We study the quantum discord in a two-spin-1/2 XXZ model in thermal equilibrium at temperature T in the presence of an external magnetic field B. Special attention is paid to the dependence of quantum discord on the temperature T and magnetic field B. It is found that quantum thermal discord is more robust than concurrence against temperature, in the sense that quantum thermal discord does not vanish at finite temperatures, but concurrence vanishes completely at a critical temperature.

https://iopscience.iop.org/article/10.1088/1674-1056/19/10/100311/pdf

Quantum thermal discord in a two-spin-1/2 XXZ model

In this paper the so-called sudden death effect of entanglement is investigated in a quantum model. The results show that one can expect the resurrection of the original entanglement to occur in a periodic way following each sudden death event. The length of the time interval for the zero entanglement depends not only on the degree of entanglement of the initial state but also on the initial state.

Sudden death of entanglement of a quantum model

Considering two artificial identical atoms interacting with two-mode thermal field through non-degenerate two-photon transitions, this paper studies the influence of atomic coherence and dipole–dipole interaction on the entanglement of two qubits. It is found that the entanglement is greatly enhanced by these mechanisms.

The influence of atomic coherence and dipole–dipole interaction on entanglement of two qubits with nondegenerate two-photon transitions

This paper proposes a scheme for realization of a three-qubit Toffoli gate operation using three four-level atoms by a selective atom-field interaction in a cavity quantum electrodynamics system. In the proposed protocol, the quantum information is encoded on the stable ground states of atoms, and atomic spontaneous emission is negligible as the large atom-cavity detuning effectively suppresses the spontaneous decay of the atoms. The influence of the dissipation on fidelity and success probability of the three-qubit Toffoli gate is also discussed. The scheme can also be applied to realize an N-qubit Toffoli gate and the interaction time required does not rise with increasing the number of qubits.

http://iopscience.iop.org/article/10.1088/1674-1056/18/1/010/pdf

Realization of Toffoli gate operation using four-level atoms in cavity QED system

The entanglement dynamics of system, where atoms A and B interact with single mode cavity fields a and b respectively, is studied The interaction between atom A and cavity a may be described by using the typical Jaynes–Cummings model, while that between the atom B and cavity b filled with a Kerr medium is of a two-photon process For a certain initial atom entanglement state, there is an entanglement sudden death effect between the two atoms The Kerr medium in the cavity b can effectively prevent the undesirable entanglement sudden death from occurring Also, from the viewpoint of the population dynamics, we discuss why the Kerr medium can do so

Control of entanglement sudden death by Kerr medium

This paper studies the entanglement properties in a system of two dipole—dipole coupled two-level atoms resonantly interacting with a single-mode thermal field. The results show that, when the temperature of the cavity is high enough (corresponding to the large value of the mean photon number), the entanglement is greatly enhanced due to the initial atomic coherence. These results are helpful for controlling the atomic entanglement by changing the initial parameters of the system.

Coherence-enhanced entanglement between two atoms at high temperature

In this paper, we propose a physical scheme to realize quantum SWAP gate by using a large-detuned single-mode cavity field and two identical Rydberg atoms. It is shown that the scheme can also be used to create multi-atom cluster state. During the interaction between atom and cavity, the cavity is only virtually excited and thus the scheme is insensitive to the cavity field states and cavity decay. With the help of our scheme it is very simple to prepare the N-atom cluster state with perfect fidelity and probability. The practical feasibility of this method is also discussed.

Efficient scheme of quantum SWAP gate and multi-atom cluster state via cavity QED ∗

In this paper, we investigate the entanglement dynamics of two entangled two-level atoms interacting with a single-mode field in a dissipative cavity. The results show that the evolution properties of the entanglement degree of two entangled atoms depend on the initial atomic entanglement degree and form, the average photon number of the cavity field, and the cavity leakage rate. When the atoms are initially in a particular entangled state, the degree of entanglement can be amplified and not influenced by the dissipation of the cavity field.

The entanglement dynamics of two entangled atoms in the dissipative cavity

Based on two atoms and two cavities initially in two pairs of atom-photon nonmaximally entangled states, we propose a relatively simple scheme to create maximally entangled photon-photon and atom-photon states via entanglement swapping using techniques of cavity QED inspired by the scheme proposed in [Phys. Rev. A 71 (2005) 044302] and [Phys. Rev. A 71 (2005) 034312]. Our scheme does not involve the measurement in Bell basis, we only require detecting the states of atoms.

Scheme for Generation of Maximally Entangled States via Entanglement Swapping

We propose a scheme for generating maximally entangled states for two or more two-level atoms in a thermal cavity. The cavity frequency is large-detuned from the atomic transition frequency, so the Hamiltonian can be expressed as an effective form. The photon-number-dependent parts in the effective Hamiltonian are cancelled with the assistance of a strong classical field, thus the scheme is insensitive to both the cavity decay and the thermal field. The scheme can be used to generate multi-atom Bell-state and Greenberger–Horne–Zeiliner (GHZ) state.

https://iopscience.iop.org/article/10.1088/0253-6102/47/3/037/pdf

Jiang Chun-Lei

Papers

Coherence-enhanced entanglement between two atoms at high temperature

Efficient scheme of quantum SWAP gate and multi-atom cluster state via cavity QED ∗

The entanglement dynamics of two entangled atoms in the dissipative cavity

Scheme for Generation of Maximally Entangled States via Entanglement Swapping

Generation of Maximally Entangled States for Many Two-Level Atoms in a Thermal Cavity