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.     

The Dynamics of Quantum Correlations in Mixed Classical Environments

We present a comparative study of the dynamics of entanglement and quantum discord in a bipartite system in the presence of mixed classical noises. In particular, the joint effects of three different types of classical noises, namely, random telegraphic noise (RTN), Ornstein–Uhlenbeck noise (OU), and static noise, are studied by combining them in two different ways. In each case, one marginal system is coupled with random telegraphic noise, and the other marginal system is coupled with either OU or static noise. We make a comparison between the behaviors of both correlations in the two setups. In the weak coupling regime, the qualitative behavior of entanglement is unaffected by switching the coupling of only one marginal system from OU to static noise, and vice versa. However, the behavior of quantum discord strongly depends on whether it is coupled with OU or static noise. On the other hand, in the strong coupling regime, the static noise is more fatal to the survival of both correlations as compared to the other two noises.     

Quantum discord in non-Markovian environments

The quantum discord dynamics of two qubits in two independent non-Markovian reservoirs has been investigated. On the condition of resonant interactions, the result shows that the quantum discord has the phenomenon of “revival”. What is more, even in the region where the entanglement is zero, the quantum discord still can reveal the quantum correlations between the two qubits. Considering the detuning, it is interesting to note that the quantum discord can be preserved when the non-Markovian and the detuning conditions are satisfied simultaneously. Finally, an intuitive physical interpretation has been given by the quasimode approach.     

Modulation of entanglement and quantum discord for circuit cavity QED states

The paper investigates the dynamic evolution behaviors of entanglement and quantum discord of coupled superconducting qubits in circuit QED system. We put emphasis on the effects of cavity field quantum state on quantum entanglement and quantum correlations dynamic behaviors of coupling superconducting qubits. The results show that, (1) generally speaking, the entanglement will appear the death and new birth because of the interaction between qubits and cavity field, on the contrary, this phenomenon will not appear in quantum discord. (2) When the cavity field is in coherent state, the entanglement survival time is controlled by the average photon number. The more the average photon number is, the longer survival time of entanglement is prolonged. Thus it has the benefit of keeping quantum correlations. (3) When the cavity field is in squeezed state, the squeezed amplitude parameters have controlling effects on quantum correlations including entanglement and quantum discord. On the one hand, the increase of squeezed amplitude parameters can prolong the survival time of entanglement, on the other hand, with the increase of squeezed amplitude parameters, the robustness of quantum discord is more and more superior to concurrence and is more advantage to keep the system quantum correlations. The further study results show that the increase of the initial relative phase of coupling superconducting qubits can also keep the quantum correlations.     

Quantum correlations dynamics of three-qubit states coupled to an XY spin chain: Role of coupling strengths

We investigate the prominent impacts of coupling strengths on the evolution of entanglement and quantum discord for a three-qubit system coupled to an XY spin-chain environment. In the case of a pure W state, more robust, even larger nonzero quantum correlations can be obtained by tailoring the coupling strengths between the qubits and the environment.For a mixed state consisting of the GHZ and W states, the dynamics of entanglement and quantum discord can characterize the critical point of quantum phase transition. Remarkably, a large nonzero quantum discord is generally retained, while the nonzero entanglement can only be obtained as the system-environment coupling satisfies certain conditions. We also find that the impact of each qubit's coupling strength on the quantum correlation dynamics strongly depends on the variation schemes of the system-environment couplings.     

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.     

Quantum Discord in Optical Coherent States

We study the dynamics of quantum correlation of optical coherent-state qubits affected by the environment. It consists in sending these states via a decohering quantum channel. The states used as the support of the encoding information are affected by an amplitude damping channel. The quantum discord is one of type of quantum correlations between the qubits. The discord and its dynamics of two qubits in non-Markovian environments are evaluated.     

Relativistic quantum correlations in bipartite fermionic states

The influences of relative motion, the size of the wave packet and the average momentum of the particles on different types of correlations present in bipartite quantum states are investigated. In particular, the dynamics of the quantum mutual information, the classical correlation and the quantum discord on the spin correlations of entangled fermions are studied. In the limit of small average momentum, regardless of the size of the wave packet and the rapidity, the classical and the quantum correlations are equally weighted. On the other hand, in the limit of large average momentum, the only correlations that exist in the system are the quantum correlations. For every value of the average momentum, the quantum correlations maximize at an optimal size of the wave packet. It is shown that after reaching a minimum value, the revival of quantum discord occurs with increasing rapidity.     

Geometric measure of quantum discord in non-Markovian environments

We study the dynamics of geometric measure of quantum discord between two non-interacting qubits each immersed in its own non-Markovian environment with a spectal distribution representing the electromagnetic field inside off-resonant high-Q cavity. We compared the dynamics of geometric measure of quantum discord with quantum discord for an initial Werner-like state and conclude three important findings. First, when there is an instantaneous disappearance in the dynamics of quantum discord at some timepoints, there is a disappearance in geometric measure of quantum discord, but not instantly. Second, the sudden change in the decay rate of geometric measure of quantum discord might not imply the sudden change in the decay rate of the dynamics of quantum discord. Third, there is a preservation for a long time in both quantum discord and geometric measure of quantum discord when the detuning and non-Markovian conditions are simultaneously satisfied.     

与XY双自旋链耦合的双量子比特系统的关联性与相干性

研究了双量子比特系统中在具有Dzyaloshinsky-Moriya相互作用的独立XY自旋链环境下的相干性与关联性动力学.推导出相干性与关联性的演化规律.发现在自旋链的临界点附近,当tt_0时,系统相干性的演化与经典关联完全相同;而在tt_0时,则与量子关联完全相同;在t_0时刻,量子关联突变为经典关联.     

Dissipative Dynamics of Quantum Discord of Two Strongly Driven Qubits

The exact dynamics of quantum discord (QD) of two strongly driven qubits, which are initially prepared in the X-type quantum states and inserted in two independent dissipative cavities or in a common dissipative cavity, are studied. The results indicate that both in the two cases, the evolution of QD is independent of the initial cavity state. For the two independent dissipative cavities, it is found that the phenomenon of sudden transition between classical and quantum decoherence exists and the transition time can be greatly delayed by suitably choosing the initial state parameter of the two qubits, the cavity mode-driving field detunning and the decay rate of the cavity. For the common dissipative cavity, it is shown that for some initial states of the two qubits, the QD can increase for a finite time at first, and then it decreases to a steady value, while for some other initial states, the QD can increase monotonously or with oscillation till a stable value is reached. Moreover, the creation of QD for the two qubits in a common cavity is discussed.     

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.     

Correlation Dynamics of Three-Qubit System Under a Classical Dephasing Environment

By starting from the stochastic Hamiltonian of the three correlated spins and modeling their frequency fluctuations as caused by dephasing noisy environments described by Ornstein-Uhlenbeck (OU) processes, we study the dynamics of quantum correlations, including entanglement and quantum discord. Of course, in this article, we use two definitions for the quantum discord (global quantum discord and quantum dissension). We prepared initially our open system with the Greenberger-Horne-Zeilinger (GHZ) and W states and present the exact solutions for evolution dynamics of entanglement and quantum discord between three spins under both Markovian and non-Markovian regime of this classical noise. By comparison the dynamics of entanglement with that of quantum discord we find that entanglement can be more robust than quantum discord against this noise. It is shown that by considering non-Markovian extensions the survival time of correlations prolong. Also, we compare the results of two definitions of the quantum discord and show that the quantum dissension is equal to the global quantum discord for GHZ state, but they are unequal for the W state.     

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.     

Controllable coupling and quantum correlation dynamics of two double quantum dots coupled via a transmission line resonator

We propose a theoretical scheme to generate a controllable and switchable coupling between two double-quantum-dot (DQD) spin qubits by using a transmission line resonator (TLR) as a bus system. We study dynamical behaviors of quantum correlations described by entanglement correlation (EC) and discord correlation (DC) between two DQD spin qubits when the two spin qubits and the TLR are initially prepared in X-type quantum states and a coherent state, respectively. We demonstrate that in the EC death regions there exist DC stationary states in which the stable DC amplification or degradation can be generated during the dynamical evolution. It is shown that these DC stationary states can be controlled by initial-state parameters, the coupling, and detuning between qubits and the TLR. We reveal the full synchronization and anti-synchronization phenomena in the EC and DC time evolution, and show that the EC and DC synchronization and anti-synchronization depends on the initial-state parameters of the two DQD spin qubits. It is shown that the initial quantum correlation may be suppressed completely when the evolution time approaches to the infinity in the presence of dissipation. These results shed new light on dynamics of quantum correlations.     

A Direct Measurement Scheme of Two Quantum-Dot Qubits Quantum Correlation via Detector Current

By using the rate equation method we have investigated a scheme for measuring the quantum discord between two double quantum dot qubits via the current in quantum point-contact devices. The current of the detectors is same in-phase as the time-dependant of the quantum discord, and we can read out the most important time point when the quantum discord or quantum correlation reaches its maximum or minimum. Furthermore, in a special initial condition, the current can reflect the quantum discord. The shot noise of the detectors is calculated numerically. Which is dominated by the measured signal in single shot experiments.     

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.

     

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.     

Quantum correlations between two non-interacting atoms under the influence of a thermal environment

By considering a double Jaynes-Cummings model, we investigate the dynamics of quantum correlations, such as the quantum discord and the entanglement, for two atoms in their respective noisy environments, and study the effect of the purity and the cavity temperature on the quantum correlations. The results show that the entanglement suffers sudden death and revival, however the quantum discord can still reveal the quantum correlations between the two atoms in the region where the entanglement is zero. Moreover, when the temperature of each cavity is high the entanglement dies out in a short time, but the quantum discord still survives for quite a long time. It means that the quantum discord is more resistant to environmental disturbance than the entanglement at higher temperatures.     

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.