The vie wW cn oncluions cotIn i kl dosmnut we duo of do. authors endlAauld not be nepee as neoeuw... more The vie wW cn oncluions cotIn i kl dosmnut we duo of do. authors endlAauld not be nepee as neoeuwily representin dw offidde policies, sidue xpramd or knpOWe, of goe Defnm dveneed ResearchProjects Agncy or thetiod Status ownnt ITh docmenh r C) 1rn dW Mt Mf Odbr 00~u
Conference on Lasers and Electro-Optics/International Quantum Electronics Conference and Photonic Applications Systems Technologies, 2004
... effect [4] and laser cooling (which also enables the gate work at room temperature). Simulati... more ... effect [4] and laser cooling (which also enables the gate work at room temperature). Simulations based on the S-matrix theory show that a nonlinear phase-shift of a tenth of πcan be obtained with realistic parameters. This work was supported in part by NSF, ARDA/ARO, and ...
Quantum computation requires a continuous supply of rapidly initialized qubits for quantum error ... more Quantum computation requires a continuous supply of rapidly initialized qubits for quantum error correction. Here, we demonstrate fast spin state initialization with near unity efficiency in a singly charged quantum dot by optically cooling an electron spin. The electron spin is successfully cooled from 5 to 0.06 K at a magnetic field of 0.88 T applied in Voigt geometry. The spin cooling rate is of order 10 9 s ÿ1 , which is set by the spontaneous decay rate of the excited state.
We review the investigation of the hole-assisted dynamical nuclear spin polarization mechanism in... more We review the investigation of the hole-assisted dynamical nuclear spin polarization mechanism in a singly charged InAs quantum dot. Using coherent dark state spectroscopy, we measure the locking of the Overhauser field to a value determined only by the laser frequencies. Importantly, we review data that the locking effect can suppress nuclear spin fluctuations. We determine the onset time of the nuclear spin narrowing effect and its persistence absent laser interactions by directly measuring the enhancement of the electron spin coherence. This nuclear field locking effect can be explained in terms of an anisotropic hyperfine coupling between the hole spin and the nuclear spins.
... first order in strain (Whitfield 1961). These deformation parameters iBAp(n, k) are important... more ... first order in strain (Whitfield 1961). These deformation parameters iBAp(n, k) are important in the electron-phonon interaction of semiconductors, and have been estimated from the transport properties (Bardeen and Shockley 1950, Herring and Vogt 1956, 1957, Aubrey, Gubler ...
The public reporting burden for this collection of information is estimated to average 1 hour per... more The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggesstions for reducing this burden, to Washington
Abstract Our studies of the determination of the transition temperature from the non-phonon mecha... more Abstract Our studies of the determination of the transition temperature from the non-phonon mechanisms is reviewed. Raising T c by replacing phonons with high frequency excitations does appear possible though the usual estimates are found to be too high.
Proceedings of the 17th International Conference on the Physics of Semiconductors, 1985
We report an investigation of the polarization of optical emission that probes the character of t... more We report an investigation of the polarization of optical emission that probes the character of the highest valence subband ho of GaAs-(AlGa)As quantum well heterostructures. The results reveal that for small in plane wavevectors k, a surprisingly large (~30%) admixture of the \( \left| {\frac{3}{2} \pm \frac{1}{2} > } \right. \) angular momentum representation is evident in ho. Further, at higher k ≈ 106cm−1 the character changes continously and tends to a completely mixed \( \left| {\frac{3}{2} \pm \frac{3}{2} > } \right.,\left| {\frac{3}{2}} \right. \pm \frac{1}{2} > \) state. Attempts to interpret these results within the effective mass approximation and coupling with Luttinger Hamiltonian yield large quantitative disagreement, especially near k=0. Several other mechanisms that could improve the theoretical understanding are found to be inadequate.
After reviewing the physical systems that may host exciton condensation, this chapter illustrates... more After reviewing the physical systems that may host exciton condensation, this chapter illustrates some recent proposals concerning the detection of coherent exciton flow. It focuses on the exciton analogues of two phenomena—Andreev reflection and the Josephson effect—which are hallmarks of superconducting behavior and stress the crucial differences between excitons and Cooper pairs. It shows that the excitonic insulator is the perfect insulator in terms of both charge and heat transport, with an unusually high resistance at the interface with a semimetal—the normal phase of the condensed state. Such behavior may be explained in terms of the coherence induced into the semimetal by the proximity of the exciton condensate. The exciton superflow may be directly probed in the case that excitons are optically pumped in a double-layer semiconductor heterostructure. The chapter proposes a correlated photon counting experiment for coupled electrostatic exciton traps.
Dynamics of entanglement of a two state system with a bath of a large but finite number of intera... more Dynamics of entanglement of a two state system with a bath of a large but finite number of interacting particles is used to show how the two state coherence is lost and can be recovered.
Submitted for the MAR11 Meeting of The American Physical Society Quantum noise of an electromagne... more Submitted for the MAR11 Meeting of The American Physical Society Quantum noise of an electromagnetically controlled two level system CHING-KIT CHAN, L.J. SHAM, Department of Physics, University of California San Diego-A coherent control of a spin is limited by both the decoherence due to coupling with the environment and noise coming from the quantized control. A quantum noise study is particularly important in fault tolerant quantum computation where a very high fidelity is demanded. Here, we present a time evolution study of a two level system interacting with a laser pulse and the electromagnetic vacuum based on the multimode Jaynes-Cummings model. We develop a diagrammatic formalism in which one can easily identify the coherent Rabi oscillation of the TLS and its relaxation from corresponding diagrams. In the small time limit (t ≪ T 1), where the noise level is small but still an issue to fault tolerant quantum computing, this method gives a quantitative evaluation of the quantum noise of the TLS under an optical control with an arbitrary pulse shape. Furthermore, this approach can be naturally extended from the Markovian to the non-Markovian regime, resulting in dynamics different from that obtained in the optical Bloch analysis. All these calculations are done without any stochastic assumption.
In a study of the ferromagnetic phase of a multilayer digital ferromagnetic semiconductor in the ... more 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.
Numerical reevaluation of the formulas yield corrections to Fig. 3 and the lower panel of Fig. 4,... more Numerical reevaluation of the formulas yield corrections to Fig. 3 and the lower panel of Fig. 4, as shown here. In Fig.
We attempt to build a Hexible and accurate theoretical model for the electronic properties of sel... more We attempt to build a Hexible and accurate theoretical model for the electronic properties of selectively doped semiconductor heterostructures based on a two-band k p effective-mass-approximation Hamiltonian that includes nonparabolicity, stress, piezoelectric, Rnite-temperature, many-body, and DX-center effects. We present quantitative self-consistent results for a variety of b-modulationdoped semiconductor heterostructures with the aim of optimizing the electronic density in the active region as a function of configuration, including [001] and [111] interfaces for device applications. The presence of DX centers leads to the prediction of saturation of the carrier density with a characteristic capacitance discontinuity as the b-doping concentration is increased. Calculated differential capacitance C-V curves indicate that spatial charge-density inhomogeneities, but not subband depopulation, lead to sharp steps in the capacitance as the gate voltage is increased.
Coherent control of a quantum system is limited both by the decoherence due to environment and th... more Coherent control of a quantum system is limited both by the decoherence due to environment and the quantum nature of the control agent. The high fidelity of control demanded by fault tolerant quantum computation and the intrinsic interest in nonclassical effects from the interplay between control and dissipation are motivations for a detailed study of the interaction dynamics between the quantum system and the macroscopic environment and control agent. We present a detailed time evolution study of a two-level system interacting with a laser pulse and the electromagnetic vacuum in the multimode Jaynes-Cummings model. A diagrammatic formalism allows easy identification of coherent dynamics and relaxation of the two-level system. We demonstrate a computational method of dynamics with precise error bounds for fast operations versus slow decoherence, spanning the Markovian and non-Markovian regimes. Comparison against an exact model solution of our results with existing approximations of the master equation shows the lack of accuracy in the latter.
A survey of the current theoretical understanding of the spin relaxation processes of electrons a... more A survey of the current theoretical understanding of the spin relaxation processes of electrons and holes in confined systems of quasi-two dimensions is given. The theoretical framework is used to form a bridge between the electronic properties and the recent time-resolved polarization measurements. Spin relaxation mechanisms surveyed include single-particle spin flips via spin-orbit interaction and electron-hole-pair spin flips via exchange interaction.
The vie wW cn oncluions cotIn i kl dosmnut we duo of do. authors endlAauld not be nepee as neoeuw... more The vie wW cn oncluions cotIn i kl dosmnut we duo of do. authors endlAauld not be nepee as neoeuwily representin dw offidde policies, sidue xpramd or knpOWe, of goe Defnm dveneed ResearchProjects Agncy or thetiod Status ownnt ITh docmenh r C) 1rn dW Mt Mf Odbr 00~u
Conference on Lasers and Electro-Optics/International Quantum Electronics Conference and Photonic Applications Systems Technologies, 2004
... effect [4] and laser cooling (which also enables the gate work at room temperature). Simulati... more ... effect [4] and laser cooling (which also enables the gate work at room temperature). Simulations based on the S-matrix theory show that a nonlinear phase-shift of a tenth of πcan be obtained with realistic parameters. This work was supported in part by NSF, ARDA/ARO, and ...
Quantum computation requires a continuous supply of rapidly initialized qubits for quantum error ... more Quantum computation requires a continuous supply of rapidly initialized qubits for quantum error correction. Here, we demonstrate fast spin state initialization with near unity efficiency in a singly charged quantum dot by optically cooling an electron spin. The electron spin is successfully cooled from 5 to 0.06 K at a magnetic field of 0.88 T applied in Voigt geometry. The spin cooling rate is of order 10 9 s ÿ1 , which is set by the spontaneous decay rate of the excited state.
We review the investigation of the hole-assisted dynamical nuclear spin polarization mechanism in... more We review the investigation of the hole-assisted dynamical nuclear spin polarization mechanism in a singly charged InAs quantum dot. Using coherent dark state spectroscopy, we measure the locking of the Overhauser field to a value determined only by the laser frequencies. Importantly, we review data that the locking effect can suppress nuclear spin fluctuations. We determine the onset time of the nuclear spin narrowing effect and its persistence absent laser interactions by directly measuring the enhancement of the electron spin coherence. This nuclear field locking effect can be explained in terms of an anisotropic hyperfine coupling between the hole spin and the nuclear spins.
... first order in strain (Whitfield 1961). These deformation parameters iBAp(n, k) are important... more ... first order in strain (Whitfield 1961). These deformation parameters iBAp(n, k) are important in the electron-phonon interaction of semiconductors, and have been estimated from the transport properties (Bardeen and Shockley 1950, Herring and Vogt 1956, 1957, Aubrey, Gubler ...
The public reporting burden for this collection of information is estimated to average 1 hour per... more The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggesstions for reducing this burden, to Washington
Abstract Our studies of the determination of the transition temperature from the non-phonon mecha... more Abstract Our studies of the determination of the transition temperature from the non-phonon mechanisms is reviewed. Raising T c by replacing phonons with high frequency excitations does appear possible though the usual estimates are found to be too high.
Proceedings of the 17th International Conference on the Physics of Semiconductors, 1985
We report an investigation of the polarization of optical emission that probes the character of t... more We report an investigation of the polarization of optical emission that probes the character of the highest valence subband ho of GaAs-(AlGa)As quantum well heterostructures. The results reveal that for small in plane wavevectors k, a surprisingly large (~30%) admixture of the \( \left| {\frac{3}{2} \pm \frac{1}{2} > } \right. \) angular momentum representation is evident in ho. Further, at higher k ≈ 106cm−1 the character changes continously and tends to a completely mixed \( \left| {\frac{3}{2} \pm \frac{3}{2} > } \right.,\left| {\frac{3}{2}} \right. \pm \frac{1}{2} > \) state. Attempts to interpret these results within the effective mass approximation and coupling with Luttinger Hamiltonian yield large quantitative disagreement, especially near k=0. Several other mechanisms that could improve the theoretical understanding are found to be inadequate.
After reviewing the physical systems that may host exciton condensation, this chapter illustrates... more After reviewing the physical systems that may host exciton condensation, this chapter illustrates some recent proposals concerning the detection of coherent exciton flow. It focuses on the exciton analogues of two phenomena—Andreev reflection and the Josephson effect—which are hallmarks of superconducting behavior and stress the crucial differences between excitons and Cooper pairs. It shows that the excitonic insulator is the perfect insulator in terms of both charge and heat transport, with an unusually high resistance at the interface with a semimetal—the normal phase of the condensed state. Such behavior may be explained in terms of the coherence induced into the semimetal by the proximity of the exciton condensate. The exciton superflow may be directly probed in the case that excitons are optically pumped in a double-layer semiconductor heterostructure. The chapter proposes a correlated photon counting experiment for coupled electrostatic exciton traps.
Dynamics of entanglement of a two state system with a bath of a large but finite number of intera... more Dynamics of entanglement of a two state system with a bath of a large but finite number of interacting particles is used to show how the two state coherence is lost and can be recovered.
Submitted for the MAR11 Meeting of The American Physical Society Quantum noise of an electromagne... more Submitted for the MAR11 Meeting of The American Physical Society Quantum noise of an electromagnetically controlled two level system CHING-KIT CHAN, L.J. SHAM, Department of Physics, University of California San Diego-A coherent control of a spin is limited by both the decoherence due to coupling with the environment and noise coming from the quantized control. A quantum noise study is particularly important in fault tolerant quantum computation where a very high fidelity is demanded. Here, we present a time evolution study of a two level system interacting with a laser pulse and the electromagnetic vacuum based on the multimode Jaynes-Cummings model. We develop a diagrammatic formalism in which one can easily identify the coherent Rabi oscillation of the TLS and its relaxation from corresponding diagrams. In the small time limit (t ≪ T 1), where the noise level is small but still an issue to fault tolerant quantum computing, this method gives a quantitative evaluation of the quantum noise of the TLS under an optical control with an arbitrary pulse shape. Furthermore, this approach can be naturally extended from the Markovian to the non-Markovian regime, resulting in dynamics different from that obtained in the optical Bloch analysis. All these calculations are done without any stochastic assumption.
In a study of the ferromagnetic phase of a multilayer digital ferromagnetic semiconductor in the ... more 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.
Numerical reevaluation of the formulas yield corrections to Fig. 3 and the lower panel of Fig. 4,... more Numerical reevaluation of the formulas yield corrections to Fig. 3 and the lower panel of Fig. 4, as shown here. In Fig.
We attempt to build a Hexible and accurate theoretical model for the electronic properties of sel... more We attempt to build a Hexible and accurate theoretical model for the electronic properties of selectively doped semiconductor heterostructures based on a two-band k p effective-mass-approximation Hamiltonian that includes nonparabolicity, stress, piezoelectric, Rnite-temperature, many-body, and DX-center effects. We present quantitative self-consistent results for a variety of b-modulationdoped semiconductor heterostructures with the aim of optimizing the electronic density in the active region as a function of configuration, including [001] and [111] interfaces for device applications. The presence of DX centers leads to the prediction of saturation of the carrier density with a characteristic capacitance discontinuity as the b-doping concentration is increased. Calculated differential capacitance C-V curves indicate that spatial charge-density inhomogeneities, but not subband depopulation, lead to sharp steps in the capacitance as the gate voltage is increased.
Coherent control of a quantum system is limited both by the decoherence due to environment and th... more Coherent control of a quantum system is limited both by the decoherence due to environment and the quantum nature of the control agent. The high fidelity of control demanded by fault tolerant quantum computation and the intrinsic interest in nonclassical effects from the interplay between control and dissipation are motivations for a detailed study of the interaction dynamics between the quantum system and the macroscopic environment and control agent. We present a detailed time evolution study of a two-level system interacting with a laser pulse and the electromagnetic vacuum in the multimode Jaynes-Cummings model. A diagrammatic formalism allows easy identification of coherent dynamics and relaxation of the two-level system. We demonstrate a computational method of dynamics with precise error bounds for fast operations versus slow decoherence, spanning the Markovian and non-Markovian regimes. Comparison against an exact model solution of our results with existing approximations of the master equation shows the lack of accuracy in the latter.
A survey of the current theoretical understanding of the spin relaxation processes of electrons a... more A survey of the current theoretical understanding of the spin relaxation processes of electrons and holes in confined systems of quasi-two dimensions is given. The theoretical framework is used to form a bridge between the electronic properties and the recent time-resolved polarization measurements. Spin relaxation mechanisms surveyed include single-particle spin flips via spin-orbit interaction and electron-hole-pair spin flips via exchange interaction.
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Papers by L. Sham