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Search Results: 1 - 10 of 78202 matches for " Pochung Chen "
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Continuous Dynamical Decoupling with Bounded Controls
Pochung Chen
Physics , 2005, DOI: 10.1103/PhysRevA.73.022343
Abstract: We develop a theory of continuous decoupling with bounded controls from a geometric perspective. Continuous decoupling with bounded controls can accomplish the same decoupling effect as the bang-bang control while using realistic control resources and it is robust against systematic implementation errors. We show that the decoupling condition within this framework is equivalent to average out error vectors whose trajectories are determined by the control Hamiltonian. The decoupling pulses can be intuitively designed once the structure function of the corresponding SU(n) is known and is represented from the geometric perspective. Several examples are given to illustrate the basic idea. From the physical implementation point of view we argue that the efficiency of the decoupling is determined not by the order of the decoupling group but by the minimal time required to finish a decoupling cycle.
Dynamical decoupling induced renormalization of the non-Markovian dynamics
Pochung Chen
Physics , 2006, DOI: 10.1103/PhysRevA.75.062301
Abstract: In this work we develop a numerical framework to investigate the renormalization of the non-Markovian dynamics of an open quantum system to which dynamical decoupling is applied. We utilize a non-Markovian master equation which is derived from the non-Markovian quantum trajectories formalism. It contains incoherent Markovian dynamics and coherent Schr\"odinger dynamics as its limiting cases and is capable of capture the transition between them. We have performed comprehensive simulations for the cases in which the system is either driven by the Ornstein-Uhlenbeck noise or or is described by the spin-boson model. The renormalized dynamics under bang-bang control and continuous dynamical decoupling are simulated. Our results indicate that the renormalization of the non-Markovian dynamics depends crucially on the spectral density of the environment and the envelop of the decoupling pulses. The framework developed in this work hence provides an unified approach to investigate the efficiency of realistic decoupling pulses. This work also opens a way to further optimize the decoupling via pulse shaping.
Quantum shape effects on Zeeman splittings in semiconductor nanostructures
Pochung Chen
Physics , 2005, DOI: 10.1103/PhysRevB.72.045335
Abstract: We develop a general method to calculate Zeeman splittings of electrons and holes in semiconductor nanostructures within the tight-binding framework. The calculation is carried out in the electron-hole picture and is extensible to the excitonic calculation by including the electron-hole Coulomb interaction. The method is suitable for the investigation of quantum shape effects and the anisotropy of the g-factors. Numerical results for CdSe and CdTe nanostructures are presented.
Theory of Coherent Optical Control of Exciton Spin Dynamics in a Semiconductor Quantum Dot
Pochung Chen,C. Piermarocchi,L. J. Sham
Physics , 2000, DOI: 10.1016/S1386-9477(01)00043-1
Abstract: We use the spin-polarized excitons in a single quantum dot to design optical controls for basic operations in quantum computing. We examine the ultrafast nonlinear optical processes required and use the coherent nonlinear optical responses to deduce if such processes are physically reasonable. The importance and construction of an entangled state of polarized exciton states in a single quantum dot is explained. We put our proposal in perspective with respect to a number of theoretical suggestions of utilizing the semiconductor quantum dots.
Quantum phase transitions of two-species bosons in square lattice
Pochung Chen,Min-Fong Yang
Physics , 2010, DOI: 10.1103/PhysRevB.82.180510
Abstract: We investigate various quantum phase transitions of attractive two-species bosons in a square lattice. Using the algorithm based on the tensor product states, the phase boundaries of the pair superfluid states with nonzero pair condensate density \emph{and} vanishing atomic condensate density are determined. Various quantum phase transitions across the phase boundaries are characterized. Our work thus provides guides to the experimental search of the pair superfluid phase in lattice boson systems.
Control of Spin Dynamics of Excitons in Nanodots for Quantum Operations
Pochung Chen,C. Piermarocchi,L. J. Sham
Physics , 2001, DOI: 10.1103/PhysRevLett.87.067401
Abstract: 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.
Field Induced Spin Supersolidity in Frustrated Spin-1/2 Spin-Dimer Models
Pochung Chen,Chen-Yen Lai,Min-Fong Yang
Physics , 2009, DOI: 10.1103/PhysRevB.81.020409
Abstract: By means of the recently developed algorithm based on the tensor product states, the magnetization process of frustrated spin-1/2 spin-dimer models on a square lattice is investigated. Various field-induced quantum phases are discovered. In particular, clear evidence of a supersolid phase over a finite regime of magnetic field is observed. Critical fields at various field-induced transitions are determined accurately. Our work not only implies that the employed numerical technique can be an efficient algorithm even for the frustrated spin systems, but also shed light on the search of the supersolid phase in real frustrated spin-dimer compounds.
Numerical Study of Spin-1/2 XXZ Model on Square Lattice from Tensor Product States
Pochung Chen,Chen-Yen Lai,Min-Fong Yang
Statistics , 2009, DOI: 10.1088/1742-5468/2009/10/P10001
Abstract: By means of the recently proposed algorithm based on the tensor product states, the magnetization process of the spin-1/2 anti-ferromagnetic XXZ model on a square lattice is investigated. In the large spin-anisotropy limit, clear evidence of a first-order spin-flip transition is observed as an external magnetic field is increased. Our findings of the critical field and the discrete jumps in various local order parameters are in good agreement with the quantum Monte Carlo data in the literature. Our results imply that this algorithm can be an accurate and efficient numerical approach in studying first-order quantum phase transitions in two dimensions.
Universal dynamics of quantum spin decoherence in a spin bath
Yi-Ya Tian,Pochung Chen,Daw-Wei Wang
Physics , 2007, DOI: 10.1103/PhysRevB.77.174434
Abstract: We systematically investigate the universal spin decoherence dynamics of a localized electron in an arbitrary nuclear spin bath, which can be even far away from equilibrium due to the weak nuclear-lattice interaction. We show that the electron spin relaxation dynamics (as well as spin pure dephasing and Hahn echo decay) can {\it always} have a universal behavior as long as the initial state is composed of a sufficiently large amount of spin eigenstates. For a given system, the pattern of the universal dynamics depends on the complicated initial condition only via a {\it single} parameter, which measures the amount of phase coherence between different spin eigenstates in the initial state. Our results apply even when the number of the involved nuclei is not large, and therefore provide a solid foundation in the comparison of theoretical/numerical results with the experimental measurement. As an example, we also show numerical results for systems of noninteracting spin bath in zero magnetic field regime, and discuss the features of universal decoherent dynamics.
Quantum and Thermal Transitions Out of the Pair-Supersolid Phase of Two-Species Bosons in Lattice
Chia-Min Chung,Shiang Fang,Pochung Chen
Physics , 2012, DOI: 10.1103/PhysRevB.85.214513
Abstract: We investigate two-species bosons in a two-dimensional square lattice by quantum Monte Carlo method. We show that the inter-species attraction and nearest-neighbor intra-species repulsion results in the pair-supersolid phase, where a diagonal solid order coexists with an off-diagonal pair-superfluid order. The quantum and thermal transitions out of the pair-supersolid phase are characterized. It is found that there is a direct first order transition from the pair-supersolid phase to the double-superfluid phase without an intermediate region. Furthermore, the melting of the pair-supersolid occurs in two steps. Upon heating, first the pair-superfluid is destroyed via a KT transition then the solid order melts via an Ising transition
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