The evolution and stability of quantum correlations in dissipative cavity

We investigate the dynamics of quantum correlations such as entanglement and quantum discord between two atoms in a lossy cavity. It is found that a stable quantum discord could be induced even when the atoms remain separable at all times. Also, we show that it is possible to amplify and protect the quantum discord under cavity decay for certain types of initial states. Moreover, entanglement decoherence-free subspaces are obtained which may be useful in quantum information and quantum computation.     

Dynamics of quantum steerability for two atoms in coupled cavities

We study the dynamics of quantum steerability between two non-interacting atoms, each of which is trapped inside one of two coupled cavities. Compared with entanglement, quantum steerability manifests sudden birth and sudden death phenomenon during the time evolution. We find that the cavity decay plays a destruction role for both steerability and entanglement. It is also shown that the survival time as well as the maximal value of steerability are sensitive to the asymmetry of the cavities. Moreover, it is found the sudden death of steerability can be controlled by the hopping rate of the coupled cavities.     

Frozen geometric discord in dissipative cavity quantum electrodynamics system

We investigate the dynamics of geometric quantum correlations for certain decoherence channels and discuss the necessary conditions for the existence of frozen geometric discord. As illustrative examples, we study the phenomenon of double sudden transitions in geometric discord for a system consisting of two noninteracting atoms inserted in two independent dissipative cavities and how the initial state parameters and decay rate of dissipative cavities affect the frozen time during which the geometric discord remains constant. We also explore the dynamics of geometric discord between two noninteracting atoms trapped in a common dissipative cavity and find that the geometric discord exhibits sudden transition between classical and quantum decoherence. Moreover, a nonzero stationary geometric discord can arise in both the independent cavity case and common cavity case.     

Controlling entanglement dynamics of two qubits coupled to a common reservoir via a coherent field

We investigate the time evolution of entanglement between two two-level atoms which are coupled to a common multimode electromagnetic reservoir and simultaneously driven by a coherent field. We find that the entanglement can always be created and maintained with a moderate intensity of the driving field during the track of approaching steady entangled states when both atoms are initially in their ground states and the reservoir is in the vacuum state or the squeezed vacuum state. We also show that the steady-state entanglement between the atoms can be enhanced by use of the coherent field when the reservoir is in the weakly squeezed vacuum state. More interestingly, in the squeezed reservoir case, the sudden death period in the time evolution of the entanglement can be removed by use of the coherent driving field.     

Enhancement of discord and entanglement with detuning for two coupled quantum dots in a driven cavity

We study the quantum discord for a system of two identical coupled quantum dots interacting with quantized cavity field in the presence of cavity as well as dot decay and detuning. The cavity is externally driven by a coherent light. These results are compared with the entanglement of the quantum dots in various parameter regimes in which system may or may not show bistability. We show that the discord in the steady state is nonzero for any nonzero cavity field amplitude. The system has higher discord in the upper branch of the bistability curve where the entanglement is zero. We also find many other interesting results including high discord and entanglement in the presence of detuning, a phenomenon which we further examine by approximating the density matrix in the appropriate limit.     

Continuous-Variable Quantum Network Coding Based on Quantum Discord

Establishing entanglement is an essential task of quantum communication technology. Beyond entanglement, quantum discord, as a measure of quantum correlation, is a necessary prerequisite to the success of entanglement distribution. To realize efficient quantum communication based on quantum discord, in this paper, we consider the practical advantages of continuous variables and propose a feasible continuous-variable quantum network coding scheme based on quantum discord. By means of entanglement distribution by separable states, it can achieve quantum entanglement distribution from sources to targets in a butterfly network. Compared with the representative discrete-variable quantum network coding schemes, the proposed continuous-variable quantum network coding scheme has a higher probability of entanglement distribution and defends against eavesdropping and forgery attacks. Particularly, the deduced relationship indicates that the increase in entanglement is less than or equal to quantum discord.     

Entanglement between two Tavis–Cummings atoms with phase decoherence

Taking the intrinsic decoherence effect into account, we investigate the dynamical behavior of entanglement between two Tavis–Cummings atoms coupled to a single mode field. We find that entanglement characters including the stationary entanglement and the so-called sudden death effect are sensitive to initial atomic state and photon numbers.     

Time evolution of entanglement and bistability of two coupled quantum dots in a driven cavity

Abstract

The time evolution of entanglement between two quantum dots (QDs) trapped inside a cavity driven by a coherent quantized field is studied. In the presence of dissipation, entanglement shows many interesting features such as sudden death and revival, and finite steady state value after sudden death. We also investigate dependence of entanglement on dot variables and its relation to bistability. It is found that entanglement vanishes when the cavity field intensity approaches the upper branch of the bistability curve. When the cavity is driven by a modulated field in the presence of dissipation, it can periodically generate entanglement, which is much larger than the maximum value attained in the steady-state for this system but the dots are never fully entangled.     

Quantifying quantumness of correlations using Gaussian Rényi-2 entropy in optomechanical interfaces

Using the Gaussian Rényi-2 entropy, we analyse the behaviour of two different aspects of quantum correlations (entanglement and quantum discord) in two optomechanical subsystems (optical and mechanical). We work in the resolved sideband and weak coupling regimes. In experimentally accessible parameters, we show that it is possible to create entanglement and quantum discord in the considered subsystems by quantum fluctuations transfer from either light to light or light to matter. We find that both mechanical and optical entanglement are strongly sensitive to thermal noises. In particular, we find that the mechanical one is more affected by thermal effects than that optical. Finally, we reveal that under thermal noises, the discord associated with the entangled state decays aggressively, whereas the discord of the separable state (quantumness of correlations) exhibits a freezing behaviour, seeming to be captured over a wide range of temperature.     

Quantum Entanglement and Correlations in Superconducting Flux Qubits

We report on the quantum correlations dissipative dynamics followed by coupled superconducting flux qubits. The coupling between the superconducting quantum register and the reservoir is described by two different mechanisms: collective and independent decoherence. By means of the Bloch?CRedfield formalism, we solve the quantum master equation and show that coupling under collective quantum noise is more robust to decoherence. This result is demonstrated for different flux qubit initial preparations, taking into account the influence due to external fields and temperature. Furthermore, we compute the entanglement and the quantum discord dissipative dynamics as controlled by external parameters. We show that the discord is more robust against decoherence effects. This fact could be harnessed in the realization of quantum computing tasks that do not need to invoke entanglement in their implementation.     

Effect of the Stark shift on entanglement in a double two-photon JC model

Considering a double two-photon JC model, we investigate the entanglement between the two two-level atoms and that between the two cavity fields, and study the effect of the Stark shift on entanglement. The results show that, on the one hand the atom–atom and cavity–cavity concurrences evolve periodically with time and the periods are affected by the Stark shift; on the other hand, the two atoms are not disentangled at any time when the Stark shift is considered, and for large values of the Stark shift parameter, the two atoms can remain in a stationary entangled state. In addition, we find that the so-called entanglement sudden death can occur under appropriate conditions on the dynamic Stark shift for a certain initial state of the system.     

Entanglement sudden birth and sudden death in a system of two distant atoms coupled via an optical element

An investigation is reported of the collective effects and the dynamics of atom–atom entanglement in a system of two distant two-level atoms which are coupled via an optical element. In the system under consideration, the two atoms, which are trapped in the foci of a lens, are coupled to a common environment being in the vacuum state and they emit photons spontaneously. A fraction of the emitted photons from each atom is thus focused on the position of the other atom. The presence of optical element between two distant atoms leads to the occurrence of delayed collective effects, such as delayed dipole–dipole interaction and delayed collective spontaneous emission, which play the crucial role in the dynamical behaviour of the entanglement. We discuss the phenomena of entanglement sudden birth, entanglement sudden death, and revival of entanglement for both cases of initial one-photon and initial two-photon unentangled atomic states. We show that the evolution of the entanglement is sensitive not only to the interatomic distance but also to the initial state of the system as well as to the properties of the optical element.     

Entanglement induced by spontaneous emission in spatially extended two-atom systems

Abstract

The role of the collective antisymmetric state in entanglement creation by spontaneous emission in a system of two non-overlapping two-level atoms has been investigated. Populations of the collective atomic states and the Wootters entanglement measure (concurrence) for two sets of initial atomic conditions are calculated and illustrated graphically. Calculations include the dipole-dipole interaction and a spatial separation between the atoms that the antisymmetric state of the system is included throughout even for small interatomic separations. It is shown that spontaneous emission can lead to a transient entanglement between the atoms even if the atoms were prepared initially in an unentangled state. It is found that the ability of spontaneous emission to create transient entanglement relies on the absence of population in the collective symmetric state of the system. For the initial state of only one atom excited, entanglement builds up rapidly in time and reaches a maximum for parameter values corresponding roughly to zero population in the symmetric state. On the other hand, for the initial condition of both atoms excited, the atoms remain unentangled until the symmetric state is depopulated. A simple physical interpretation of these results is given in terms of the diagonal states of the density matrix of the system. We also study entanglement creation in a system of two non-identical atoms of different transition frequencies. It is found that the entanglement between the atoms can be enhanced compared to that for identical atoms, and can decay with two different time scales resulting from the coherent transfer of the population from the symmetric to the antisymmetric state. In addition, it was found that a decaying initial entanglement between the atoms can display a revival behaviour.     

Decay and survivability of entanglement in two-dimensional ionic motions

This contribution describes some new features of quantum entanglement in a single three-level trapped ion confined in a two-dimensional (2D) harmonic potential. The dynamical properties of quantum entanglement are studied in terms of the reduced-density linear entropy considering two specific initial states of the field. Allowing the instantaneous position of the centre-of-mass motion of the ion to be explicitly time dependent, it is shown that either sudden death of entanglement or survivability of quantum entanglement can be obtained with a specific choice of the initial state parameters. The difference in evolution picture corresponding to the multi-quanta processes is discussed.     

On the quantumness of correlations in nuclear magnetic resonance

Nuclear magnetic resonance (NMR) was successfully employed to test several protocols and ideas in quantum information science. In most of these implementations, the existence of entanglement was ruled out. This fact introduced concerns and questions about the quantum nature of such bench tests. In this paper, we address some issues related to the non-classical aspects of NMR systems. We discuss some experiments where the quantum aspects of this system are supported by quantum correlations of separable states. Such quantumness, beyond the entanglement-separability paradigm, is revealed via a departure between the quantum and the classical versions of information theory. In this scenario, the concept of quantum discord seems to play an important role. We also present an experimental implementation of an analogue of the single-photon Mach-Zehnder interferometer employing two nuclear spins to encode the interferometric paths. This experiment illustrates how non-classical correlations of separable states may be used to simulate quantum dynamics. The results obtained are completely equivalent to the optical scenario, where entanglement (between two field modes) may be present.     

Non-classical correlations in the general state of two SC-qubit with a phase damping: non-local correlation and geometric discord

A system consists of two charged qubits are initially prepared in a maximum entangled Bell state and having no mutual interaction, where each qubit interacts independently with a superconducting transmission line resonator. An analytical solution of the time evolution of the final state of the system with the effect of a phase decoherence is found. In previous works, quantum correlations are only investigated in X-state for the models which are as our model. In this work, the analytical formulas of the geometric quantum discord (GQD) and measurement-induced non-locality (MIN) are introduced for a general state of two-qubit (non-X-state). Quantum correlations are studied via GQD and MIN with quantum entanglement (QE). It is found that a sudden disappearance only occurs for QE, while MIN and GQD still exist. Due to the increase in the amplitude of the coherent states, the intervals of the sudden disappearance of QE increase and MIN and GQD decrease. It is interesting to note that initial correlations can be lost and they reach their stationary correlations with the increase in the intrinsic decoherence. The stationary correlation of MIN can be destroyed, it reach zero value, when both the decoherence effect and detuning are present simultaneously. By starting with different types of Bell-like states, the stationary correlations as well as the time intervals of sudden disappearance have notable changes. It is possible to control the quantum correlations with certain parameter sets.     

Decay of two-qubit correlation in correlated stochastic dephasing

Time evolutions of quantum correlation, classical correlation and total correlation of two qubits are investigated when the two qubits are placed under the influence of classical phase fluctuations with correlation. Stochastic variables that describe the phase fluctuations are correlated and subject to the stationary Gauss–Markov process. The model includes the local and global dephasing models. It is shown that the quantum correlation measured by the quantum discord is increased by the correlation between the stochastic variables in the initial time region while the classical correlation and the total correlation are not. Furthermore the entanglement, the optimal fidelity of the quantum teleportation and the violation of the Bell inequality are investigated.     

Researching the Link Between the Geometric and Rènyi Discord for Special Canonical Initial States Based on Neural Network Method

Quantum correlation which is different to the entanglement and classical correlation plays important role in quantum information field. In our setup, neural network method is adopted to simulate the link between the Rènyi discord (α = 2) and the geometric discord (Bures distance) for special canonical initial states in order to show the consistency of physical results for different quantification methods. Our results are useful for studying the differences and commonalities of different quantizing methods of quantum correlation.     

Creating quantum entanglement between multi-atom Dicke states and two cavity modes

We present a scheme to create quantum entanglement between multi-atom Dicke states and two cavity modes by passing N three-level atoms in Λ configuration through a resonant two-mode cavity one by one. We further show that such a scheme can be used to generate arbitrary two-mode N-photon entangled states, arbitrary superposition of Dicke states, and a maximal entangled state of Dicke states. These states may find applications in the demonstration of quantum non-locality, high-precision spectroscopy and quantum information processing.     

Classification and supermultiplet structure of doubly excited states

Based upon the analysis of electron correlations in hyperspherical coordinates for two-electron atoms for arbitrary L, S and π states, a classification scheme for all doubly excited states is presented. A new set of correlation quantum numbers K, T and A, is introduced. Here (K, T) describes wave functions in hyperspherical coordinates, it is shown that states with identical (K, T)^A have isomorphic correlations. This isomorphism is shown to be the origin of the supermultiplet structure of doubly excited states.