Nonclassicality and Decoherence of Photon-Subtraction Squeezing-Enhanced Thermal State

We introduce a kind of non-Gaussian state—photon-subtracted squeezing-enhanced thermal state (PSSETS) characteristics by two-squeezing parameters (λ,r). Its normalization factor is a Legendre polynomial of two-squeezing parameters and average photon number of the thermal state. The nonclassicality is investigated by using the negativity of Wigner function (WF). It is shown that the single PSSETS always has negative values when . The decoherence effect on PSSETS is then included by analytically deriving the time evolution of WF. For the single PSSETS, the characteristic time is longer than that of photon-subtracted squeezing thermal state.     

Nonclassicality and Decoherence of Generalized Photon-Modulated Squeezed Thermal State

In this paper, we introduce the generalized photon-modulated squeezed thermal states (GPMSTS) obtained by repeatedly acting the combination of Bosonic creation and annihilation operation on the squeezed thermal state. We investigate the nonclassical properties of GPMSTS and its decoherence in a thermal environment according to the negative region of Wigner function. It is found that the normalization factor of GPMSTS is a Legendre polynomial. Especially, the decoherence effect is discussed by deriving the time evolution of the Wigner function.     

Nonclassicality and Decoherence of Photon-added Thermo-invariant Coherent State

By introducing a kind of new quantum state—Photon-added thermo invariant coherent state (PATCS), we discuss its nonclassicality in terms of the negativity of Wigner function (WF) after deriving its analytical expression. It is found that the Wigner function is related to Lagurre-Gaussian function. We then study the effect of decoherence (a thermal environment) on the PATCS according to its WF (also related to Lagurre-Gaussian function). It is shown that it is not possible for WF to present the negative region when the decay time $\kappa t>\frac{1}{2}\ln \frac{2\bar{n}+2}{2\bar{n}+1}$\kappa t>\frac{1}{2}\ln \frac{2\bar{n}+2}{2\bar{n}+1} .     

The Nonclassicality and Decoherence of a Superposition State Generated in a Resonant Cavity

We discuss nonclassicality of a superposition of coherent states in terms of sub-Poissonian photon statistics as well as the negativity of the Wigner function. We derive an analytic expression for the Wigner function from which we find that the function has some negative region in phase space. We obtain a compact form of the Wigner function when decoherence occurs and study the effect of decoherence on the state. We demonstrate the behaviour of the nonclassicality indicator.     

Entanglement of Photon-Subtracted Two-Mode Squeezed Thermal State and Its Decoherence in Thermal Environments

Using a non-Gaussian operation—photon subtraction from two-mode squeezed thermal state (PS-TMSTS), we construct a kind of entangled state. A Jacobi polynomial is found to be related to the normalization factor. The negativity of Wigner function (WF) is used to discuss its nonclassicality. The investigated entanglement properties turn out that the symmetrical PS-TMSTS may be more effective than the non-symmetric for quantum teleportation. Then the time evolution of WF is used to examine the decoherence effect, which indicates that the characteristic time of single PS-TMSTS depends not only on the average photon number of environment, but also on the average photon number of thermal state and the squeezing parameter.     

Decoherence,the Density Matrix,the Thermal State and the Classical World

Decoherence has been the basis for understanding the emergence of the classical world from its quantum underpinnings. Unfortunately the calculations establishing decoherence overshoot and, based on assumptions that break down, predict that with the passage of time the off-diagonal elements of the density matrix become arbitrarily small. It has been recognized by some authors that the thermal state, assumed to hold for systems in equilibrium, places a bound on off diagonal terms. In this article we establish—preserving the conservation of energy, as is not the case for previous work—that indeed the thermal state is an attractor under scattering. Moreover, the bound on the off-diagonal terms present in the thermal state does not contradict everyday experience.     

Decoherence Within a Simple Model for the Environment

This article examines the decoherence of a macroscopic body using a simple model of the environment and following the evolution of the pure state for the whole system. We found that decoherence occurs for very general initial conditions and were able to confirm a number of widely accepted features of the process.     

压缩真空态与压缩单光子态的相干叠加态的非经典性质

通过对量子比特态进行压缩操作,获得压缩真空态与压缩单光子态按一定的权重比例叠加而成的相干叠加态,此相干叠加态与压缩参数和权重因子有关,研究其非经典性和非高斯性.研究结果表明,通过压缩使得量子比特态的光子数分布范围变宽了.特别是从亚泊松分布和Wigner函数负定性情况分析了该量子态的非经典属性,从Wigner函数的边缘分布角度观察了该量子态的非高斯性.发现调节权重因子和压缩参数可以改善该态的非经典性和非高斯性.     

Two-Mode Excited Entangled Coherent State: Nonclassicality and Entanglement

Two-mode excited entangled coherent states (TME-ECSs) are introduced by operating repeatedly the photon-excited operator on the ECSs. It is shown that the normalization constant is related to the product of two Laguerre polynomials. The influence of the operation on nonclassical behaviour of the ECSs is investigated in terms of cross-correlation function, anti-bunching effect and the negativity of Wigner function, which show that nonclassical properties can be enhanced. In addition, inseparability properties of the TME-ECSs are discussed by using Bell inequality and concurrence. It is found that the degree of quantum entanglement of even ECSs increases with the increase of the total excited photon number, and the violation of Bell inequality can be present for both even and odd case only when the total excited photon numbers are even and odd, respectively.     

Entanglement and Decoherence in Two-Dimensional Coherent State Superpositions

A detailed investigation of entanglement in the generalized two-dimensional nonorthogonal states, which are expressed in the framework of superposed coherent states, is presented. In addition to quantifying entanglement of the generalized two-dimensional coherent states superposition, necessary and sufficient conditions for maximality of entanglement of these states are found. We show that a large class of maximally entangled coherent states can be constructed, and hence, some new maximally entangled coherent states are explicitly manipulated. The investigation is extended to the mixed system states and entanglement properties of such mixed states are investigated. It is shown that in some cases maximally entangled mixed states can be detected. Furthermore, the effect of decoherence, due to both cavity losses and noisy channel process, on such entangled states are studied and its features are discussed.     

Decoherence Suppression in Phase Decoherence Environment Using Weak Measurement and Quantum Measurement Reversal

Decoherence suppression from disturbance of the environment is an essential task in quantum information processing. We investigate decoherence suppression of a qubit system interacting with a heat bath with phase decoherence by employing the weak measurement (WM) and quantum measurement reversal (QMR) operation. We show explicitly that the qubit decoherence can be efficiently completely suppressed by means of the combination WM and QMR, which is independent of the form of the spectral density of the reservoir and the form of initial input state.     

Nonclassicality of Coherent Photon-Subtracted Two Single-Modes Squeezed Vacuum State

We introduce a two-mode non-Gaussian state, generated by nonlocal coherent photon-subtraction (CPS) from two single-mode squeezed vacuum states (TSSV). Its normalized factor is turned out to be related with a Legendre polynomial. We further investigate the nonclassical properties of the CPS-TSSV through cross-correlation function, antibunching effect, photon number distribution, wave function and Wigner function. It is shown that the CPS operation can produce the vortex state and can exhibit the highly nonclassical properties.     

Time-Evolution of Photon-Number Distribution and Density Operator of Squeezed Thermal State in the Thermal Environment

By virtue of the thermal entangled state representation, we analytically study time-dependent evolution of photon-number distribution and density operator of squeezed thermal state (STS) in the thermal environment. It is found that the initial density operator of STS still keeps squeezing and thermal within the thermal environment. At long times, such a state decays to thermal, a Gaussian classical state, as a result of decoherence. Moreover, the oscillations of photon-number distribution slowly disappear with increasing t, but the change of oscillations is completely different from that of STS in amplitude dissipative channel.     

扩散过程中弱相干光场的退相干

研究了扩散过程中弱相干光场量子特性的演化.利用正规乘积、反正规乘积和Weyl编序算符内的积分技术,采用热纠缠态表象求解密度矩阵主方程,利用Kraus算符给出扩散过程中密度算符解的表达式,导出初态为弱相干态的量子态密度算符演化规律.讨论了扩散对光场压缩效应和反聚束效应的影响.结果表明:随着扩散过程的进行,弱相干场压缩深度和压缩范围均在减小;扩散初期光场呈反聚束效应,扩散时间大于一定值后反聚束效应消失.     

Nonclassicality of Single-Variable Hermite Polynomial State and Its Fidelity with Squeezed Vacuum State

The nonclassicality of the single-variable Hermite polynomial state (SHPS) and its fidelity with squeezed vacuum state are studied in this paper. It is found that the SHPS can be considered as a single mode photon subtracted squeezed vacuum state. A compact expression for the Wigner function (WF) is also derived analytically by using the Weyl-ordered operator invariance under similar transformations. Especially, the nonclassicality is discussed in terms of the negativity of the WF. At last, the fidelity as a non-Gaussianity measure for this state is also discussed.     

Stability of Pairwise Entanglement in Decoherence Environment

Consider the dynamics of a bipartite entangled system in the decoherence environment, we investigate the stability of pairwise entanglement under decoherence.We find that with the same initial entanglement, the lifetime of entanglement in pure states and some mixed states is the longest.We call these special entangled states as Decoherence Path States (DPS).Besides, we present simple analytic evolution equations of the entanglement in these states.The lifetimes can also be obtained easily.Furthermore, we also study the stability of the nearest neighbor entanglement in the ground state of an antiferromagnetic spin-1/2 ring.Coincidentally, the conclusion is that it is as stable as Decoherence Path States.Thus the nearest neighbor entanglement in the ground state is not maximized but it is the most stable.This interesting result links the energy and entanglement in a spin system from a new point of view.     

Entanglement and Decoherence of Coupled Superconductor Qubits in Contact with a Common Environment

The sudden death of entanglement is investigated for non-Markovian dynamic process of a pair interacting flux qubits under a thermal bath. The entangling evolution of the coupled qubits interacting with non-Markov environment is investigated in terms of concurrence. The results show that, for initially two-qubit entangled states, the sudden death of entanglement (ESD) always happens in the thermal reservoir, where the appearance of entanglement sudden death strongly depends on the environment. Especially, ESD of the qubits is easier to occur for non-Markovian process than for Markovian one.     

Entanglement Dynamics of Multipartite Systems Under Decoherence from a Spin Environment

The entanglement evolution of multipartite quantum states under a spin environment is analyzed. Due to interaction, the extent to which the entanglement vanishes depends not only on the size of system and the structure of quantum states, but also on the exchange couplings with environment. The decoherence-free subspaces have been identified by using the linear entropy.     

Decoherence of a Quantum Nonlinear Oscillator Under a Non-zero Temperature Thermal Bath

The characteristic time τ_D for decoherence process of a quantum nonlinear oscillator system under a non-zero temperature thermal bath is studied by expanding the linear entropy. By numerical analysis, it is shown that at a non-zero temperature, the quantum coherence decays much faster than at zero temperature. Moreover, the non-zero temperature thermal bath will bring a crucial suppression to the quantum effects of the observables, which causes these quantum effects to become unable to persist up to the Ehrenfest time but is insufficient to destroy the quantum-classical transition.