control mechanisms for possible device applications.

cond-mat.mes-hall↗

arXiv2002

Spin separation in digital ferromagnetic heterostructures

In a study of the ferromagnetic phase of a multilayer digital ferromagnetic semiconductor in the mean-field and effective-mass approximations, we find the exchange interaction to have the dominant energy scale of the problem, effectively controlling the spatial distribution of the carrier spins in the digital ferromagnetic heterostructures. In the ferromagnetic phase, the majority and minority carriers tend to be in different regions of the space (spin separation). Hence, the charge distribution of carriers also changes noticeably from the ferromagnetic to the paramagnetic phase. An example of a design to exploit these phenomena is given.

cond-mat.mes-hall↗

arXiv2001

Theory of Fast Quantum Control of Exciton Dynamics in Semiconductor Quantum Dots

Optical techniques for the quantum control of the dynamics of multiexciton states in a semiconductor quantum dot are explored in theory. Composite bichromatic phase-locked pulses are shown to reduce the time of elementary quantum operations on excitons and biexcitons by an order of magnitude or more. Analytic and numerical methods of designing the pulse sequences are investigated. Fidelity of the operation is used to gauge its quality. A modified Quantum Fourier Transform algorithm is constructed with only Rabi rotations and is shown to reduce the number of operations. Application of the designed pulses to the algorithm is tested by a numerical simulation.

cond-mat.mes-hall↗

arXiv2001

A theory of ferromagnetism in planar heterostructures of (Mn,III)-V semiconductors

A density functional theory of ferromagnetism in heterostructures of compound semiconductors doped with magnetic impurities is presented. The variable functions in the density functional theory are the charge and spin densities of the itinerant carriers and the charge and localized spins of the impurities. The theory is applied to study the Curie temperature of planar heterostructures of III-V semiconductors doped with manganese atoms. The mean-field, virtual-crystal and effective-mass approximations are adopted to calculate the electronic structure, including the spin-orbit interaction, and the magnetic susceptibilities, leading to the Curie temperature. By means of these results, we attempt to understand the observed dependence of the Curie temperature of planar $δ$-doped ferromagnetic structures on variation of their properties. We predict a large increase of the Curie Temperature by additional confinement of the holes in a $δ$-doped layer of Mn by a quantum well.

cond-mat↗

arXiv2001

Control of Spin Dynamics of Excitons in Nanodots for Quantum Operations

This work presents a step furthering a new perspective of proactive control of the spin-exciton dynamics in the quantum limit. Laser manipulation of spin-polarized optical excitations in a semiconductor nanodot is used to control the spin dynamics of two interacting excitons. Shaping of femtosecond laser pulses keeps the quantum operation within the decoherence time. Computation of the fidelity of the operations and application to the complete solution of a basic quantum computing algorithm demonstrate in theory the feasibility of quantum control.

cond-mat.mes-hall↗

arXiv2000

Thermoelectric properties of junctions between metal and strongly correlated semiconductor

We propose a junction of metal and rare-earth compound semiconductor as the basis for a possible efficient low-temperature thermoelectric device. If an overlayer of rare earth atoms differing from the bulk is placed at the interface, very high values of the figure of merit ZT can be reached at low temperature. This is due to sharp variation of the transmission coefficient of carriers across the junction at a narrow energy range, which is intrinsically linked to the localized character of the overlayer f-orbital.

cond-mat.str-el↗