Publications (84)160.94 Total impact
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ABSTRACT: We present analytical solutions to three qubits and a singlemode cavity coupling system beyond the rotatingwave approximation (RWA). The zeroth order approximation gives correct solutions when the qubits are far detuned from the cavity. The first order approximation, called generalized rotatingwave approximation (GRWA), produces an effective solvable Hamiltonian with the same form as the ordinary RWA one and exhibits substantial improvements of energy levels over the RWA even on resonance. Based on these analytical eigensolutions, we study both the bipartite entanglement and genuine multipartite entanglement (GME). The dynamics of the concurrence and the GME using the GRWA are in consistent with the numerical ones. Interestingly, the well known sudden death of entanglement occurs in the bipartite entanglement dynamics but not in GME dynamics.  [Show abstract] [Hide abstract]
ABSTRACT: The twomode quantum Rabi model with bilinear coupling is studied using extended squeezed states. We derive $G$functions for each Bargmann index $q$% . They share a common structure with the $G$function of the onephoton and twophoton quantum Rabi models. The regular spectrum is given by zeros of the $G$function while the conditions for the presence of doubly degenerate (exceptional) eigenvalues are obtained in closed form through the lifting property. The simple singularity structure of the $G$function allows to draw conclusions about the distribution of eigenvalues along the real axis and to understand the spectral collapse phenomenon when the coupling reaches a critical value.  [Show abstract] [Hide abstract]
ABSTRACT: In this paper, the finite size Dicke model of arbitrary number of qubits is solved analytically in an unified way within extended coherent states. For the $N=2k$ or $2k1$ Dicke models ($k$ is an integer), the $G$function, which is only an energy dependent $k \times k$ determinant, is derived in a transparent manner. The regular spectrum is completely and uniquely given by stable zeros of the $G$function. The closedform exceptional eigenvalues are also derived. The level distribution controlled by the pole structure of the $G$functions suggests nonintegrability for $N>1$ model at any finite coupling in the sense of recent criterion in literature. A preliminary application to the exact dynamics of genuine multipartite entanglement in the finite $N$ Dicke model is presented using the obtained exact solutions.  [Show abstract] [Hide abstract]
ABSTRACT: Using extended coherent states, an analytical exact study has been carried out for the quantum Rabi model (QRM) with two arbitrary qubits in a very concise way. The $G$functions with $2 \times 2$ determinants are generally derived. For the same coupling constants, the simplest $G$function, resembling that in the onequbit QRM, can be obtained. Zeros of the $G$function yield the whole regular spectrum. The exceptional eigenvalues, which do not belong to the zeros of the $G$ function, are obtained in the closed form. The Dark states in the case of the same coupling can be detected clearly in a continuedfraction technique. The present concise solution is conceptually clear and practically feasible to the general twoqubit QRM and therefore has many applications.  [Show abstract] [Hide abstract]
ABSTRACT: In this work, the anisotropic quantum Rabi model with different coupling strengths of the rotatingwave and counterrotating wave terms is studied by using two kinds of extended coherent states (ECS). By the first kind of ECS, we can derive a socalled $G$function, by which both the regular and exceptional solutions can be given. The exceptional solution are just corresponding to the crossing points of two energy levels with different parities, so is doubly degenerate. By the second kind of ECS, a general scheme for the eigensolutions is derived analytically in a unified way. The zeroorder approximation is just the adiabatic approximation, and the firstorder approximation is actually a generalized rotatingwave approximation. The algebraic formulae for the eigensolutions are given explicitly in two approximations. The generalized rotatingwave approximations work well in a wide range of two different coupling strengths and the qubit detunings.  [Show abstract] [Hide abstract]
ABSTRACT: In this work, an analytical exact study has been carried out for the twomode quantum Rabi model (QRM) using extended squeezed states. The concise Gfunctions are derived for each Bargmann index q. It shares a common structure with those in the onepohton [Phys. Rev. Lett. 107 , 100401(2011)] and twophoton [Phys. Rev. A 86, 023822(2012)] QRMs. Zeros of the Gfunction yield the whole regular spectrum. The exceptional eigenvalues, which do not belong to the zeros of the G function, are obtained in the closedform. The necessary and sufficient condition for their occurrence is given in a transparent manner. The present solution is derived in a simple physical way, and is therefore conceptional clear and practically feasible to the treatments of many physics processes.  [Show abstract] [Hide abstract]
ABSTRACT: A variational approach based on the multicoherentstate ansatz with asymmetric parameters is employed to study the ground state of the spinboson model. Without any artificial approximations except for the finite number of the coherent states, we find the robust Gaussian critical behavior in the whole subOhmic bath regime. The converged critical coupling strength can be estimated with the $1/N$ scaling, where $N $ is the number of the coherent states. It is strongly demonstrated the breakdown of the wellknown quantumtoclassical mapping for $1/2<s<1$. In addition, the entanglement entropy displays more steep jump around the critical points for the Ohmic bath than the subOhmic bath.  [Show abstract] [Hide abstract]
ABSTRACT: We show analytically that the collapse and revival in the population dynamics of the atomcavity coupled system under the rotating wave approximation (RWA), valid only at very weak coupling, is an artifact as the atomcavity coupling is increased. Even the firstorder correction to the RWA is able to bring about the absence of the collapse in the dynamics of atomic population inversion thanks to intrinsic oscillations resulting from the transitions between two levels with the same atomic quantum number. The removal of the collapse is valid for a wide range of coupling strengths which are accessible to current experimental setups. In addition, based on our analytical results that greatly improve upon the conventional RWA, even the strongcoupling power spectrum can now be explained with the help of the numerically exact energy levels.  [Show abstract] [Hide abstract]
ABSTRACT: The generalized rotatingwave approximation (GRWA) is presented for the twoqubit and cavity coipling system . The analytical expressions in the zeroth order approximation recover the previous adiabatic ones. The counterrotatingwave 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.  [Show abstract] [Hide abstract]
ABSTRACT: The spinboson model is analytically studied using displaced Fock states (DFS) without discretization of the continuum bath. In the orthogonal displaced Fock basis, the groundstate wavefunction can be systematically improved in a controllable way. Interestingly, the zerothorder DFS reproduces exactly the well known SilbeyHarris results. In the framework of the secondorder 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 subOhmic bath regime $0<s<1$, compared with that by the exactly solvable generalized SilbeyHarris ansatz. It is strongly suggested that the system with subOhmic 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 quantumclassical Mapping for $% 1/2<s<1$.  [Show abstract] [Hide abstract]
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 is the necessary and sufficient conditions for the occurrence of isolated exceptional solutions, which are not doubly degenerate as in the onequbit quantum Rabi model.  [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 continuedfraction technique.  [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 continuedfraction technique. 
Article: Analytical exact solutions to the finitesize Dicke model: Extended coherent states approaches
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ABSTRACT: Using extended coherent states, an analytically exact study has been carried out for arbitrary finitesize Dicke model. For $N=2k$ or $2k1$ 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 pseudosolutions 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. 
Article: Analytical exact solutions to the finitesize Dicke model: Extended coherent states approaches
[Show abstract] [Hide abstract]
ABSTRACT: Using extended coherent states, an analytically exact study has been carried out for arbitrary finitesize Dicke model. For $N=2k$ or $2k1$ 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 pseudosolutions 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.  [Show abstract] [Hide abstract]
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 onequbit QRM.  [Show abstract] [Hide abstract]
ABSTRACT: Using wormtype quantum Monte Carlo simulations, we investigate bosonic mixtures on the triangular lattice of two species of bosons, which interact via nearestneighbour 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 groundstate phase diagram that contains double solid, double superfluid (2SF), supersolid (SS), solidsuperfluid (SolidSF) and counterflow supersolid (CSS) states. The SS, SolidSF 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.  [Show abstract] [Hide abstract]
ABSTRACT: Developed originally for the Holstein polaron, the Davydov D1 ansatz is an efficient, yet extremely accurate trial state for timedependent variation of the spinboson model [N. Wu, L. Duan, X. Li, and Y. Zhao, J. Chem. Phys. 138, 084111 (2013)]. In this work, the DiracFrenkel timedependent 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 BornMarkov approximation or the rotatingwave approximation. In the strong coupling regime finitetime 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.  [Show abstract] [Hide abstract]
ABSTRACT: We have performed largescale dynamical simulations on the currentdriven threedimensional 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 firstorder phase transition. It is suggested that the proliferation of dislocations mediates the dynamical melting in both cases.  [Show abstract] [Hide abstract]
ABSTRACT: The counter rotatingwave term (CRT) effects from the systembath 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 rotatingwave approximation are performed. For the two independent baths, beyond the weak systembath 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 nonMarkovian 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.
Publication Stats
612  Citations  
160.94  Total Impact Points  
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Institutions

20142015

Nanjing University
Nanching, Jiangsu Sheng, China


20082014

Zhejiang Normal University
 Department of Physics
Jinhua, Zhejiang Sheng, China


20012008

Zhejiang University
 Department of Physics
Hanghsien, Zhejiang Sheng, China


20032007

National Institute for Materials Science
 ___Computational Materials Science Center
Tsukuba, Ibarakiken, Japan
