arXiv subjects

Shou Zhang

Publications and source records attributed to Shou Zhang.

At least 37 records · Page 2Linked to original sources

arXiv2018

Simulating Z_2 topological insulators via a one-dimensional cavity optomechanical cells array

We propose a novel scheme to simulate Z_2 topological insulators via one-dimensional (1D) cavity optomechanical cells array. The direct mapping between 1D cavity optomechanical cells array and 2D quantum spin Hall (QSH) system can be achieved by using diagonalization and dimensional reduction methods. We show that the topological features of the present model can be captured using a 1D generalized Harper equation with an additional SU(2) guage structure. Interestingly, spin pumping of effective photon-phonon bosons can be naturally derived after scanning the additional periodic parameter, which means that we can realize the transition between different QSH edge states.

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arXiv2018

Qubit-assisted squeezing of the mirror motion in a dissipative optomechanical cavity system

We investigate a hybrid system consisting of an atomic ensemble trapped inside a dissipative optomechanical cavity assisted with the perturbative oscillator-qubit coupling. It is shown that such a hybrid system is very suitable for generating stationary squeezing of the mirror motion in the long-time limit under the unresolved sideband regime. Based on the approaches of master equation and covariance matrix, we discuss the respective squeezing effects in detail and find that in both approaches, simplifying the system dynamics with adiabatic elimination of the highly dissipative cavity mode is very effective. In the approach of master equation, we find that the squeezing is a resulting effect of cooling process and is robust against the thermal fluctuations of the mechanical mode. While in the covariance matrix approach, we can obtain the analytical result of the steady-state mechanical position variance from the reduced dynamical equation approximately. Finally, we compare the two different approaches and find that they are completely equivalent for the stationary dynamics. The scheme may be meaningful for the possible ultraprecise quantum measurement involved mechanical squeezing.

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arXiv2018

Spontaneous PT symmetry breaking in non-Hermitian coupled cavities array

We study the effects of the position of the passive and active cavities on the spontaneous parity-time (PT) symmetry breaking behavior in non-Hermitian coupled cavities array model. We analyze and discuss the energy eigenvalue spectrums and PT symmetry in the topologically trivial and nontrivial regimes under three different cases in detail, i.e., the passive and active cavities are located at, respectively, the two end positions, the second and penultimate positions, and each position in coupled cavities array. The odevity of the number of cavities is further considered to check the effects of the non-Hermitian terms applied on the PT symmetric and asymmetric systems. We find that the position of the passive and active cavities has remarkable impacts on the spontaneous PT symmetry breaking behavior, and in each case the system exhibits distinguishable and novel spontaneous PT symmetry breaking characteristic, respectively. The effects of the non-Hermitian terms on the $\mathcal{PT}$ symmetric and asymmetric systems due to the odevity are comparatively different in the first case while qualitatively same in the second case.

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arXiv2018

Optomechanical cooling beyond the quantum backaction limit with frequency modulation

In the usual optomechanical cooling, even if the system has no thermal component, it still has a quantum limit-known as the quantum backaction limit (QBL)-on the minimum phonon number related to shot noise. By studying the side-band cooling regime in optomechanical system (OMS), we find that the cooling can be improved significantly when the frequency modulation (FM) that can suppress the Stokes heating processes is introduced into the system. We analyze and demonstrate the reasons of the phonon number below the QBL redefined in the whole stable region of the standard OMSs. The above analyses are further checked by numerically solving the differential equations of second order moments derived from the quantum master equation with broad system parameters, ranging from weak coupling (WC) to ultra-strong coupling (USC) and resolved side-band (RSB) to unresolved side-band (USB) regimes. Comparing with the cases of those without FM, the stable ground-state cooling can also be achieved even in the conventional unstable region.

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arXiv2016

Fast adiabatic quantum state transfer and entanglement generation between two atoms via dressed states

Recently, a new method, which can significantly speed up adiabatic quantum state transfer by using dressed states, was proposed by Baksic \emph{et~al.} [Phys. Rev. Lett. \textbf{116}, 230503 (2016)]. Assisted by quantum Zeno dynamics, we develop this dressed-state method to achieve shortcuts to complete and fractional stimulated Raman adiabatic passage for speeding up adiabatic two-atom quantum state transfer and maximum entanglement generation, respectively. By means of some numerical simulations, we determine the parameters used in the scheme which can guarantee the feasibility and efficiency both in theory and experiment. Besides, we give strict numerical simulations to discuss the scheme's robustness, and the results show the scheme is robust against the variations in the parameters, atomic spontaneous emissions and the photon leakages from the cavity.

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arXiv2016

Dressed-state scheme for a fast CNOT gate

A highly feasible dressed-state scheme, which greatly speeds up the adiabatic population transfer of a quantum system, is applied for implementing a fast CNOT gate in a cavity QED system. The cavity QED system consists of two identical five-level atoms which are respectively trapped in two optical cavities connected by a fiber. With the help of quantum Zeno dynamics, the CNOT gate is constructed by three steps with six pulses. Because the adiabatic condition is not necessary to be considered and all of the pulses are smoothly turned on and off, the scheme is fast and feasible in experiment. Numerical simulations indicate that the average fidelity for constructing the CNOT gate is quite high and the gate operation time is relatively short. Moreover, the effects on the fidelity of the atomic spontaneous emission and the photon leakages from the cavities are discussed by the master equation and the result shows the scheme is robust against the decoherence.

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arXiv2016

Modulation of entanglement between two oscillators separated in space with an optical parametric amplifier

We propose a scheme to modulate the entanglement between two oscillators separated in space via the squeezing cavity field generated by the optical parametric amplifier instead of injecting the squeezing field directly with the assistance of Coulomb interaction. We show that the Coulomb interaction between the oscillators is the essential reason for the existence of entanglement. Due to the gain of the optical parametric amplifier and the phase of the pump driving the optical parametric amplifier can simultaneously modulate the squeezing cavity field, the radiation pressure interaction between the cavity field and the oscillator is modulated accordingly. We find that there is competing effect between the radiation pressure interaction and the Coulomb interaction for the oscillator which these two interactions act on simultaneously. Therefore, the modulation of entanglement can be achieved with the assistance of Coulomb interaction. The results of numerical simulation show that the present scheme has stronger robustness against the temperature of environment compared with previous schemes in experimentally feasible regimes.

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arXiv2016

Fast generations of tree-type three-dimensional entanglement via Lewis-Riesenfeld invariants and transitionless quantum driving

Recently, a novel three-dimensional entangled state called tree-type entanglement, which is likely to have applications for improving quantum communication security, was prepared via adiabatic passage by Song et al. [Phys. Rev. A 93, 062321 (2016)]. Here we propose two schemes for fast generations of tree-type three-dimensional entanglement among three spatially separated atoms via shortcuts to adiabatic passage. With the help of quantum Zeno dynamics, two kinds of different but equivalent methods, Lewis-Riesenfeld invariants and transitionless quantum driving, are applied to construct shortcuts to adiabatic passage. The comparisons between the two methods are discussed. The strict numerical simulations show that the tree-type three-dimensional entangled states can be fast prepared with quite high fidelities and the two schemes are both robust against the variations in the parameters, atomic spontaneous emissions and the cavity-fiber photon leakages.

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arXiv2016

Fast generation of three-dimensional entanglement between two spatially separated atoms via invariant-based shortcut

A scheme is proposed for the fast generation of three-dimensional entanglement between two atoms trapped in two cavities connected by a fiber via invariant-based shortcut to adiabatic passage. With the help of quantum Zeno dynamics, the technique of invariant-based shortcut to adiabatic passage is applied for the generation of two-atom three-dimensional entanglement. The numerical simulation results show that, within a short time, the scheme has a high fidelity and is robust against the decoherence caused by the atomic spontaneous emission, photon leakage, and the variations in the parameters selected. Moreover, the scheme may be possible to be implemented with the current experimental technology.

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arXiv2016

Steady-state mechanical squeezing in a double-cavity optomechanical system

We study the physical properties of double-cavity optomechanical system in which the mechanical resonator interacts with one of the coupled cavities and another cavity is used as an auxiliary cavity. The model can be expected to achieve the strong optomechanical coupling strength and overcome the optomechanical cavity decay, simultaneously. Through the coherent auxiliary cavity interferences, the steady-state squeezing of mechanical resonator can be generated in highly unresolved sideband regime. The validity of the scheme is assessed by numerical simulation and theoretical analysis of the steady-state variance of the mechanical displacement quadrature. The scheme provides a platform for the mechanical squeezing beyond the resolved sideband limit and addresses the restricted experimental bounds at present.

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arXiv2016

Robust entanglement between a movable mirror and atomic ensemble and entanglement transfer in coupled optomechanical system

We propose a scheme for the creation of robust entanglement between a movable mirror and atomic ensemble at the macroscopic level in coupled optomechanical system. In experimentally accessible parameter regimes, we show that critical temperature of the bipartite continuous variable entanglement in our scheme can be raised from previous 24 K [Vitali {\it et al.}, Phys. Rev. Lett. \textbf{98}, 030405 (2007)] and 20 K [Genes {\it et al.}, Phys. Rev. A \textbf{77}, 050307(R) (2008)] to 32 K. We also investigate the entanglement transfer based on this coupled system. The scheme can be used for the realization of quantum memories for continuous variable quantum information processing and quantum-limited displacement measurements.

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arXiv2016

Direct conversion of a three-atom W state to a Greenberger-Horne-Zeilinger state in spatially separated cavities

State conversion between Greenberger-Horne-Zeilinger (GHZ) state and W state is an open challenging problem because they cannot be converted to each other only by local operations and classical communication. Here we propose a cavity quantum electrodynamics method based on interference of polarized photons emitted by the atoms trapped in spatially separated optical cavities that can convert a three-atom W state to a GHZ state. We calculate the success probability and fidelity of the converted GHZ state when the cavity decay, atomic spontaneous decay, and photon leakage of the cavities are taken into account for a practical system, which shows that the proposed scheme is feasible and within the reach of current experimental technology.

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arXiv2016

Multi-qubit non-adiabatic holonomic controlled quantum gates in decoherence-free subspaces

Non-adiabatic holonomic quantum gate in decoherence-free subspaces is of greatly practical importance due to its built-in fault tolerance, coherence stabilization virtues, and short run-time. Here we propose some compact schemes to implement two- and three-qubit controlled unitary quantum gates and Fredkin gate. For the controlled unitary quantum gates, the unitary operator acting on the target qubit is an arbitrary single-qubit gate operation. The controlled quantum gates can be directly implemented using non-adiabatic holonomy in decoherence-free subspaces and the required resource for the decoherence-free subspace encoding is minimal by using only two neighboring physical qubits undergoing collective dephasing to encode a logical qubit.

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arXiv2015

Steady-state mechanical squeezing in a hybrid atom-optomechanical system with a highly dissipative cavity

Quantum squeezing of mechanical resonator is important for studying the macroscopic quantum effects and the precision metrology of weak forces. Here we give a theoretical study of a hybrid atom-optomechanical system in which the steady-state squeezing of the mechanical resonator can be generated via the mechanical nonlinearity and cavity cooling process. The validity of the scheme is assessed by simulating the steady-state variance of the mechanical displacement quadrature numerically. The scheme is robust against dissipation of the optical cavity, and the steady-state squeezing can be effectively generated in a highly dissipative cavity.

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arXiv2015

Generation of three-dimensional entanglement between two spatially separated atoms via shortcuts to adiabatic passage

We propose a scheme for generating three-dimensional entanglement between two atoms trapped in two spatially separated cavities reapectively via shortcuts to adi- abatic passage based on the approach of Lewis-Riesenfeld invariants in cavity quan- tum electronic dynamics. By combining Lewis-Riesenfeld invariants with quantum Zeno dynamics, we can generate three dimensional entanglement of the two atoms with high fidelity. The Numerical simulation results show that the scheme is ro- bust against the decoherences caused by the photon leakage and atomic spontaneous emission.

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arXiv2015

Preparation of entanglement between atoms in spatially separated cavities via fiber loss

We propose a scheme to prepare a maximally entangled state for two Lambda-type atoms trapped in separate optical cavities coupled through an optical fiber based on the combined effect of the unitary dynamics and the dissipative process. Our work shows that the fiber loss, as well as the atomic spontaneous emission and the cavity decay, is no longer undesirable, but requisite to prepare the distributed entanglement, which is meaningful for the long distance quantum information processing tasks. Originating from an arbitrary state, the desired state could be prepared without precise time control. The robustness of the scheme is numerically demonstrated by considering various parameters.

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arXiv2015

Deterministic SWAP gate using shortcuts to adiabatic passage

We theoretically propose an alternative method to realize a deterministic SWAP gate using shortcuts to adiabatic passage based on the approach of Lewis- Riesenfeld invariants in cavity quantum electronic dynamics (QED). By combining Lewis-Riesenfeld invariants with quantum Zeno dynamics, the SWAP gate can be achieved deterministically. The strict numerical results show that our scheme is a fast and robust approach to achieve SWAP gate.

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arXiv2015

Simplified scheme for entanglement preparation with Rydberg pumping via dissipation

Inspired by recent work [A. W. Carr and M. Saffman, Phys. Rev. Lett. 111, 033607 (2013)], we propose a simplified scheme to prepare the two-atom maximally entangled states via dissipative Rydberg pumping. Compared with the former scheme, the simplified one involves less classical laser fields and Rydberg interactions, and the asymmetric Rydberg interactions are avoided. Master equation simulations demonstrate that the fidelity and the Clauser-Horne-Shimony-Holt correlation of the maximally entangled state could reach up to 0.999 and 2.821, respectively, under certain conditions. Furthermore, we extend the physical thoughts to prepare the three-dimensional entangled state, the numerical simulations show that, in theory, both the fidelity and the Negativity of the desired entanglement could be very close to unity under certain conditions.

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