Liu Ji-Bing

Optical bistability via quantum interference in a four-level atomic medium

We theoretically investigate optical bistability (OB) behaviour in a four-level atomic system confined in a unidirectional ring cavity. It is shown that OB can be controlled by adjusting the intensity of the coupling field and the value of the cooperativity parameter. More importantly, our numerical results show that the threshold intensity of OB can be reduced without decreasing the range of the hysteresis loop via adjusting the effect of spontaneously generated coherence (SGC) or the relative phase between two applied fields. The influence of the frequency detuning of the coupling field on the OB behaviour is also discussed.

Controllable Optical Bistability and Multistability in a Four-Level Atomic System with Closed-Loop Configuration

We theoretically investigate optical bistability (OB) and multistability (OM) behaviour of a closed-loop configuration atomic system driven by a degenerate coupling field and a degenerate probe field inside a unidirectional ring cavity. It is found that the OB and OM behaviour can be controlled by adjusting the intensity and the frequency detuning of the coupling field, respectively. Interestingly, our numerical results show that it is easy to realize the transition from OB to OM or vice versa by adjusting the intensity of the coupling field under a appropriate frequency detuning. The effect of the atomic cooperation parameter on the OB behaviour is also discussed.

Sudden Death, Birth and Stable Entanglement in a Two-Qubit Heisenberg XY Spin Chain

1College of Physics and Electronic Science, Hubei Normal University, Huangshi 435002, China 2Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, China 3State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan, 430071, China (Dated: August 31, 2008) Abstract Taking the decoherence effect due to population relaxation int account, we investigate the entanglement properties for two qubits in the Heisenberg XY interaction a nd subject to an external magnetic field. It is found that the phenomenon of entanglement sudden death (E SD) as well as sudden birth(ESB) appear during the evolution process for particular initial states . The influence of the external magnetic field and the spin environment on ESD and ESB are addressed in detail. I t is shown that the concurrence, a measure of entanglement, can be controlled by tuning the parameters of the spin chain, such as the anisotropic parameter, external magnetic field, and the coupling streng th with their environment. In particular, we find that a critical anisotropy constant exists, above which ESB vanishes while ESD appears. It is also notable that stable entanglement, which is independent of differen t initial states of the qubits, occurs even in the presence of decoherence.Taking the decoherence effect due to population relaxation into account, we investigate the entanglement properties for two qubits in the Heisenberg XY interaction and subject to an external magnetic field. It is found that the phenomenon of entanglement sudden death ( ESD) as well as sudden birth ( ESB) appear during the evolution process for particular initial states. The influence of the external magnetic field and the spin environment on ESD and ESB are addressed in detail. It is shown that the concurrence, a measure of entanglement, can be controlled by tuning the parameters of the spin chain, such as the anisotropic parameter, external magnetic field, and the coupling strength with their environment. In particular, we find that a critical anisotropy constant exists, above which ESB vanishes while ESD appears. It is also notable that stable entanglement, which is independent of different initial states of the qubits, occurs even in the presence of decoherence.

Master equation describing the diffusion process for a coherent state

The evolution of a pure coherent state into a chaotic state is described very well by a master equation, as is validated via an examination of the coherent states evolution during the diffusion process, fully utilizing the technique of integration within an ordered product (IWOP) of operators. The same equation also describes a limitation that maintains the coherence in a weak diffusion process, i.e., when the dissipation is very weak and the initial average photon number is large. This equation is dρ/ dt = −κ[a+aρ −a+ρa − aρa+ + ρaa+]. The physical difference between this diffusion equation and the better-known amplitude damping master equation is pointed out.

N −Qubit W State of Spatially Separated Atoms via Fractional Adiabatic Passage

An alternative scheme is presented to realize an N-qubit W state with distant atoms trapped in spatially separated optical cavities coupled by optical fibers via a fractional adiabatic passage. The present scheme is tolerant to device parameter nonuniformity and does not need to accurately control the experimental time by employing the fractional stimulated Raman adiabatic passage. In addition, the excited states of atoms are adiabatically eliminated and no additional qubit is required. Cavity decay, fiber losses and atomic spontaneous emission can all be greatly suppressed in the present scheme.

Entanglement evolution in an anisotropic two-qubit Heisenberg XYZ model with Dzyaloshinskii–Moriya interaction

This paper investigates the entanglement evolution of a two-qubit anisotropic Heisenberg XYZ chain in the presence of Dzyaloshinskii–Moriya interaction. The time evolution of the concurrence is studied for the initial pure entangled states cos θ |00 + sin θ |11 and cos |01 + sin |10 at zero temperature. The influences of Dzyaloshinskii–Moriya interaction D, anisotropic parameter Δ and environment coupling strength γ on entanglement evolution are analysed in detail. It is found that the effect of noisy environment obviously suppresses the entanglement evolution, and the Dzyaloshinskii–Moriya interaction D acts on the time evolution of entanglement only when the initial state is cos |01 + sin |10. Finally, a formula of steady state concurrence is obtained, and it is shown that the stable concurrence, which is independent of different initial states and Dzyaloshinskii–Moriya interaction D, depends on the anisotropic parameter Δ and the environment coupling strength γ.

Ultraslow Optical Solitons in a Coupled Double Quantum-Well Nanostructure

We demonstrate the formation of ultraslow dark optical solitons with a four-level scheme in an asymmetric semiconductor double quantum well (SDQW) structure based on intersubband transitions by using only a low-intensity pulsed laser radiation. With appropriate conditions we show numerically that the dark optical soliton can travel with a ultraslow group velocity Vg/c ~ ? 10?3. Such a semiconductor system is much more practical than its atomic counterpart because of its flexible design and the controllable interference strength. This nonlinear optical process in the SDQW solid-state material may be used for the control technology of optical delay lines and optical buffers.

Entanglement Dynamics of Two-Qubit System in Different Types of Noisy Channels

In this paper, we study entanglement dynamics of a two-qubit extended Werner-like state locally interacting with independent noisy channels, i.e., amplitude damping, phase damping, and depolarizing channels. We show that the purity of initial entangled state has direct impacts on the entanglement robustness in each noisy channel. That is, if the initial entangled state is prepared in mixed instead of pure form, the state may exhibit entanglement sudden death (ESD) and/or be decreased for the critical probability at which the entanglement disappear.

The effect of intersubsystem coupling on the geometric phase in a composite system driven by decohering quantized fields

We investigate the behaviour of the geometric phase of a bipartite system with intersubsystem coupling via a quantum jump approach, where one of the subsystems is driven by a one- or two-mode quantized field which is also subjected to decoherence. The results show that the first correction due to the decohering field is only quadratic in decaying rate λ when the evolution of the system is adiabatic, and the lowest order correction is related to the first order in λ when the evolution of the system is non-adiabatic. Moreover, an interesting interpretation of this phenomenon is given.

Decoherence Effect in An Anisotropic Two-Qubit Heisenberg XYZ Model with Inhomogeneous Magnetic Field

Taking the decoherence effect into account, the entanglement evolution of a two-qubit anisotropic Heisenberg XYZ chain in the presence of inhomogeneous magnetic field is investigated. The time evolution of concurrence is studied for the initial state cos θ |01 +sin θ|10 at zero temperature. The influences of inhomogeneous magnetic field, anisotropic parameter and decoherence on entanglement dynamic are addressed in detail, and a concurrence formula of the steady state is found. It is shown that the entanglement sudden death (ESD) and entanglement sudden birth (ESB) appear with the decoherence effect, and the stable concurrence depends on the uniform magnetic field B, anisotropic parameter Δ and environment coupling strength γ, which is independent of different initial states and nonuniform magnetic field b.