Population Dynamics of Excited Atoms in Dissipative Cavities

Population dynamics of excited atoms in dissipative cavities is investigated in this work. We present a method of controlling populations of excited atoms in dissipative cavities. For the initial state |ee〉_AB|00〉_ab, the repopulation of excited atoms can be obtained by using atom-cavity couplings and non-Markovian effects after the atomic excited energy decays to zero. For the initial state |gg〉_AB|11〉_ab, the two atoms can also be populated to the excited states from the initial ground states by using atom-cavity couplings and non-Markovian effects. And the stronger the atom-cavity coupling or the non-Markovian effect is, the larger the number of repopulation of excited atoms is. Particularly, when the atom-cavity coupling or the non-Markovian effect is very strong, the number of repopulation of excited atoms can be close to one in a short time and will tend to a steady value in a long time.     

双光子Jaynes-Cummings模型中的纠缠演化和热纠缠态现象

文中利用共生纠缠度研究了双光子Jaynes-Cummings模型中原子与腔纠缠的时间演化和有限温度下系统热纠缠态.结果表明,腔场中原子与腔展现出周期性的纠缠演化过程,演化周期随原子与腔耦合强度的增大而减小;在有限温度下,系统的共生纠缠度随温度的升高而降低,当趋近临界温度时,系统纠缠现象消失,这一临界温度值与原子-腔耦合强度成正比.对于典型的实验数据,临界温度大约在～10-5 K数量级.     

Deterministic Generation of a Four-Atom Entangled State in a Two-Dimensional Coupled-Cavity System

In this paper, we study the dynamics of a two-dimensional coupled cavity system under one-excitation condition. We show that the dynamics behaviors are strongly affected by the atom-cavity detuning (Δ) and cavity-cavity hopping υ. As a typical application, we apply this system to generate a four-atom W state. Notably, the entangled state generation is deterministic. As a basic research, the result will contribute to the understanding of more complex systems.     

Scalable preparation of three-atom and four-atom W states via atom-cavity-laser interaction

We propose the optical generation of W states for three atomic and four atomic qubits, with each qubit trapped in a separate cavity and coupled to the cavity laser. A single-photon source and two classical fields are employed in the present scheme. By encoding the quantum information of each qubit on the degenerate ground states of the atom, we obtain the atomic entanglement that is relatively stable against spontaneous emission. It is demonstrated that the three- and four-atomic W states can be produced deterministically via a proper manipulation of the atom-cavity interaction sequence and time. Generalization of the present scheme to prepare multi-atomic W states is also discussed.     

Additivity Relations of Quantum Correlations for Three-qubit Entangled State in Atom-cavity System

Taking advantage of quantum discord and negativity respectively, four additivity relations of quantum correlations are studied in the atom-cavity system, for three identical two-level atoms resonantly interact with a single-mode high-Q cavity simultaneously. Through numerical analysis, we find that in this model of atom-cavity system, four additivity relations of negativity are always satisfied well, no matter tripartite are in the pure states or the mixed states, while the results of quantum discord are not always satisfied. What’s more, if and only if the mixed three-atom states evolve into the pure states again, four additivity inequality relations of quantum discord and negativity become equalities simultaneously.     

Preparation of Six-Atom Cluster State via Cavity Quantum Electrodynamics

A cavity quantum electrodynamics scheme for preparing a six-atom cluster state is proposed. In the scheme, the atom-cavity detuning is much bigger than the atom-cavity coupling strength and the time needed to complete the whole procedure is much shorter than the Rydberg-atom lifespan. Thus the scheme is insensitive to the cavity decay and the atom radiation.     

A scheme of quantum repeaters with single atom and cavity-QED

To implement long-distance quantum communication, quantum repeaters have been proposed. The distribution and storage of quantum entanglement are essential to implement quantum repeaters. Here, we propose a new quantum repeaters protocol which is based on single atom-cavity QED. We use simple long-life two-level atoms to store quantum entanglement unlike three-level atoms which are commonly used in other quantum repeaters proposals. The property of long life-time (T₁) and transverse decay time (T₂) between excited level and ground level, such as rare-earth atoms, may store quantum entanglement as long as possible. Modulations of cavity mode and rate of coupling between cavity mode and output mode are also key steps to our scheme. And the efficiency of our protocol is analyzed by quantum trajectory theory.     

Accelerated and Robust Generation of W State by Parametric Amplification and Inverse Hamiltonian Engineering

A scheme to realize the accelerated and robust generation of W state in a cavity quantum electrodynamics (QED) system by combining parametric amplification (PA) with inverse Hamiltonian engineering (IHE) is proposed. The atom-cavity coupling strength can be exponentially enhanced via parametrically squeezing the cavity mode, which facilitates the generation of W state. Moreover, the evolution of the system can be optimized with suppressing the populations of the intermediate states. Numerical simulations show that the scheme is fast and high-fidelity, immune to systematic parameter deviations, robust against spontaneous emission of atoms, and decay of cavities. Therefore, this scheme may provide some useful applications in entanglement generation.     

Preparation of Genuinely Entangled Six-Atom State via Cavity Quantum Electrodynamics

A cavity quantum electrodynamics scheme for preparing a genuinely entangled state [A. Borras, et al., J. Phys. A 40 (2007) 13407] on six two-level atoms is proposed. In the scheme, the atom-cavity detuning is much bigger than the atom-cavity coupling strength and the necessary preparation time is much shorter than the Rydberg-atom lifespan. Hence the scheme has two distinct features, i.e., insensitive to the cavity decay and the atom radiation.     

Quantum coherence and non-Markovianity of an atom in a dissipative cavity under weak measurement

Quantum coherence and non-Markovianity of an atom in a dissipative cavity under weak measurement are investigated in this work. We find that: the quantum coherence obviously depends on the initial atomic state, the strength of the weak measurement and its reversal, the atom-cavity coupling constant and the non-Markovian effect. It is obvious that the weak measurement effect protects the coherence better. The quantum coherence is preserved more efficiently for larger atom-cavity coupling. The stronger the non-Markovian effect is, the more slowly the coherence reduces. The quantum coherence can be effectively protected by means of controlling these physical parameters.     

The effect of atomic motion and two-quanta JCM on the information entropy

We study the interaction between a moving two-level atom and a single-mode field. The coupled atom-cavity system with atomic center-of-mass motion included is modeled by considering the dependence of the atomic motion along z-axis. At exact resonance between the internal atomic transition and the cavity eigenfrequency, an exact solution of the system is obtained and periodically modulated Rabi oscillations and regular translational motion are observed. We focused on the dynamics of both field Wehrl entropy and Wehrl phase distribution. The influence of the atomic motion on the evolution of von Neumann entropy and Wehrl entropy is examined. The results show that the atomic motion and the field-mode structure play important roles in the evolution of the von Neumann entropy, Wehrl entropy and Wehrl PD.     

原子在弱相干场光纤耦合腔系统中的纠缠特性

研究由两个相同的二能级原子分别处于用单模光纤耦合的两弱相干光场系统的共生纠缠特性, 通过数值计算研究了光纤模-腔模与原子-腔模的耦合强度比、弱相干光场的强度和两光场相对相位差等因素对系统纠缠演化的影响. 结果表明: 两腔中的两原子之间、两光场之间和每个腔中的原子与光场之间的纠缠随时间呈现周期或准周期性演化, 两腔场之间的纠缠与腔中的两原子的纠缠可以相互转换, 与两原子之间和两光场之间的纠缠相比, 每个腔中光场与原子之间的纠缠随时间变化的周期缩短. 光纤模-腔模与原子-腔模的耦合强度比与两腔中光场相位差对系统纠缠的影响很大, 较小的光纤模-腔模与原子-腔模的耦合强度之比可以获得较大的系统纠缠度. 关键词： 弱相干场 光纤耦合腔 耦合强度 量子纠缠     

Environment induced entanglement in cavity-QED

The effects of dissipative dynamics on the magnitude of entanglement generated In atom-photon interactions inside cavities is studied. We present some concrete examples of environment Induced entanglement in alom-photon interactions. We consider various dissipative atom-cavity systems and show that their collective dynamics can be used to maximize entanglement for intermediate values of the cavity leakage parameter κ. We first consider the interaction of a single two-level atom with one of two coupled microwave cavities and show analytically that the atom-cavity entanglement increases with cavity leakage. We next consider a system of two atoms passing successively through a cavity and derive the expression for the maximum value of in terms of the Rabi angle gt, for which the two-atom entanglement can be Increased. Finally, numerical investigation of micromaser dynamics also reveals the increase of two-atom entanglement with stronger cavity-environment coupling for experimentally attainable values of the micromaser parameters.     

Robust Scheme for Long-Distance Teleportation of an Unknown Atomic State

A scheme is presented for the long-distance teleportation of an unknown atomic state between two separated cavities. The scheme is based on the Raman coupling and cavity decay. In the scheme, the effective atom-cavity coupling strength is much smaller than the cavity decay rate and thus cavities of high quality factor are unnecesssary.     

Generation of maximally entangled state from a single-photon state in fiber-connected cavities

Starting from the physical model consisting of two remotely located atoms trapped in high finesse fiber-connected cavities, generation of maximally entangled state is firstly realized theoretically from a initial single-photon state. Result shows that under the regime of strong atom-cavity coupling strength, the averaged fidelity can remarkably exceed 0.9980. The distant creation of maximally entangled state may provides certain opportunities and be helpful in the accomplishment of scalable quantum networks.     

制备n原子GHZ类态

我们提出一个方案通过极化光子的干涉来制备囚禁在相距很远的腔中的n原子GHZ类态。在该方案中,原子腔耦合强度小于腔衰减率。因此对腔的品质因子的要求大大降低了,而且,该方案的保真度不受探测器效率和原子自发衰变的影响。 中文关键词：纠缠制备;GHZ类态;腔QED     

Purification of entangled states for two atoms via cavity decay

In this paper a scheme is proposed for the purification of entangled states for two atoms trapped in two distant cavities via cavity decay. In the scheme, the atoms have no probability of being populated in the excited state and thus the atomic spontaneous emission is suppressed. This scheme is valid no matter when the cavity decay rate is larger or smaller than the effective atom-cavity coupling strength. The fidelity of the final state is not affected by the imperfection of the photodetectors.     

Quantum logic operations on two distant atoms trapped in two optical-fibre-connected cavities

Based on the coupling of two distant three-level atoms in two separate optical cavities connected with two optical fibres, schemes on the generation of several two-qubit logic gates are discussed under the conditions of Δ = δ - 2ν cos πk/2 >> g/2 and ν ~ g. Discussion and analysis of the fidelity, gate time and experimental setups show that our schemes are feasible with current optical cavity, atomic trap and optical fibre techniques. Moreover, the atom-cavity- fibre coupling can be used to generate an N-qubit nonlocal entanglement and transfer quantum information among N distant atoms by arranging N atom-cavity assemblages in a line and connecting each two adjacent cavities with two optical fibres.     

Description of charge-exchange resonances in deformed nuclei

The (p, n) and (n, p) transition strength functions with excitation of the GT resonance, other 1⁺ states, spin-dipole with λ^π=0^- andE1 charge-exchange resonances in deformed nuclei in the regions 156≦A≦168 and 236≦A≦240 are calculated in the RPA. It is shown that the GT resonance has a maximum at 18–20 MeV, and in the region of 5–6 MeV around maximum (60–70)% of strength is concentrated. The spindipole resonance with λ^π=0^-, 1^- and 2^? strength is distributed within 14–33 MeV and theE1 strength within 25–29 MeV. The latter is splitted withΔE equal to 0.6–2 MeV into two peaks withI ^π I=1^-0 and 1^?1. In the region of 4–7 MeV around maximum 73–77% ofE1 strength is concentrated. The total (n,p) transition strength is 10–200 times as small as the total (p, n) transition strength.     

Engineering Multi-atom Entangled States via Cavity Decay

A scheme is proposed for the generation of entangled states for multiple atoms trapped in a cavity by detecting photon decay. The scheme is valid no matter when the effective atom-cavity coupling strength is larger than the cavity decay rate or not, which is of importance in view of experiment. The fidelity of entanglement is insensitive to the inefticiency of the photo-detector. The scheme does not require a photon to be initially injected into the cavity.