Entropy Exchange and Entanglement in Superconducting Charge Qubit Inside a Resonant Cavity with Intrinsic Decoherence

By considering the intrinsic decoherence effect, we investigate the entropy exchange and entanglement in the interacting system of a superconducting charge qubit coupled to a single-mode optical cavity. We found that although the intrinsic decoherence leads to an irreversible evolution of the interacting system due to a suppression of coherent quantum features through the decay of off-diagonal matrix elements of the density operator, and has an apparently influence on the partial entropies of the two-component subsystems, it dose not destroy entropy exchange behavior. In addition, the lower bound of the concurrence, as the measure of entanglement of the coupling system, is calculated. It is shown that the evolution of entanglement is sensitive to the change of the intrinsic decoherence.     

Decoherence of a Double Quantum Dot Charge Qubit Approached with Redfield Equation

In this paper, we investigate the decoherence time of a double quantum dot （DQD） charge qubit in three kinds of baths through solving dynamics of the qubit. The dynamics of the qubit is investigated with Redfield master equation. It is shown that the decoherence time of the qubit in Ohmic bath has the same order of magnitude as the experiments reported. When the environment is modeled with the supra-Ohmic bath the decoherence time of the qubit is shorter than the experimental result. And when modeled with the sub-Ohmic bath the decoherence time of the qubit is longer than the experimental result.     

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.     

Measurement-induced disturbance in Heisenberg XY spin model with Dzialoshinskii–Moriya interaction under intrinsic decoherence

Quantum correlations measured by measurement-induced disturbance （MID） in a two-qubit Heisenberg XY spin model with Dzialoshinskii-Moriya （DM） interaction under intrinsic decoherence are investigated. MID is studied un-der various circumstances and the influences of the external dependencies on the final quantum state which has stable MID are discussed. Two kinds of initial quantum states are considered as well as different conclusions. MID appears to decay periodically during the processing of intrinsic decoherence; both DM interaction and intrinsic decoherence have a negative impact on the correlations. The MID of the stable state depends on several factors, except the parameter of the intrinsic decoherence. Moreover, we find a special initial state that is able to maintain the maximum quantum correlations during the processing of intrinsic decoherence.     

Effect of Noise on Berry‘s Phase for Quantum Computing

Based on a new type of two-level-system reservoir, we investigate the impact of dissipation on Berry‘‘s phase of a spin trapped in a periodical external field. It is found that the existence of environmental noise could lead to a decaying term in the matrix of Berry‘s phase as the sign of a decoherence process, which is in agreement with the result of Nazir et al. (Phys. Rev. A 65 (2002) 042303). Particularly, in comparison with a specialized case of the traditional Leggett dissipation model, we only shows the dependence of time but not temperature in decaying tem. A concrete case is exhibited by using the one-dimensional Ohmic function.     

Entanglement Swapping of Pair Coherent States with Phase Decoherence

We investigate the entanglement swapping of continuous state and the two-mode squeezed vacuum which is exposed variable using the pair coherent state as the input in a phase decoherence environment as the quantum channel. By adopting the log-negativity as the measure of entanglement, we analyze how entanglement of the two initial states and the phase decoherence environment affect the entanglement swapping quality.     

Quantum correlation of a three-particle W-class state under quantum decoherence

We investigate the quantum characteristics of a three-particle W-class state and reveal the relationship between quan- tum discord and quantum entanglement under decoherence. We can also identify the state for which discord takes a maximal value for a given decoherence factor, and present a strong bound on quantum entanglement-quantum discord. In contrast, a striking result will be obtained that the quantum discord is not always stronger than the entanglement of formation in the case of decoherence. Furthermore, we also theoretically study the variation trend of the monogamy of quantum correlations for the three-particle W-class state under the phase flip channel, and find that the three-particle W-class state could transform from polygamous into monogamous, owing to the decoherence.     

Quantum correlation of a three-particle-class state under quantum decoherence

We investigate the quantum characteristics of a three-particle W-class state and reveal the relationship between quantum discord and quantum entanglement under decoherence.We can also identify the state for which discord takes a maximal value for a given decoherence factor,and present a strong bound on quantum entanglement–quantum discord.In contrast,a striking result will be obtained that the quantum discord is not always stronger than the entanglement of formation in the case of decoherence.Furthermore,we also theoretically study the variation trend of the monogamy of quantum correlations for the three-particle W-class state under the phase flip channel,and find that the three-particle W-class state could transform from polygamous into monogamous,owing to the decoherence.     

Comparative studies of open and master equation qubit via Bloch equation of Redfield form

In this paper, we investigate the dynamics of an open qubit model by solving two sets of its reduced dynamical equations. One set of the equations is the well-known Bloch equations and the other is the widely investigated master equations of Redfield form. Both of them are obtained from the perturbation approximation which demands the system of interest weakly coupled to its environment. It is shown that the qubit has a longer decoherence and relaxiation time as the dynamics is described by the Redfield equantions.     

Controlling quantum discord dynamics in cavity QED systems by applying a classical driving field with phase decoherence

We investigate a two-level atom interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence and find that a stationary quantum discord can arise in the interaction of the atom and cavity field as the time turns to infinity.We also find that the stationary quantum discord can be increased by applying a classical driving field.Furthermore,we explore the quantum discord dynamics of two identical non-interacting two-level atoms independently interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence.Results show that the quantum discord between two atoms is more robust than entanglement under phase decoherence and the classical driving field can help to improve the amount of quantum discord of the two atoms.     

Influence of Noises on Remote State Preparation Using GHZ State

Using a quantum channel consisting of a GHZ state exposed to noisy environment, we investigate how to remotely prepare an entangled state and a qubit state, respectively. By solving the master equation in the Lindblad form, the influence of the various types of noises on the GHZ state is first discussed. Then we use the fidelity to describe how close the remotely prepared state and the initial state are. Our results show that the fidelity is a function of the decoherence rates and the angles of the initial state. It is found that for each of the two RSP schemes, the influence of the noise acting simultaneously in x, y, and z directions on the average fidelity is the strongest while the influence of the noise acting in x or z direction on the average fidelity is relatively weaker.     

Dense coding with a two-qubit Heisenberg XYZ chain under the influence of phase decoherence

We investigate the joint effects of phase decoherence,Dzyaloshinskii-Moriya(DM) interaction and inhomogeneity of the external magnetic field(b) on dense coding in a two-qubit anisotropic Heisenberg XY Z spin chain.Analytical expressions are obtained for the dense coding capacity.It is found that valid dense coding is always possible with this model when the system is initially prepared in the maximum entangled state.Moreover,optimal dense coding can be implemented for this initial state as long as the mean spin-spin coupling constant J + of the XY plane is larger than b and the DM interaction despite the intrinsic decoherence.Non-maximal entangled initial states are found to be undesirable for dense coding with this model.     

Remote Preparation of an Entangled State in Nonideal Conditions

In this paper, the theoretical investigation of remote preparation of an entangledstate is studied in nonideal conditions. Our studies include two parts. In the first part, we consider the remote state preparation （RSP） of an entangled state through two equally noisy quantum channel states, namely, a mixture of Bell states. Studies show there is a particular mixed-state channel for which all pure entangled states remain entangled after this inexact RSP. In the second part, we suppose that noises which quantum channels suffer from can be expressed as the Lindblad operators. The master equation of the system can be expressed in the Lindblad form. Through solving the master equation, we calculate the fidelity as a function of decoherence rates and parameters of the state to be prepared. For a given entangled state, we investigate the influence of different types of noises on the fidelity.     

Comparative studies of open qubit via Bloch equation and master equation of Redfield form

In this paper, we investigate the dynamics of an open qubit model by solving two sets of its reduced dynamical equations. One set of the equations is the well-known Bloch equations and the other is the widely investigated master equations of Redfield form. Both of them are obtained from the perturbation approximation which demands the system of interest weakly coupled to its environment. It is shown that the qubit has a longer decoherence and relaxiation time as the dynamics is described by the Redfield equantions.     

Effects of inhomogeneous couplings between atoms and acavity field on entanglement dynamics

Based on the concept of concurrence, we have investigated the entanglement dynamics of two two-level atoms coupled to a single-mode cavity field with inhomogeneous couplings. We find that, for some initial states, the inhomogeneous couplings not only induce but also enhance the entanglement in the process of its evolution. In addition, considering the intrinsic decoherence proposed by Milburn, we also find that a proper value of inhomogeneous couplings can enhance the stationary entanglement, and as a result, the destructive effect of intrinsic decoherence on entanglement can be moderated by the inhomogeneous couplings.     

Entropy squeezing for a two-level atom in two-mode Raman coupled model with intrinsic decoherence

In this paper, we investigate the entropy squeezing for a two-level atom interacting with two quantized fields through Raman coupling. We obtain the dynamical evolution of the total system under the influence of intrinsic decoherence when the two quantized fields are prepared in a two-mode squeezing vacuum state initially. The effects of the field squeezing factor, the two-level atomic transition frequency, the second field frequency and the intrinsic decoherence on the entropy squeezing are discussed. Without intrinsic decoherence, the increase of field squeezing factor can break the entropy squeezing. The two-level atomic transition frequency changes only the period of oscillation but not the strength of entropy squeezing. The influence of the second field frequency is complicated. With the intrinsic decoherence taken into consideration, the results show that the stronger the intrinsic decoherence is, the more quickly the entropy squeezing will disappear. The increase of the atomic transition frequency can hasten the disappearance of entropy squeezing.     

Quantum Discord Behavior about Two-Qubit Heisenberg XYZ Model with Decoherence

We investigate the properties of quantum discord dynamics of a two-qubit Heisenberg XYZ system which is influenced by the environmental decoherence under an external nonuniform magnetic field. It shows that the influence of the parameters on the system heavily rely on the selection of the initial states. One point shows that the environmental decoherence cannot entirely destroy the quantum correlation, and properly controlling the parameters can inhibit the decoherence. Moreover, it presents that the inhomogeneous magnetic field cannot affect the steady quantum discord （QD）, while the uniform magnetic field and the anisotropy coupling constant will change the steady QD. These investigations imply that one can obtain larger steady QD values by reasonably adjusting parameters on quantum correlation in solid state systems.     

Quantum State Transfer in Engineered Spin Chain under Influence of Spatially Distributed Environment

It has been shown that a quantum state could be perfectly transferred via a spin chain with engineered ＂always-on interaction＂. In this paper, we study a more realistic problem for such a quantum state transfer （QST） protocol, how the efficacy of QST is reduced by the quantum decoherence induced by a spatially distributed environment. Here, the environment is universally modeled as a bath of fermions located in different positions. By making use of the irreducible tensor method in angular momentum theory, we investigate the effect of environment on the efficiency of QST for both cases at zero and finite temperatures. We not only show the generic exponential decay of QST efficiency as the number of sites increase, but also find some counterintuitive effect, the QST can be enhanced as temperature increases in some cases.     

Entanglement Evolution of Three-Qubit States under Local Decoherence

By using negativity as entanglement measure, we have investigated the effect of local decoherence from a non-Markovian environment on the time evolution of entanglement of three-qubit states including the GHZ state, the W state, and the Werner state. From the results, we find that the entanglement dynamics depends not only on the coupling strengths but also on the specific states of concern. Specifically, the entanglement takes different behaviors under weak or strong coupling and it varies with the quantum states under study. The entanglement of the GHZ state and the Werner state can be destroyed completely by the local decoherence, while the entanglement of the W state can survive through the local decoherence partially.     

The effect of quantum noise on the restricted quantum game

It has recently been established that quantum strategies have great advantage over classical ones in quantum games. However, quantum states are easily affected by the quantum noise resulting in decoherence. In this paper, we investigate the effect of quantum noise on the restricted quantum game in which one player is restricted in classical strategic space, another in quantum strategic space and only the quantum player is affected by the quantum noise. Our results show that in the maximally entangled state, no Nash equilibria exist in the range of It has recently been established that quantum strategies have great advantage over classical ones in quantum games. However, quantum states are easily affected by the quantum noise resulting in decoherence. In this paper, we investigate the effect of quantum noise on the restricted quantum game in which one player is restricted in classical strategic space, another in quantum strategic space and only the quantum player is affected by the quantum noise. Our results show that in the maximally entangled state, no Nash equilibria exist in the range of 0 〈 p ≤ 0.422 （p is the quantum noise parameter）, while two special Nash equilibria appear in the range of 0.422 〈 p 〈 1. The advantage that the quantum player diminished only in the limit of maximum quantum noise. Increasing the amount of quantum noise leads to the increase of the classical player＇s payoff and the reduction of the quantum player＇s payoff, but is helpful in forming two Nash equilibria.