Narrow Results

For bipartite quantum states we obtain lower bounds on two important entanglement measures, concurrence and negativity, studying the inequalities for the expectation value of a projector on some subspace of the Hilbert space. Several applications, including analysis of stability of entanglement under various perturbations of a state, are discussed.

In this presentation, we considered the atomic decay (but not for the cavity) for a two-level atom interacting with a single mode of electromagnetic field. The exact solution of the master equation in the case of a high-Q cavity with atomic decay is found. The effects of the atomic damping for a thermal reservoir on the temporal evolution of partial entropies of the atom or the field, and the total entropy as a quantitative entanglement, was measured. The degree of entanglement, through the sum of the negative eigenvalues of the partially transposed density matrix and the negative mutual information, has been studied and compared with other measures.

In this paper, we use the negativity and geometric quantum discord (GQD) to investigate the quantum phase transition (QPT) in the anisotropic spin-1/2 XY model with staggered Dzyaloshinskii–Moriya (DM) interaction using the quantum renormalization-group method, and the results show that the negativity and GQD can both obtain the quantum critical points associated with QPTs after several iterations of the renormalization. In addition to this, we discuss how the strength of the DM interaction and anisotropic parameter make the effect on the negativity and GQD for different RG steps. At last, we give out the relationship between negativity and GQD, and verify that with anisotropy parameter and the strength of DM interaction increased, the squared negativity and the normalization of GQD tend to equal in this model.

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 analytically investigate the thermal entanglement of three-mixed-spin (1/2, 1, 1/2) XXZ model with the DM interaction under an external magnetic field B. Two different cases are considered: one subsystem (1/2, 1/2) consists of two spin-1/2 fermions and the other subsystem (1/2, 1) contains a spin-1/2 fermion and a spin-1 boson. It is shown that the DM interaction parameter D, the external magnetic field strength B and coupling constant J have different effects on Fermi and mixed Fermi–Bose systems. All of the factors mentioned above can be utilized to control entanglement switch of any two particles in mixed spins model.

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.

Our aim is to present an analytical solution for the quantum optics model that has two two-level atoms interacting together and interacting with an intensity-dependent quantized field. The quantized field is initially in binomial state and based on the Lamb–Dicke parameter and associated Laguerre polynomials. While two atoms are symmetric and initially in superposition state. Besides, the quantum entanglement between the quantum system components under the effect of the parameters of detuning, coupling, binomial, Lamb–Dicke and the initial state of the model is discussed. Several phenomena have been clearly observed and analyzed, such as entanglement sudden death (ESD) followed by entanglement sudden birth (ESB) in the concurrence and the negativity curves, and long-lived quantum coherence in the von Neumann entropy curve. In addition, the presence of revivals and collapses in the atomic population inversion curves has been figured out. Our analysis aims to help the scientists to determine the critical values required of quantum system parameters to control entanglement dynamics.

The entanglement characteristics of the three-qutrit spin-one Heisenberg chain in the presence of one impurity site is investigated by using the concept of negativity. The results show that the existence of impurity can lead to an enhancement of both bipartite and pairwise entanglement for certain ranges of the impurity parameter, and the amount of entanglement can be controlled and tuned by varying the impurity parameter J₁. The results also show that the thermal fluctuations always suppress the entanglement and as a result there exists a threshold temperature T_th after which the entanglement vanishes. T_th increases as J₁ increases except the case for that of the pairwise negativity which measures entanglement between the two norm spins in the chain.

By using the concept of negativity, we determine entanglement features of the tetrahedral lattice with Heisenberg interactions. It is found that except for the case of (1/2-1/2-1-1) mixed spins, there is no pairwise entanglement between any two spins at any temperature when B = 0. When B increases after B = 0, the ground-state negativities exhibit square wave or stepwise structures, and experience sharply descents to lower values at some energy-level crossing points. For thermal entanglement, the negativities exhibit ridge-like or staircase structures as B increases, and there are regions in the B–T plane where an increase of temperature may help to enhance the entanglement.

Quantum information protocols are often designed in the ideal situation with no decoherence. However, in real setup, these protocols are subject to the decoherence and thus reducing fidelity of the measurement outcome. In this work, we analyze the effect of state-dependent bath on the quantum correlations and the fidelity of a single qubit teleportation. We model our system-bath interaction as qubits interacting with a common bath of bosons, and the state dependence of the bath is generated through a projective measurement on the joint state in thermal equilibrium. The analytic expressions for the time evolution of entanglement, discord and average fidelity of quantum teleportation are calculated. It is shown that due to the presence of initial system-bath correlations, the system maintains quantum correlations for long times. Furthermore, due to the presence of finite long-time entanglement of the quantum channel, the average fidelity is shown to be higher than its classical value.

In this paper, we study the interaction of Nitrogen Vacancies in Diamond (NVD) with quantized cavity field. The system is explored analytically and the effect of the system parameters is analyzed. The stability of a quantum system with influencing factors is investigated using the Mandal Parameter. The generated correlation between the NVD and the quantized cavity field is quantified using the negativity. This study also investigates geometric phase and its dependence on the system parameters. The results show that this system holds great potential applications in quantum computation and quantum memory. Additionally, the features of the system can be controlled by the system parameters.

We study the thermal entanglement of a 2-qutrit spin chain with nonlinear coupling in the presence of nonuniform magnetic field. Thermal entanglement of an arbitrary (finite-dimensional) m-partite system vanishes at some finite threshold temperature T_s. We investigate the dependence of T_s on the system's parameters, i.e. the nonlinear coupling and the magnetic field, for this 2-qutrit system. In addition, we compare two lower bounds of I-concurrence for this system and also study its dense coding capacity as a function of system's parameters.

The pairwise entanglement dynamics in a multipartite system consisting of three two-level atoms A, B, C and a single-mode cavity field a is studied via negativity. Three atoms are arranged in such a way that atoms BC are embedded in and locally interact with the cavity while atom A is located in a spatially separate place outside of the cavity. Initially, atom-pair AB is prepared in a Bell-like state while atom C in a superposition of ground and excited state, |g_C〉 and |e_C〉. It shall be shown that all the pairwise negativities of the total system including atoms and cavity undergo qualitatively different evolutions. The so-called entanglement sudden death is observed for atom-pair AB under certain conditions and the entanglement transfer among all the possible degrees of freedom of the whole system is also discussed.

We studied the entanglement properties of the system consisting of two atoms in Bell states interacting with the two-mode odd-even entangled coherent field by utilizing the complete quantum theory. The influences of the initial Bell state of the atoms, the intensity of field, the coupling strength of interaction between atoms and the initial degree of the entanglement between the two-mode fields on the atomic entropy and on negativity are discussed by using numerical calculations. It turns out that the two atoms can keep their initial maximal entangled state while the two-atom and the two-mode field keep their initial disentangled state if the two atoms are initially in |β₁₁〉. Therefore, for the initial atomic state |β₀₀〉 or |β₀₁〉, the increasing of the field intensity can be a help to prepare the atom-field entanglement, and for |β₁₀〉, it is beneficial for the atom–atom entanglement. Meanwhile, for the initial atomic state |β₁₀〉 and |β₀₁〉, strong coupling strength of interaction between atoms is beneficial for the atom–atom entanglement, but for |β₀₀〉, it can be helpful for the atom-field entanglement.

The quantum nonlocal correlation between an atom and coherent field is described quantitatively in terms of multi-photon and phase damping processes. Especially, considering a two-level atom interacts with a single-mode quantized field in a coherent state inside a phase-damped cavity, and taking into account the number of multi-photon transitions and phase damping effect, the entanglement is investigated during the time evolution as a function of involved' parameters in the system. The results show that the enhancement of the transitions are very useful in generating a high amount of entanglement. Due to the significance of how a system is quantum correlated with its environment in the construction of a scalable quantum computer, the entanglement dynamics between the bipartite system with its environment is evaluated and investigated during the dissipative process. Finally, the physical interpretation of the correlation behaviors between the subsystems is explained through the statistical properties of the field.

Heat capacity of a mixed-three-spin (1/2,1,1/2) antiferromagnetic XXX Heisenberg chain is precisely investigated by use of the partition function of the system for which, spins (1,1/2) have coupling constant $J_1$ and spins (1/2,1/2) have coupling constant $J_2$. We verify tripartite entanglement for the model by means of the convex roof extended negativity (CREN) and concurrence as functions of temperature T, homogeneous magnetic field B and the coupling constants $J_1$ and $J_2$. As shown in our previous work, [H. A. Zad, Chin. Phys. B25 (2016) 030303.] the temperature, the magnetic field and the coupling constants dependences of the heat capacity for such spin system have different behaviors for the entangled and separable states, hence, we did some useful comparisons between this quantity and negativities of its organized bipartite (sub)systems at entangled and separable states. Here, we compare the heat capacity of the mixed-three-spin (1/2,1,1/2) system with the CREN and the tripartite concurrence (as measures of the tripartite entanglement) at low temperature. Ground state phase transitions, and also, transition from ground state to some excited states are explained in detail for this system at zero temperature. Finally, we investigate the heat capacity behavior around those critical points in which these quantum phase transitions occur.

We consider a one-dimensional, mixed spin Heisenberg XXX model with an homogeneous external magnetic field and Dzyaloshinskii–Moriya interaction. Alternating spin-$\frac{1}{2}$ and spin-1 particles are forming the chain. The effect of the different parameters of the system on the bipartite thermal entanglement is studied. The type of chain used (mixed) and the size of the chain ($N=5$) allow to study three types of bipartite entanglement, the qubit–qubit, qubit–qutrit and qutrit–qutrit thermal entanglement.

Our study employs a connected correlation matrix to quantify quantum entanglement. The matrix encompasses all necessary measures for assessing the degree of entanglement between particles. We begin with a three-qubit state and involve obtaining a mixed state by performing partial tracing over one qubit. Our goal is to exclude the non-connected sector by focusing on the connected correlation. This suggests that the connected correlation is deemed crucial for capturing relevant entanglement degrees. The study classifies mixed states and observes that separable states exhibit the lowest correlation within each class. We demonstrate that the entanglement measure monotonically increases concerning the correlation measure. This implies that connected correlation serves as an effective measure of quantum entanglement. Finally, our proposal suggests that interpreting quantum entanglement from a local perspective is possible. The observable is described as a vector with locality but violates freedom of choice.

In this talk, we discuss the effect of BMS symmetries on quantum entanglement and its implications in the context of the black hole information paradox. In particular, we illustrate the gravitational memory effect for linear uniformly accelerated observers in a physical process involving a BMS shock-wave without planar/spherical symmetry. This classical memory is accompanied by a quantum memory that modulates the quantum entanglement between the opposing Rindler wedges in quantum field theory. A corresponding phenomenon across the Schwarzschild black hole horizon suggests that the Negativity measure of entanglement between infalling and outgoing Hawking pair should be degraded due to an infalling BMS shockwave while there should be linear order generation of Negativity between two outgoing Hawking particles. Implications are discussed.