Robustness investigations on quantum entanglement and quantum discord of coupled qubits in squeezed vacuum reservoir

We investigate the quantum discord of coupled qubits in squeezed vacuum reservoir and compare it with the quantum entanglement of system. We find that the quantum discord and entanglement perform completely oppositely with the change of squeezed parameters. Quantum discord survives longer with the increase of squeezed amplitude parameter and entanglement death faster on the contrary. Under high squeezed amplitude parameter, the quantum discord can keep nonzero which indicate that the quantum discord is more robust than entanglement. We also find that the purity reduction of the initial quantum state will lead to the decay of concurrence or quantum discord. However, the quantum discord damps remarkably more slowly and survives longer than concurrence.     

Investigations on quantum correlation of coupled qubits in squeezed vacuum reservoir

In this paper, we investigate the quantum correlation of coupled qubits which are initially in maximally entangled mixed states in squeezed vacuum reservoir. We compare and analyse the effects of squeezed parameters on quantum discord and quantum concurrence. The results show that in squeezed vacuum reservoir, the quantum discord and quantum concurrence perform completely opposite behaviors to the change of squeezed parameters. Quantum discord survives longer with the increase of squeezed amplitude parameter, but entanglement death turns faster on the contrary. The results also indicate that the classical correlation of the system is smaller than quantum discord in vacuum reservoir, while it is bigger than quantum discord in squeezed vacuum reservoir. The quantum discord and classical correlation are more robust than quantum concurrence in the two reservoir environments, which indicates that the entanglement actually is easily affected by decoherence and quantum discord has stronger ability to avoid decoherence in squeezed vacuum reservoir.     

Investigations into quantum correlation of coupled qubits in a squeezed vacuum reservoir

In this paper,we investigate the quantum correlation of coupled qubits which are initially in maximally entangled mixed states in a squeezed vacuum reservoir.We compare and analyze the effects of squeezed parameters on quantum discord and quantum concurrence.The results show that in a squeezed vacuum reservoir,the quantum discord and quantum concurrence perform with completely opposite behaviors with the change of squeezed parameters.Quantum discord survives longer with the increase of squeezed amplitude parameter,but entanglement death is faster on the contrary.The results also indicate that the classical correlation of the system is smaller than quantum discord in a vacuum reservoir,while it is bigger than quantum discord in a squeezed vacuum reservoir.The quantum discord and classical correlation are more robust than quantum concurrence in the two reservoir environments,which indicates that the entanglement actually is easily affected by decoherence and quantum discord has a stronger ability to avoid decoherence in a squeezed vacuum reservoir.     

Measurement-induced nonlocality and geometric quantum discord in two SC-charge qubits

By using geometric quantum discord and measurement-induced nonlocality, quantum correlations are investigated for two superconducting (SC) charge qubits that share a large Josephson junction where the field is assumed to be prepared initially in a coherent state. It is found that the difference between measure measurement-induced nonlocality and geometric quantum discord, of the final state of the two SC-charge qubits system which is especial case of X-states, is equal to a constant value. It is found that the quantum correlations and entanglement of the qubits are very sensitive to the mean number of the coherent photons. The entanglement exists in small intervals of death quantum discord and measurement-induced nonlocality. This is further evidence in support of the fact that quantum correlation and entanglement are not synonymous.     

Prepare Multiphoton Binomial State Utilizing Hybrid Circuit QED

We utilized hybrid circuit QED to prepare a multiphoton binomial state in accordance with the optimal quantum control method, and further investigated the quantum correlation dynamics of the hybrid coupling superconducting qubits. The results showed that the quantum entanglement was so weak that the duration was too short to be used as quantum information resource in the optical field of binomial state, while the geometric measure of quantum discord (GMQD) was fully adequate to the practical demand. With the increase of binomial optical field parameters M and r, the GMQD evolved more and more frequently, and collapse and revival occurred periodically. When the optical field of binomial state tended to coherent state, the collapse and revival period of the GMQD extended as the average photon number increased.     

Preparation of multi-photon Fock states and quantum entanglement properties in circuit QED

We demonstrate the controllable generation of multi-photon Fock states in circuit quantum electrodynamics （circuit QED）. The external bias flux regulated by a counter can effectively adjust the bias time on each superconducting flux qubit so that each flux qubit can pass in turn through the circuit cavity and thereby avoid the effect of decoherence. We further investigate the quantum correlation dynamics of coupling superconducting qubits in a Fock state. The results reveal that the lower the photon number of the light field in the number state, the stronger the interaction between qubits is, then the more beneficial to maintaining entanglement between qubits it will be.     

Dynamics control of geometric quantum discord for two coupling qubits in a squeezed vacuum reservoir

We investigate the dynamics of geometric quantum discord of coupled qubits in a squeezed vacuum reservoir. The results show that there is distinct difference between the dynamics of geometric quantum discord and that of quantum entanglement near (or away from) the decoherence free subspace. We also find that the squeezed vacuum reservoir with high squeezed amplitude is more suitable for geometric quantum discord to survive. The robustness of geometric quantum discord is stronger than that of quantum entanglement.     

The effect and control on remaining the quantum correlation by the coupling symmetry between qubits and the bath

We study the dynamic evolution of quantum correlation of two interacting coupled qubits system in non-Markov environment, and quantify the quantum correlation using concurrence and quantum discord. We find that although both of them are physical quantities which measure the system characteristics of the quantum correlations, the quantum discord is more robust than concurrence, since it can keep a positive value even when the ESD happens. The quantum correlation of quantum system not only depends on the initial state but also strongly depends on the coupling ways between qubits and environment. For the given initial state, by keeping the coupling between qubits and environment in completely symmetric, we can completely avoid the effect the decoherence influenced on the quantum correlation and effectively prolong the survival time of quantum discord and concurrence. We also find that the stronger the interaction between qubits is, the more conducive the death of the quantum correlation is resisted.     

Non-Markovian dynamics of two non-coupled qubits interacting with two separate reservoirs with different spectral densities

The dynamics of two non-coupled qubits independently interacting with their reservoirs is solved by the time convolutionless projection operator method. We study two-qubit quantum correlation dynamics for two different types of spectral densities, which are a Lorentzian distribution and an Ohmic spectral density with a Lorentzian-Drude cutoff function. For two qubits initially prepared in the initial Bell state, quantum discord can keep longer time and reach larger values in nonMarkovian reservoirs for the first spectral distribution or by reducing the cutoff frequency for the second case. For the initial Bell-like state, the dynamic behaviors of quantum discord and entanglement are compared. The results show that a long time of quantum correlation can be obtained by adjusting some parameters in experiment and further confirm that the discord can capture quantum correlation in addition to entanglement.     

Quantum Discord of Two Atoms Interacting with a Single-mode Thermal Field

This paper investigates the quantum discord and entanglement of two atoms when they simultaneously interact with a single-mode thermal field. The results show that, the two atoms which are initially in separate states can be entangled by a thermal field. However, with increase of the mean photon number, the value of the entanglement decreases and disappears when the temperature of the cavity is high enough (corresponding to the large value of the mean photon number). In stark contrast, the quantum discord does not decrease, but gradually reaches stable value at high temperature. In addition, when the two atoms are initially the Werner mixed state, we have found that, a large amount of quantum discord is exist even in the region where the entanglement is zero, which is a strong signature for the presence of non classical correlations. These results indicate that, the quantum discord is more resistant to the environment’s disturbance than the entanglement for higher temperatures.     

Non-Classical Correlations and Transfer of Quantum Information in a Superconducting Qubit System with Dynamical Decoupling Pulses

We investigate the dynamics of non-classical correlations(entanglement and quantum discord) of the system consisting of two non-interacting superconducting qubits coupling with a common data bus, where the system is driven by the dynamical decoupling pulses. It is found that the non-classical correlations between two superconducting qubits can be increased by appling a train of dynamical decoupling pulses. Furthermore, we also explore the influence of the dynamical decoupling pulses on the information flowing between superconducting qubits and data bus by making use of the trace distance. It is shown that the dynamical decoupling pulses can protect quantum information of two superconducting qubits and force information to flow back to the superconducting qubits from the data bus.

     

Tavis-Cummings模型中的几何量子失协特性

采用几何量子失协的计算方法,通过改变两原子初始状态、腔内光子数和偶极-偶极相互作用强度,研究了Tavis-Cummings模型中的几何量子失协特性.结果表明:几何量子失协都是随时间周期性振荡的,选取适当的初态可以使两原子一直保持失协状态,增加腔内光子数和偶极相互作用对几何量子失协有积极的影响.     

Dynamics of Quantum Discord of Two-Qubit Coupled with a Vacuum Cavity

The dynamical behaviors of quantum discord between two atoms coupled with a vacuum cavity are investigated. If the two qubits are initially prepared in two extended Werner-like states, the quantum discord and entanglement can be numerically calculated. There are remarkable differences between the time evolutions of the quantum discord and entanglement under the same conditions. These results imply that quantum discord is not zero for some unentangled states and in some regions entanglement can disappear completely. A large amount of quantum discord exists between the two-qubit. Thus, the quantum discord is more robust than entanglement for the quantum system exposed to the environment. The quantum discord shows sudden change and its existence depends on the initial state of the system. This property of quantum discord may have important implications for experimental characterization of quantum phase transitions.     

Entanglement Dynamics of Three Superconducting Charge Qubits Coupled to a Cavity

We investigate the entanglement dynamics of a quantum system consisting of three superconducting charge qubits (SCQs) interacting with a microwave field. For separable and entangled states of the SCQs, the evolutions are studied under various photon numbers of cavity field. The results show that the amplitude and period of the bipartite entanglement square concurrences can be controlled by the choice of initial states of SCQs and photon numberof cavity field, respectively. This simple model of a quantum register allows us to understand the dynamic process of the quantum storage of information carried by charge qubit.     

Regulation of Entanglement and Geometric Quantum Discord of Hybrid Superconducting Qubits for Circuit QED

We investigate the dynamic evolution behaviors of entanglement and geometric quantum discord (GQD) of hybrid qubits in a circuit QED system. Under certain initial conditions, interactions between qubits and that between the qubit and the cavity can suppress the decay of entanglement and GQD of qubits. Under the initial condition |ψ _Q (0)〉 _I =sinα|↓↑〉+cosα|↑↓〉, such decay was avoided by increasing the coupling strength ratio between qubits and that between the qubit and the cavity. Under the aforementioned condition, the survival time of entanglement and GQD was prolonged by decreasing the coupling strength ratio between qubits and that between the qubit and the cavity.     

Entanglement and Quantum Discord Dynamics of Two Atoms in a Broadband Squeezed Vacuum Bath

We study the dynamics of entanglement and quantum discord between two two-level atoms that interact with a common squeezed reservoir. It is shown that the degree of entanglement and quantum discord are very sensitive to the degree of two-photon correlation for large values of the mean photon number. The squeezed vacuum environment can drive the system to a stationary state with high entanglement and quantum discord for certain X-type states. Furthermore, sudden change happens to the dynamics of quantum discord while the entanglement is remained almost unchanged.     

Quantum discord dynamics of two qubits in single-mode cavities

The dynamics of quantum discord for two identical qubits in two independent single-mode cavities and a common single-mode cavity are discussed. For the initial Bell state with correlated spins, while the entanglement sudden death can occur, the quantum discord vanishes only at discrete moments in the independent cavities and never vanishes in the common cavity. Interestingly, quantum discord and entanglement show opposite behavior in the common cavity, unlike in the independent cavities. For the initial Bell state with anti-correlated spins, quantum discord and entanglement behave in the same way for both independent cavities and a common cavity. It is found that the detunings always stabilize the quantum discord.     

Geometrical quantum discord and negativity of two separable and mixed qubits

We studied quantum correlation and quantum entanglement of a quantum system in which a coherent state light field interacts with two qubits that are initially prepared in a separable and mixed state.The influence of mean photon number of the coherent field and distribution probability of the atom on the geometrical quantum discord and the negativity are discussed.Our results show that the mean photon number of light field and distribution function of the atom can regulate and control the quantum correlation and quantum entanglement.     

Quantifying Quantumness with Sudden Birth or Death of Entanglement for Two-Qubits System

Quantumness of the correlations between two qubits, coupled to a cavity field whenever the system is open or closed, is investigated when sudden death (or birth) of the entanglement occurs. It is found that quantum correlation not exists only in the entangled state. It’s found that the dephasing parameters and the purity of the initial states play an important role in the dynamics behaviors of the quantum correlations, including entanglement. Quantumness of the correlations and entanglement, due to dephasing of the cavity, are damped until the same their values which are contained in their initial states.     

Quantum Entanglement and Quantum Discord of a Two-Qubit System in a Dissipative Cavity

The goal of this work is to investigate quantum entanglement and quantum discord of a pair of two-level atoms which is driven by an external classical field and interacts with a cavity field. After extracting density matrix of the atom-atom subsystem, it is shown that we have stronger quantum discord by increasing atom-field coupling constant for the case in which there is no entanglement. Moreover, for the atom-atom subsystem it is realized that quantum entanglement and quantum discord cannot increase, they decrease after passing some times due to cavity dissipation. Also quantum entanglement and quantum discord decrease faster by increasing atom-field coupling constant.