Biomedical subjects

L J Sham

Publications and source records attributed to L J Sham.

14 recordsLinked to original sources

PubMed2005

Ultrafast quenching of ferromagnetism in InMnAs induced by intense laser irradiation.

Time-resolved magneto-optical Kerr spectroscopy of ferromagnetic InMnAs reveals two distinct demagnetization processes--fast (<1 ps) and slow (approximately 100 ps). Both components diminish with increasing temperature and are absent above the Curie temperature. The fast component rapidly grows with pump power and saturates at high fluences (>10 mJ/cm(2)); the saturation value indicates a complete quenching of ferromagnetism on a subpicosecond time scale. We attribute this fast dynamics to spin heating through p-d exchange interaction between photocarriers and Mn ions, while the approximately 100 ps component is interpreted as spin-lattice relaxation.

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PubMed2005

Theory of control of the 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 interconverted via a cavity-assisted Raman process. This Raman process can be made to generate or annihilate an arbitrarily shaped single-photon wave packet 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 wave packet with arbitrarily specified shape and average photon number. Numerical study of errors from noise and system parameters on the operations shows high fidelity and robust tolerance.

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PubMed2005

Stimulated and spontaneous optical generation of electron spin coherence in charged GaAs quantum dots.

We report on the coherent optical excitation of electron spin polarization in the ground state of charged GaAs quantum dots via an intermediate charged exciton (trion) state. Coherent optical fields are used for the creation and detection of the Raman spin coherence between the spin ground states of the charged quantum dot. The measured spin decoherence time, which is likely limited by the nature of the spin ensemble, approaches 10 ns at zero field. We also show that the Raman spin coherence in the quantum beats is caused not only by the usual stimulated Raman interaction but also by simultaneous spontaneous radiative decay of either excited trion state to a coherent combination of the two spin states.

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PubMed2005

Coherent transport in a homojunction between an excitonic insulator and semimetal.

From the solution of a two-band model, we predict that the thermal and electrical transport across the junction of a semimetal and an excitonic insulator will exhibit high resistance behavior and low entropy production at low temperatures, distinct from a junction of a semimetal and a normal semiconductor. This phenomenon, ascribed to the dissipationless exciton flow which dominates over the charge transport, is based on the much longer length scale of the change of the effective interface potential for electron scattering due to the coherence of the condensate than in the normal state.

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PubMed2004

Coherently photoinduced ferromagnetism in diluted magnetic semiconductors.

Ferromagnetism is predicted in undoped diluted magnetic semiconductors illuminated by intense sub-band-gap laser radiation. The mechanism for photoinduced 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 1K.

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PubMed2004

Spin accumulation in forward-biased MnAs/GaAs Schottky diodes.

We describe a new means for all-electrical generation of spin polarization in semiconductors. In contrast with spin injection of electrons by tunneling through a reverse-biased Schottky barrier, we observe 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 nonequilibrium carrier polarization.

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PubMed2004

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(pi/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 electromagnetically induced transparency and pulse shaping, optimize the quantum efficiency of the gate operation.

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PubMed2003

An all-optical quantum gate in a semiconductor quantum dot.

We report coherent optical control of a biexciton (two electron-hole pairs), confined in a single quantum dot, that shows coherent oscillations similar to the excited-state Rabi flopping in an isolated atom. The pulse control of the biexciton dynamics, combined with previously demonstrated control of the single-exciton Rabi rotation, serves as the physical basis for a two-bit conditional quantum logic gate. The truth table of the gate shows the features of an all-optical quantum gate with interacting yet distinguishable excitons as qubits. Evaluation of the fidelity yields a value of 0.7 for the gate operation. Such experimental capability is essential to a scheme for scalable quantum computation by means of the optical control of spin qubits in dots.

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PubMed2002

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.

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PubMed2002

Spin polarization of semiconductor carriers by reflection off a ferromagnet.

We present a theory of generation or alteration of the electron spin coherence and population in an n-doped semiconductor by reflection at the interface with a ferromagnet. The dependence of the spin reflection on the Schottky barrier height and the doping concentration in the semiconductor was computed for a generic model. The theory provides an explanation for the spontaneous electron spin coherence and nuclear polarization in the semiconductor interfaced with a ferromagnet and associated phenomena recently observed by time-resolved Faraday rotation experiments. The study also points to an alternative approach to spintronics different from spin injection.

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PubMed2002

Biexciton quantum coherence in a single quantum dot.

Nondegenerate (two-wavelength) two-photon absorption using coherent optical fields is used to show that there are two different quantum mechanical pathways leading to formation of the biexciton in a single quantum dot. Of specific importance to quantum information applications is the resulting coherent dynamics between the ground state and the biexciton from the pathway involving only optically induced exciton/biexciton quantum coherence. The data provide a direct measure of the biexciton decoherence rate which is equivalent to the decoherence of the Bell state in this system, as well as other critical optical parameters.

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PubMed2001

Rabi oscillations of excitons in single quantum dots.

Transient nonlinear optical spectroscopy, performed on excitons confined to single GaAs quantum dots, shows oscillations that are analogous to Rabi oscillations in two-level atomic systems. This demonstration corresponds to a one-qubit rotation in a single quantum dot which is important for proposals using quantum dot excitons for quantum computing. The dipole moment inferred from the data is consistent with that directly obtained from linear absorption studies. The measurement extends the artificial atom model of quantum dot excitonic transitions into the strong-field limit, and makes possible full coherent optical control of the quantum state of single excitons using optical pi pulses.

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PubMed2001

Control of exciton dynamics in nanodots for quantum operations.

We present a theory to further a new perspective of proactive control of exciton dynamics in the quantum limit. Circularly polarized optical pulses in a semiconductor nanodot are used to control the dynamics of two interacting excitons of opposite polarizations. 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 minimal quantum computing algorithm demonstrate in theory the feasibility of quantum control.

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PubMed2000

Polariton-biexciton transitions in a semiconductor microcavity.

The influence of four-particle correlations on the nonlinear optics of a semiconductor microcavity is determined by a pump-and-probe investigation. Experiments are performed on a nonmonolithic microcavity which contains a ZnSe quantum well. In this system the biexciton binding energy exceeds both the normal-mode splitting between exciton and cavity mode and all damping constants. Oscillatory spectral features below the excitonic resonance are observed in the response for counterpolarized beams. Comparison with model calculations shows that in this case the coherent nonlinearity is dominated by biexciton-exciton interactions beyond the Hartree-Fock approximation.

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