Yu-Yu Zhang

Chongqing University, Ch’ung-ch’ing-shih, Chongqing Shi, China

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Publications (15)24.18 Total impact

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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: 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. · 1.77 Impact Factor
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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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    Chen Wang, Yu-Yu Zhang, Qing-Hu Chen
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    ABSTRACT: Quantum and classical pairwise correlations in two typical collective spin systems (i.e., the Dicke model and the Lipkin-Meshkov-Glick model) are discussed. These correlations in the thermodynamical limit are obtained analytically and in a finite-size system are calculated numerically. Large-size scaling behavior for the quantum discord itself is observed, which has never been reported in another critical system. A logarithmic diverging behavior for the first derivative of the quantum discord is also found in both models, which might be universal in the second-order quantum phase transition. It is suggested that the pronounced maximum or minimum of first derivative of quantum discord signifies the critical point. Comparisons between the quantum discord and the scaled concurrence are performed. It is shown that the quantum discord is very small in one phase and robust in the other phase, while the scaled concurrence shows maximum at the critical point and decays rapidly when away from the the critical point.
    Physical Review A 02/2012; 85(5). · 3.04 Impact Factor
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    Yu-Yu Zhang, Qing-Hu Chen, Shi-Yao Zhu
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    ABSTRACT: he effect of the counter-rotating terms in the Jaynes-Cummings model is investigated with an extended coherent-state approach. The counter-rotating terms greatly modify the vacuum Rabi splitting. Two peaks with different heights in the weak coupling regime and more than two peaks in the intermediate coupling regime are predicted. The collapses and revivals in the evolution of the atomic population inversion disappear in the intermediate coupling regime, but reappear in the strong coupling regime. This reappearance is similar to that under the rotating-wave approximation, attributed to the summation of periodic cosine functions of the evolution.
    06/2011;
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    ABSTRACT: By using extended bosonic coherent states, the solution to the Jaynes-Cummings model without the rotating-wave approximation can be mapped to that of a polynomial equation with a single variable. The solutions to this polynomial equation can give all eigenvalues and eigenfunctions of this model with all values of the coupling strength and the detuning exactly, which can be readily applied to recent circuit quantum electrodynamic systems operating in the ultra-strong coupling regime.
    EPL (Europhysics Letters) 11/2010; 89(4). · 2.26 Impact Factor
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    ABSTRACT: The dipole-coupled two-level atoms(qubits) in a single-mode resonant cavity is studied by extended bosonic coherent states. The numerically exact solution is presented. For finite systems, the first-order quantum phase transitions occur at the strong interatomic interaction. Similar to the original Dicke model, this system exhibits a second-order quantum phase transition from the normal to the superradiant phases. Finite-size scaling for several observables, such as the average fidelity susceptibility, the order parameter, and concurrence are performed for different interatomic interactions. The obtained scaling exponents suggest that interatomic interactions do not change the universality class. Comment: 13 pages, 5 figures
    Physical Review A 08/2010; · 3.04 Impact Factor
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    ABSTRACT: The instability in the Dicke model with a rotating-wave approximation for finite N atoms is investigated in terms of the Berry phase and the fidelity. It can be identify by the discontinuity of these quantities as a function of the atom-field coupling parameter. Involving an additional field interaction, it is observed that the instability is not eliminated even for strong interaction of the bosonic fields, contrarily to the previous results limited to the minimal coupling regions.
    Optics Communications 01/2010; 283(18):3459-3463. · 1.44 Impact Factor
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    Yu-Yu Zhang, Qing-Hu Chen, Ke-Lin Wang
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    ABSTRACT: We propose a general extended coherent state approach to the qubit (or fermion) and multi-mode boson coupling systems. The application to the spin-boson model with the discretization of a bosonic bath with arbitrary continuous spectral density is described in detail, and very accurate solutions can be obtained. The quantum phase transition in the nontrivial sub-Ohmic case can be located by the fidelity and the order-parameter critical exponents for the bath exponents $s<1/2$ can be correctly given by the fidelity susceptibility, demonstrating the strength of the approach. Comment: 4 pages, 3 figures
    Physical review. B, Condensed matter 12/2009;
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    ABSTRACT: We study the two-site Hubbard-Holstein model by using an extended phonon coherent state. For the nontrivial singlet bipolarons, the double occupancy probability, the fidelity and the entanglement entropy are calculated to characterize the ground-state properties in both two-site and single-site bipolaron-dominated regimes. We use the localized minimum of the fidelity to define a crossover and plot the bipolaron phase diagram, which separates the large and small entanglement region. Furthermore, the relation between the bipolaron entanglement and the correlation functions demonstrates that the large entanglement corresponds to the large magnitude of lattice deformations induced by electrons.
    Journal of Physics Condensed Matter 10/2009; 21(41):415601. · 2.36 Impact Factor
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    ABSTRACT: The instability, so-called the quantum-phase-like transition, in the Dicke model with a rotating-wave approximation for finite $N$ atoms is investigated in terms of the Berry phase and the fidelity. It can be marked by the discontinuous behavior of these quantities as a function of the atom-field coupling parameter. Involving an additional field $A^{2}$ term, it is observed that the instability is not eliminated beyond the characteristic atom-field coupling parameter even for strong interaction of the bosonic fields, contrarily to the previous studies. Comment: 6 pages, 6 figures,
    01/2009;
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    ABSTRACT: Within the numerically exact solution to the Dicke model proposed previously, we study the quantum criticality in terms of the ground-state (GS) energy, fidelity, and the order parameter. The finite size scaling analysis for the average fidelity susceptibility (FS) and second derivative of GS energy are performed. The correlation length exponent is obtained to be nu=2/3 , which is the same as that in Lipkin-Meshkov-Glick model obtained previously, suggesting the same universality. It is observed that average FS and second derivative of GS energy show similar critical behavior, demonstrating the intrinsic relation in the Dicke model. The scaling behavior for the order parameter and the singular part of the GS energy at the critical point are also analyzed and the obtained exponents are consistent with the previous scaling hypothesis in /N expansion scheme.
    Physical Review A 01/2009; 80(2). · 3.04 Impact Factor
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    ABSTRACT: We revisit the two-site Hubbard-Holstein model by using extended phonon coherent states. The nontrivial singlet bipolaron is studied exactly in the whole coupling regime. The ground-state (GS) energy and the double occupancy probability are calculated. The linear entropy is exploited successfully to quantify bipartite entanglement between electrons and their environment phonons, displaying a maximum entanglement of the singlet-bipolaron in strong coupling regime. A dramatic drop in the crossover regime is observed in the GS fidelity and its susceptibility. The bipolaron properties is also characterized classically by correlation functions. It is found that the crossover from a two-site to single-site bipolaron is more abrupt and shifts to a larger electron-phonon coupling strength as electron-electron Coulomb repulsion increases. Comment: 6 pages, 6 figures
    09/2008;
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    Ke-Wei Sun, Yu-Yu Zhang, Qing-Hu Chen
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    ABSTRACT: Quantum phase transition in the one-dimensional period-two and uniform quantum compass model are studied by using the pseudo-spin transformation method and the trace map method. The exact solutions are presented, the fidelity, the nearest-neighbor pseudo-spin entanglement, spin and pseudo-spin correlation functions are then calculated. At the critical point, the fidelity and its susceptibility change substantially, the gap of pseudo-spin concurrence is observed, which scales as $1/N$ (N is system size). The spin correlation functions show smooth behavior around the critical point. In the period-two chain, the pseudo-spin correlation functions exhibit a oscillating behavior, which is absent in the unform chain. The divergent correlation length at the critical point is demonstrated in the general trend for both cases. Comment: 5 pages, 6 figures
    Physical review. B, Condensed matter 09/2008;
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    ABSTRACT: By using extended bosonic coherent states, a technique to solve the Dicke model exactly is proposed in the numerical sense. The accessible system size is two orders of magnitude higher than that reported in the literature. Finite-size scaling for several observables, such as the ground-state energy, Berry phase, and concurrence, is analyzed. A scaling exponent for ground-state energy is found. An existing discrepancy in the scaling exponent of the concurrence is reconciled.
    Physical Review A 01/2008; 78(5). · 3.04 Impact Factor

Publication Stats

35 Citations
24.18 Total Impact Points

Institutions

  • 2012
    • Chongqing University
      Ch’ung-ch’ing-shih, Chongqing Shi, China
  • 2008–2012
    • Zhejiang University
      • Department of Physics
      Hang-hsien, Zhejiang Sheng, China
  • 2009
    • Zhejiang Normal University
      Jinhua, Zhejiang Sheng, China