Narrow Results

In this paper, we investigate the behaviors of quantum correlation quantified by trace norm measurement-induced nonlocality (TMIN) for two uniformly accelerated atoms coupled with electromagnetic vacuum fluctuation in the Minkowski vacuum. We firstly discuss the solving process of master equation that governs the system evolution based on the Pauli matrix presentation for two-qubit state. Similar to [Y. Q. Yang et al., Phys. Rev. A94, 032337 (2016)], we analyze the degradation, creation, revival and enhancement of quantum correlation for different initial states and polarizations of two atoms, and the influence of interatomic separation and acceleration on quantum correlation. Compared with the entanglement dynamics discussed in [Y. Q. Yang et al., Phys. Rev. A94, 032337 (2016)], it is found that quantum correlation exhibits better robustness than entanglement. This may be helpful for quantum information processing.

In this work, we investigate the dynamics of quantum-memory-assisted entropic uncertainty relation for a two-level atom coupled with fluctuating electromagnetic field in the presence of a perfectly reflecting plane boundary. The solution of the master equation that governs the system evolution is derived. We find that entropic uncertainty and mixedness increase to a stable value with evolution time, but quantum correlation reduces to zero with evolution time. That is, the mixedness is positively associated with entropic uncertainty, however, increasing quantum correlation can cause the decrease of the uncertainty. The tightness of entropic uncertainty grows at first and then declines to zero with evolution time. In addition, entropic uncertainty fluctuates to relatively stable values with increasing the atom’s distance from the boundary, especially for short evolution time, which suggests a possible way of testing the vacuum fluctuating and boundary effect. Finally, we propose an effective method to control the uncertainty via quantum weak measurement reversal.

In this paper, we explore the dynamics of quantum correlation for two circularly accelerated atoms interacting with a bath of fluctuating massless scalar field with a reflecting plane boundary. First, we derive the master equation that governs the system evolution. Then we analyze the behaviors of quantum correlation for various conditions and compare the behaviors of quantum correlation with that of the static atoms immersed in a thermal bath with a boundary. It is found that the dynamics of quantum correlation for circularly accelerated atoms present some features distinct from those of static atoms immersed in a thermal bath.

Entanglement and atomic coherence are investigated for a system that consists of a four-level atom (FLA) coupled with a nonlinear field. We explore the influence of field deformation and photon transition on the dynamical behavior of the quantum measures when the quantized field is initially in a coherent state or in a Schrödinger cat state with and without energy dissipation. The results indicate that the entanglement and coherence in the FLA–field system can be controlled and manipulated through the nature and initial state of the quantized field.

Recent work has relatively comprehensively studied the quantum discord, which is supposed to account for all the nonclassical correlations present in a bipartite state (including entanglement), and provide computational speedup and quantum enhancement even in separable states. Firstly, we introduce several different indicators of nonclassical correlations, including their definitions and interpretations, mathematical properties, and the relationship between them. Secondly, we review two major topics of quantum discord. One is the remarkable behavior at quantum phase transitions. The pairwise quantum discord for nearest neighbors as well as distant spin pairs can perfectly signal the critical behavior of many physical models, even at finite temperatures. The other is quantum discord dynamics in open systems, especially for "system-spin environment" models. Quantum discord is more robust than entanglement against external perturbations. It can be created, greatly amplified or protected under certain conditions, and presents promising applications in quantum technologies such as quantum computers.

The dynamics of geometric discord (GD) and its transfer in a dissipative system consisting of two independent atom-cavity-reservoir subsystems under the strong coupling and the weak coupling regimes is studied. It is shown that the GD of the atoms and the cavities oscillatorily decays to zero while the reservoirs begin to present nonzero geometric quantum discord already immediately after t = 0 in the strong coupling regime. However, in the weak coupling regime, the GD between the atoms progressively decays becoming zero and the discord between the reservoirs arises from zero to a steady value, while the cavities remain almost uncorrelated during the evolution. We also show that the amount of GD contained in atoms and reservoirs depends on the purity p and it is proportional to p, the smaller the value of p the smaller the amount of GD. It is worth noting that, in both strong coupling and the weak coupling regimes, the results show that GD initially stored in the atoms will eventually be completely transferred to the reservoirs, independent of the parameters, but the transfer is mediated via the cavities in the strong coupling regime, while it is almost directly in the weak coupling regime.

In this paper, we investigate the dynamical behaviors of quantum correlations witnessed by geometric discord and negativity when two three-level spin-1 atoms exist in the optical lattice. The results show that the GD can detect the critical point K = J at finite temperature associated with the quantum phase transition which separates the superfluid phase from the Mott insulator phase, while the negativity cannot. In addition, the system undergoes an entanglement sudden death (ESD), but the GD always exists, meanwhile, the GD is more robust than negativity against temperature T.

We investigate the influence of the composite effect and information backflow effect in non-Markovian channel on the dynamics of quantum correlation including quantum entanglement and quantum discord. It is found that, the composite effect of independent channels is not only harmful to the maintenance of quantum correlation but also unfavorable for the maintenance of classic correlation. In a non-Markovian channel, by regulating the discord between qubit and the center frequency of cavity model, the time of quantum correlation and classical correlation of the system can be effectively prolonged. Thus, the quantum information processing can be achieved more easily under larger detuning.

Quantum correlations are essential for quantum information processing (QIP). Measurement-induced nonlocality (MIN) is a good measure of quantum correlation, and is favored for its conceptual implication and potential application. We investigated here the particular behaviors of the geometric and entropic measures of MIN in the two-qubit Heisenberg XY model and revealed the effects of anisotropic parameter γ and the external magnetic field B on them. Our results showed that both γ and B can serve as efficient controlling parameters for tuning MIN in the XY model.

Quantum correlations are considered analytically in a model that contains two coupled superconducting charge qubits sharing a large Josephson junction. Dynamical properties for the reduced two-qubit state are addressed in the quantum evolution when the coupling between the field and the qubits is either constant or time-dependent. In the latter case, adiabatic and nonergodic features are present for extremely low and fast evolutions, respectively. Finally, the case where the coupling drops to zero is also considered.

We investigate in this paper measurement-induced disturbance (MID) and negativity in a two-spin-qutrit model by considering the influence of the external magnetic field, nonlinear coupling parameter, the uniaxial field and temperature. It is shown that all of these parameters play a significant role in negativity and MID. We make an explicit comparison between the negativity and MID and disclose some interesting results. By the way, we find that negativity is a better measure than MID to detect the sudden point in a finite temperature, which is obviously different from the previous findings.

Discord and entanglement characterize two kinds of quantum correlations, and discord captures more correlation than entanglement in the sense that even separable states may have nonzero discord. In this paper, we propose a new kind of quantum correlation that we call as oblique discord. A zero-discord state corresponds to an orthonormal basis, while a zero-oblique-discord state corresponds to a basis which is not necessarily orthogonal. Under this definition, the set of zero-discord states is properly contained inside the set of zero-oblique-discord states, and the set of zero-oblique-discord states is properly contained inside the set of separable states. We give a characterization of zero-oblique-discord states via quantum mapping, provide a geometric measure for oblique discord, and raise a conjecture, which if it holds, then we can define an information-theoretic measure for oblique discord. Also, we point out that the definition of oblique discord can be properly extended to some different versions just as the case of quantum discord.

We use the example of playing a 2-player game with entangled quantum objects to investigate the effect of quantum correlation. We find that for simple game scenarios it is classical correlation that is the central feature and that these simple quantum games are not sensitive to the quantum part of the correlation. In these games played with quantum objects it is possible to transform a game such as Prisoner's Dilemma into the game of Chicken. We show that this behavior, and the associated enhanced equilibrium payoff over playing the game with quantum objects in nonentangled states, is entirely due to the classical part of the correlation.

Generalizing these games to the pure strategy 2-player quantum game where the players have finite strategy sets and a projective joint measurement is made on the output state produced by the players, we show that a given quantum game of this form can always be reproduced by a classical model, such as a communication channel. Where entanglement is a feature of the these 2-player quantum games the matrix of expected outcomes for the players can be reproduced by a classical channel with correlated noise.

In this paper, we consider a hidden variable theoretical description of successive measurements of non-commuting spin observables on an input spin-s state. Although these spin observables are non-commuting, they act on different states, and so the joint probabilities for the outputs of successive measurements are well-defined. We show that, in this scenario, hidden variable theory (HVT) leads to Bell-type inequalities for the correlation between the outputs of successive measurements. We account for the maximum violation of these inequalities by quantum correlations (i.e. the correlations of successive measurements on a quantum state) by varying the spin value and the number of successive measurements. Our approach can be used to obtain a measure of the deviation of Quantum Mechanics from the theory obeying realism and time-locality, in terms of the amount of classical information needed to be transferred between successive measurements in order to simulate the above-mentioned correlations in successive measurements.

We study the effect of external electric bias on the quantum correlations in the array of optically excited three coupled semiconductor quantum dots. The correlations are characterized by the quantum discord and concurrence and are observed using excitonic qubits. We employ the lower bound of concurrence for thermal density matrix at different temperatures. The effect of the Förster interaction on correlations will be studied. Our theoretical model detects nonvanishing quantum discord when the electric field is on while concurrence dies, ensuring the existence of nonclassical correlations as measured by the quantum discord.

We show that the sudden change of quantum correlation can occur even when only one part of the composite entangled state is exposed to a noisy environment. Our results are illustrated through the action of different noisy environments individually on a single qubit of quantum system. Composite noise on the whole of the quantum system is thus not the necessarily condition for the occurrence of sudden transition for quantum correlation.

The characterization of quantum discord (QD) and geometric discord (GD) has mostly concentrated on two-qubit states since the minimization in both discords is a daunting task for high-dimensional states. Numerical calculations of both discords are carried out for a generic bipartite state. When one-dimensional orthogonal projectors for a local measurement on n-dimensional Hilbert space are realized by the generators and the Euler angles of SU(n), the optimal measurements have a figure of merit that includes n(n - 1) Euler parameters. As an representative example, such projectors and two kinds of algorithms are used to estimate both discords for two-qutrit mixed states in recent literature. The generalized negativity as a measure of quantum entanglement is calculated for reference purposes. For those states with one parameter the discords and the negativity respectively display the nonlinear and the linear function of the parameter, with different turning points. However, they are positively correlated in the suitable ranges of the parameter for those states. The hierarchy of those quantities is discussed as well. Those shed new light on the understanding of QDs and quantum entanglement of mixed states in high-dimensions.

We have investigated the quantum discord (QD) of the thermal density matrix of spin-1/2 Heisenberg chains with Dzyaloshinskii–Moriya (DM) interaction. With fermionization technique, we study the mutual effect of DM interaction and the external magnetic field on the QD and the entanglement. Our analysis implies that the DM interaction can enhance the QD while the external magnetic field will shrink the QD. By a comparison between the entanglement and the QD, we find that the QD is more robust to the temperature and to the external magnetic field than the entanglement of formation (EoF) in the sense that the EoF takes a zero value while the QD does not for high temperature and strong external magnetic field. This point confirms the conclusion that there exist some separable states containing non-zero QD.

We give the analytic expression for the information-theoretical symmetric discord of one type of two-qubit X states which depend on only four real parameters. As an application, we study the symmetric discord in a model, which shows that the numerical and analytic results are well consistent.

The correlations between a pair of spins in a many-spin state encoded in the diagonal and off-diagonal spin–spin correlation functions. These spin functions determine the quantum correlation measures, like pair-wise concurrence, quantum discord and other measures of quantum information. We show that for isotropic and translationally invariant states, the quantum correlations depend only on the diagonal spin correlation function. The pair concurrence shows a strict short-ranged behavior. The distribution of concurrence for a random W-like state exhibits a long tail for both time-reversal invariant states and for states that break the time reversal. The quantum discord can be related to the diagonal spin correlation function. As the spin function is long range close to a critical point, analogously the quantum discord exhibits a long range behavior. For the isotropic state, the conditional entropy distribution is a Dirac delta function, whereas it has a twin-peak structure for the anisotropic model.