Qing-Hu Chen

Nanyang Technological University, Tumasik, Singapore

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Publications (69)131.8 Total impact

  • Source
    Yu-Yu Zhang, Qing-Hu Chen
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    ABSTRACT: The generalized rotating-wave approximation (GRWA) is presented for the two-qubit and cavity coipling system . The analytical expressions in the zeroth order approximation recover the previous adiabatic ones. The counterrotating-wave terms can be eliminated by performing the first order corrections. An effective solvable Hamiltonian with the same form as the ordinary RWA one are then obtained, giving a significantly accurate eigenvalues and eigenstates. Energy levels in the present GRWA are in accordant with the numerical exact diagonalization ones in the a wide range of coupling strength. The atomic population inversion in the GRWA is in quantitative agreement with the numerical results for different detunings in the ultrastrong coupling regime.
    10/2014;
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    Shu He, Liwei Duan, Qing-Hu Chen
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    ABSTRACT: The spin-boson model is analytically studied using displaced Fock states (DFS) without discretization of the continuum bath. In the orthogonal displaced Fock basis, the ground-state wavefunction can be systematically improved in a controllable way. Interestingly, the zeroth-order DFS reproduces exactly the well known Silbey-Harris results. In the framework of the second-order DFS, the magnetization and the entanglement entropy are exactly calculated. It is found that the magnetic critical exponent $\beta$ is converged to $0.5$ in the whole sub-Ohmic bath regime $0<s<1$, compared with that by the exactly solvable generalized Silbey-Harris ansatz. It is strongly suggested that the system with sub-Ohmic bath is always above its upper critical dimension, in sharp contrast with the previous findings. This is the first evidence of the violation of the quantum-classical Mapping for $% 1/2<s<1$.
    10/2014;
  • Hui Wang, Shu He, Liwei Duan, Yang Zhao, Qing-Hu Chen
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    ABSTRACT: Using extended coherent states, an analytically exact study has been carried out for the quantum Rabi model with two equivalent qubits. Compact transcendental functions of one variable have been derived leading to exact solutions. The energy spectrum is clearly identified and analyzed. Also obtained analytically are the necessary and sufficient conditions for the occurrence of isolated exceptional solutions, which are not doubly degenerate as in the one-qubit quantum Rabi model.
    EPL (Europhysics Letters) 05/2014; 106(5):54001. · 2.26 Impact Factor
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    Qing-Hu Chen, Liwei Duan, Shu He
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    ABSTRACT: Using extended coherent states, an analytically exact study has been carried out for the quantum Rabi model with two different qubits. We have derived two G functions, both are $2 \times 2$ determinants, much too simpler than that with $8 \times 8$ determinant existing in the recent literature. Zeros of each G function yield the whole regular spectrum. Exceptional solutions in one G function are regular in other G function, which provides a simple and convenient way to obtain the necessary and sufficient condition for the occurrence of the exceptional eigenvalue. For the case of the same couplings, G functions can be reduced to a formalism without any determinant. Previous exceptional solution for $E=m$ (m is an integer) for the same coupling case is actually neither an exceptional solution, nor singularity in our G function in the present scheme. Previous special Dark states with a special condition for two qubit frequencies, independent of the coupling, can be detected clearly in a continued-fraction technique.
    05/2014;
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    Qing-Hu Chen, Shu He, Liwei Duan
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    ABSTRACT: Using extended coherent states, an analytically exact study has been carried out for arbitrary finite-size Dicke model. For $N=2k$ or $2k-1$ Dicke model (% $k$ is an integer), analytical exact solutions can be obtained from the derived G function, which is only a $k \times k$ determinant. Zeros of each G function yield not only the complete regular spectrum but also very few pseudo-solutions in practical calculations. The regular spectrum is completely given by common zeros of all G functions. Exceptional solutions in one G function are regular in other G function, which provides a simple and convenient way to obtain the necessary and sufficient condition for the occurrence of the exceptional eigenvalue. The present analytical exact solutions are just counterparts to the numerical exact solutions by Chen et al [Phys. Rev. A 78 051801(2008)] and Liu et al [Phys. Rev. A 80, 165308(2009)], but may be of more academic and practical value.
    04/2014;
  • Qing-Hu Chen, Liwei Duan, Shu He
    [Show abstract] [Hide abstract]
    ABSTRACT: Using extended coherent states, an analytically exact study has been carried out for the quantum Rabi model with two different qubits. We have derived two G functions, both are $2 \times 2$ determinants, much too simpler than that with $8 \times 8$ determinant existing in the recent literature. Zeros of each G function yield the whole regular spectrum. Exceptional solutions in one G function are regular in other G function, which provides a simple and convenient way to obtain the necessary and sufficient condition for the occurrence of the exceptional eigenvalue. For the case of the same couplings, G functions can be reduced to a formalism without any determinant. Previous exceptional solution for $E=m$ (m is an integer) for the same coupling case is actually neither an exceptional solution, nor singularity in our G function in the present scheme. Previous special Dark states with a special condition for two qubit frequencies, independent of the coupling, can be detected clearly in a continued-fraction technique.
    04/2014;
  • Qing-Hu Chen, Shu He, Liwei Duan
    [Show abstract] [Hide abstract]
    ABSTRACT: Using extended coherent states, an analytically exact study has been carried out for arbitrary finite-size Dicke model. For $N=2k$ or $2k-1$ Dicke model (% $k$ is an integer), analytical exact solutions can be obtained from the derived G function, which is only a $k \times k$ determinant. Zeros of each G function yield not only the complete regular spectrum but also very few pseudo-solutions in practical calculations. The regular spectrum is completely given by common zeros of all G functions. Exceptional solutions in one G function are regular in other G function, which provides a simple and convenient way to obtain the necessary and sufficient condition for the occurrence of the exceptional eigenvalue. The present analytical exact solutions are just counterparts to the numerical exact solutions by Chen et al [Phys. Rev. A 78 051801(2008)] and Liu et al [Phys. Rev. A 80, 165308(2009)], but may be of more academic and practical value.
    03/2014;
  • Source
    Hui Wang, Shu He, Liwei Duan, Qing-Hu Chen
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    ABSTRACT: An analytical exact study for the quantum Rabi models with two identical qubits is performed by extended coherent states. Concise transcendental functions with only one variable responsible for the exact solutions have been derived. The structure of spectrum is clearly identified and analyzed. The necessary and sufficient condition for the occurrence of the isolated exceptional solutions is analytically obtained. These exceptional solutions are essentially different from the Juddian solutions with doubly degenerate eigenvalues in the one-qubit QRM.
    01/2014;
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    Jian-Ping Lv, Qing-Hu Chen, Youjin Deng
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    ABSTRACT: Using worm-type quantum Monte Carlo simulations, we investigate bosonic mixtures on the triangular lattice of two species of bosons, which interact via nearest-neighbour intraspecies ($V$) and onsite interspecies ($U$) repulsions. For the case of symmetric hopping amplitude ($t_A/V=t_B/V$) and $U/V=1$, we determine a rich ground-state phase diagram that contains double solid, double superfluid (2SF), supersolid (SS), solid-superfluid (Solid-SF) and counterflow supersolid (CSS) states. The SS, Solid-SF and CSS states exhibit spontaneous symmetry breaking among the three sublattices of the triangular lattice and between the two species, which leads to nonzero crystalline density wave order in each species. We furthermore show that the CSS and the SS states are present for $t_A/V \neq t_B/V$, and the latter even survives up to $t_A/V \rightarrow \infty$ or $t_B/V \rightarrow \infty$ limit. The effects induced by the variation of $U/V$ and by the imbalance of particle numbers of the two species are also explored.
    11/2013; 89(1).
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    Liwei Duan, Hui Wang, Qing-Hu Chen, Yang Zhao
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    ABSTRACT: Developed originally for the Holstein polaron, the Davydov D1 ansatz is an efficient, yet extremely accurate trial state for time-dependent variation of the spin-boson model [N. Wu, L. Duan, X. Li, and Y. Zhao, J. Chem. Phys. 138, 084111 (2013)]. In this work, the Dirac-Frenkel time-dependent variational procedure utilizing the Davydov D1 ansatz is implemented to study entanglement dynamics of two qubits under the influence of two independent baths. The Ohmic spectral density is used without the Born-Markov approximation or the rotating-wave approximation. In the strong coupling regime finite-time disentanglement is always found to exist, while at the intermediate coupling regime, the entanglement dynamics calculated by Davydov D1 ansatz displays oscillatory behavior in addition to entanglement disappearance and revival.
    The Journal of Chemical Physics 07/2013; 139(4):044115. · 3.12 Impact Factor
  • Fei Qi, Huan Liu, Qing-Hu Chen
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    ABSTRACT: We have performed large-scale dynamical simulations on the current-driven three-dimensional frustrated anisotropic XY model with sparse weak columnar defects. Below the matching field, a moving Bose glass phase with superconducting coherence but without spatial order is observed at low temperatures. For very small columnar defect concentrations, a moving Bragg glass phase with both superconducting coherence and hexagonal Bragg peaks can be formed. Both the moving Bose glass and Bragg glass phases dynamically melt into a moving smectic via a first-order phase transition. It is suggested that the proliferation of dislocations mediates the dynamical melting in both cases.
    Physics of Condensed Matter 07/2013; 86(7). · 1.28 Impact Factor
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    Chen Wang, Qing-Hu Chen
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    ABSTRACT: The counter rotating-wave term (CRT) effects from the system-bath coherence on the dynamics of quantum correlation of two qubits in two independent baths and a common bath are systematically investigated. The hierarchy approach is extended to solve the relevant spin boson models with the Lorentzian spectrum, the exact dynamics for the quantum entanglement and quantum discord (QD) are evaluated, and the comparisons with previous ones under the rotating-wave approximation are performed. For the two independent baths, beyond the weak system-bath coupling, the CRT essentially changes the evolution of both entanglement and QD. With the increase of the coupling, the revival of the entanglement is suppressed dramatically and finally disappears, and the QD becomes smaller monotonically. For the common bath, the entanglement is also suppressed by the CRT, but the QD shows quite different behaviors, if initiated from the correlated Bell states. In the non-Markovian regime, the QD is almost not influenced by the CRT and generally finite in the long time evolution at any coupling, while in the Markovian regime, it is significantly enhanced with the strong coupling.
    New Journal of Physics 03/2013; · 4.06 Impact Factor
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    Yu-Yu Zhang, Qing-Hu Chen, Yang Zhao
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    ABSTRACT: The generalized rotating-wave approximation with counter-rotating interactions has been applied to a biased qubit-oscillator system. Analytical expressions are explicitly given for all eigenvalues and eigenstates. For a flux qubit coupled to superconducting oscillators, spectra calculated by our approach are in excellent agreement with experiment. Calculated energy levels for a variety of biases also agree well with those obtained via exact diagonalization for a wide range of coupling strengths. Dynamics of the qubit has also been examined, and results lend further support to the validity of the analytical approximation employed here. Our approach can be readily implemented and applied to superconducting qubit-oscillator experiments conducted currently and in the near future with a biased qubit and for all accessible coupling strengths.
    Physical Review A 10/2012; 87(3). · 3.04 Impact Factor
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    ABSTRACT: The Jaynes-Cummings model without the rotating-wave approximation can be solved exactly by an extended Swain ansatz with conserved parity. Analytical approximations are then performed at different levels. The well-known rotating-wave approximation (RWA) is naturally covered in the present zeroth- and first-order approximations. A first-order correction to the RWA can be obtained in a second-order approximation, by which the effect of the counter-rotating-wave term emerges clearly. Concise analytical expressions are given explicitly and can be applicable up to the ultrastrong-coupling regime. A preliminary application to vacuum Rabi splitting is shown to be very successful.
    Physical Review A 09/2012; 86(3). · 3.04 Impact Factor
  • Qing-Hu Chen, Chen Wang, Shu He, Tao Liu, Ke-Lin Wang
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    ABSTRACT: Within extended coherent states, a recent exact solution to the quantum Rabi model (QRM) [ Phys. Rev. Lett. 107 100401 (2011)] can be recovered in an alternative simpler and more physical way, without use of any extra conditions. In the same framework, the two-photon QRM is solved exactly by treating extended squeezed states on an equal footing. Concise transcendental functions responsible for the exact solutions are derived. The isolated Juddian solutions are also analytically obtained in terms of degeneracy. Both the extended coherent states and squeezed states employed here are essentially Fock states in the space of the corresponding Bogoliubov operators, which result in free-particle number operators. The present approach can be summarized concisely in a unified way and easily extended to various spin-boson systems with multiple levels, even multiple modes.
    Physical Review A 08/2012; 86(2). · 3.04 Impact Factor
  • Tao Liu, Yu-Yu Zhang, Qing-Hu Chen, Ke-Lin Wang
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    ABSTRACT: Although the oscillator strength sum rule forbids the phase transition in ideal non-interacting two-level atoms systems, we present the possibility of the quantum phase transition in the coupled two-level atoms in a cavity. The system undergoes the superradiant phase transition in the thermodynamics limit and this transition is account for the atom–atom attractive interaction, exhibiting a violation of the sum rule. The bosonic coherent state technique has been adopted to locate the quantum critical point accurately in the finite-size system. We predict the existence of the superradiant phase transition as the number of atoms increases, satisfying all the constraints imposed by the sum rule.
    Physics Letters A 05/2012; 376(s 26–27):1962–1965. · 1.63 Impact Factor
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    Qing-Hu Chen, Chen Wang, Shu He, Tao Liu, Ke-Lin Wang
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    ABSTRACT: The quantum Rabi model can be solved exactly by the Bargmann transformation from real coordinate to complex variable recently [Phys. Rev. Lett. \textbf{107}, 100401 (2011)]. By the extended coherent states, we recover this solution in an alternative simpler and perhaps more physical way without uses of any extra conditions, like Bargmann conditions. In the same framework, the two-photon Rabi model are solved exactly by extended squeeze states. Transcendental functions have been derived with the similar form as those in one-photon model. Both extended coherent states and squeeze states are essentially Fock states in the space of the corresponding Bogoliubov operators. The present approach could be easily extended to study the exact solvability or integrability of various spin-boson systems with multi-level, even multi-mode.
    04/2012;
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    ABSTRACT: An effective scheme within two displaced bosonic operators with equal positive and negative displacements is extended to study qubit-oscillator systems analytically in an unified way. Many previous analytical treatments, such as generalized rotating-wave approximation (GRWA) [Phys. Rev. Lett. 99, 173601 (2007)] and an expansion in the qubit tunneling matrix element in the deep strong coupling regime [Phys. Rev. Lett. 105, 263603 (2010)] can be recovered straightforwardly in the present scheme. Moreover, further improving GRWA and extension to the finite-bias case are implemented easily. The analytical expressions are then derived explicitly and uniquely, which work well in a wide range of the coupling strengthes, detunings, and static bias including the recent experimentally accessible parameters.
    Chinese Physics B 04/2012; 22(6). · 1.15 Impact Factor
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    ABSTRACT: On the basis of resistively-shunted junction dynamics, we study vortex dynamics in two-dimensional Josephson junction arrays with asymmetrically single and bimodulated periodic pinning potential for the full range of vortex density f. The ratchet effect occurring at a certain range of temperature, current, and f, is observed in our simulation. We explain the microscopic behavior behind this effect by analyzing the vortex distribution and interaction. The reversal of the ratchet effect can be observed at several f values for a small driven current. This effect is stronger when the asymmetric potential is simultaneously introduced in two directions.
    Physics Letters A 03/2012; 376(16):1371–1374. · 1.63 Impact Factor
  • Source
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    ABSTRACT: The Jaynes-Cummings model without the rotating-wave approximation can be solved exactly by extended Swain's ansatz with the conserved parity. The analytical approximations are then performed at different levels. The well-known rotating-wave approximation is naturally covered in the present zero and first approximations. The effect of the counter rotating-wave term emerges clearly in the second order approximation. The concise analytical expressions are given explicitly and can be applicable up to the ultra-strong coupling regime. The preliminary application to the vacuum Rabi splitting is shown to be very successful.
    03/2012;

Publication Stats

294 Citations
131.80 Total Impact Points

Institutions

  • 2014
    • Nanyang Technological University
      • Division of Materials Science
      Tumasik, Singapore
  • 2008–2014
    • Zhejiang Normal University
      Jinhua, Zhejiang Sheng, China
  • 2001–2014
    • Zhejiang University
      • Department of Physics
      Hang-hsien, Zhejiang Sheng, China
  • 2011–2012
    • China University of Mining Technology
      Suchow, Jiangsu Sheng, China
  • 2009
    • Southwest University of Science and Technology
      Mien-yang-hsien, Sichuan, China
  • 2003–2007
    • National Institute for Materials Science
      • ___Computational Materials Science Center
      Tsukuba, Ibaraki-ken, Japan