Quantum Engtanglement And Quantum Steering In A Dissipative Optomechanical System

Quantum entanglement is an important resource in quantum information and quantum computing.It is a special kind of quantum correlation,which can be used to deal with many problems which can not be handled by classical computing.So many physicists have attracted a lot of interest in this research.However quantum Einstein-Podolsky-Rosen steering manifests a type of quantum correlations intermediate between entanglement and Bell nonlocality.The remarkable feature of quantum steering is its directionality.If Alice and Bob share entanglement state,the change of Bob can remotely steer Alice to a particular state,which makes the quantum steering widely used in quantum key distribution,quantum teleportation and so on.In this thesis we propose a scheme for realizing quantum entanglement and quantum steering in a dissipative optomechanical system,which compose of an atomic ensemble located inside a single-mode cavity with a movable mirror.The cavity mode is driven by a short laser pulse.We firstly study the influence of a fast damped auxiliary mode on the creation of bipartite entanglement in a dissipative three-mode optomechanical system.Then we show how the reduction of fluctuations of the modes and the creation of the entanglement depend on the type of the coupling between the auxiliary and the other modes.A detailed calculation of the inseparability parameter is presented for three different types of couplings possible in the system:(a)nonlinear parametric-type,(b)linear-type,and(c)linear-type coupling to one of the two modes and parametric-type coupling to the other mode.Our calculations reveal the necessity of having a parametric-type coupling in the system.We find,however,that the presence of the linear-type rather than the parametric-type coupling protects the modes from excess noise.In particular,we show that in(a)-(c)cases a strong linear-type coupling between some of the mode leads to a significant reduction of the fluctuations of the modes,and in the(b)and(c)cases the presence of a strong linear-type coupling between the auxiliary and the other modes results in an entanglement between the modes.Then we consider a dissipative three-mode system and show how a fast damping of one of the modes may work as a mechanism for producing bipartite steering correlations between other two modes.We focus on the possibility of achieving a steady-state bipartite quantum steering via fast dissipation of one of the modes.We specify our treatment to three cases.In the first,we assume the mirror as a fast damped "auxiliary" mode which directly couples to the cavity mode.In the second,we assume the atomic mode as a fast damped "auxiliary" mode which directly couples to the cavity mode.In the third,we take the cavity mode as a fast damped "auxiliary" mode which directly couples to both atomic and mirror modes.We adiabatically eliminate the fast damped mode and obtain modified equations of motion for the operators of the remaining two modes.The equations are then solved for the steady-state and the solutions are used to derive explicit analytical expressions for the variances of the quadrature components and correlations between the modes.We find that that the cavity and mirror modes,which are coupled by the parametric-type interaction,can be entangled but cannot exhibit quantum steering.When,in addition,the cavity mode is coupled to a fast damped atomic mode,steering correlations are created and the modes then exhibit one-way steering.When the cavity mode appears as a fast damped mode,the atomic and mirror mode may exhibit two-way steering.We demonstrate that the auxiliary mode creates an asymmetry between the variances of the quadrature components of the modes.We point out that the asymmetry is crucial for the generation of the steering correlations and quantum steering may occur only when the variance of the steering mode is larger that the variance of the steered mode.We also discuss the effect of the thermal excitation of the modes and show that the scheme is quite robust against the thermal excitation of the modes if the fluctuations of the steering mode are larger than the fluctuations of the steered mode.