Fidelity of Interference Between Two Bose-Einstein Condensates with Collision and Dissipation

Interference between the two Bose-Einstein condensates with collision and dissipation is investigated. Itis found that when the two condensates are initially in the coherent state, the interference intensity is affected by thecollision and dissipation, but for the initial Fock state, it is only related to the dissipation. Whether the initial stateis in the coherent state or in a Fock state, the fidelity time has nothing to do with collision. For the initial coherentstate, the fidelity loss rate is zero, but for the initial Fock state, it is determined by the initial particle number of thetwo condensates and dissipation.     

Quantum teleportation of one－ and two－photon superposition states

Quantum teleportation of one- and two-photon superposition states based on EPR entanglement of continuous-wave two-mode squeezed state is discussed. The fidelities of teleportation axe deduced for two different input quantum states. The dependence of the fidelity on the parameters of EPR entanglement and the gain of the classical channels are shown numerically. Comparing with the teleportation of Fock state and coherent state, it is pointed out that for given EPR entanglement and classical gain, the higher the nonclassicality of the input state, the lower the accessible fidelity of teleportation.     

Generating a Squeezed‐Coherent‐Cat State in a Double‐Cavity Optomechanical System

A squeezed‐coherent‐cat state (SCCS) in a mechanical system not only plays an important role for macroscopic quantum coherence, but also can be a carrier for quantum information. A scheme to generate a SCCS in a two‐mode optomechanical system is proposed, in which the modulated hopping interaction of two cavities is introduced. The two cavity modes couple with the same mechanical mode with linear and quadratic interaction, respectively. The SCCS is analytically deduced under an appropriate initial state, and the average phonon number and the parameter of squeeze are numerically calculated. Wigner function shown the properties of superposition and squeezing is plotted. Including the dissipation of the environment, the results show that a high quality mechanical resonator and a low noise environment are required to obtain high fidelity.     

Long phase coherence time and number squeezing of two Bose-Einstein condensates on an atom chip

We measure the relative phase of two Bose-Einstein condensates confined in a radio frequency induced double-well potential on an atom chip. We observe phase coherence between the separated condensates for times up to approximately 200 ms after splitting, a factor of 10 longer than the phase diffusion time expected for a coherent state for our experimental conditions. The enhanced coherence time is attributed to number squeezing of the initial state by a factor of 10. In addition, we demonstrate a rotationally sensitive (Sagnac) geometry for a guided atom interferometer by propagating the split condensates.     

基于量子测量的Fock态制备和自相位调制的影响

提出一种制备Fock态的新方案研究在包含有自相位调制的Kerr介质中两束光的相互作用,信号光场和探测光场初始处于相干态,经由非线性相互作用后演化成为纠缠态.若对探测光场的正交位相分量实行第一类量子测量,信号光场的光子数分布会受到调制.重复上述过程,发现信号光场最终演化成为一个纯的Fock态.这种制备Fock态的原理是基于互相位调制,而自相位调制则起着阻碍Fock态形成的作用.     

基于拉曼耦合Jaynes-Cumming-Paul模型的原子布居数反转动力学演化

本文主要研究了拉曼耦合Jaynes-Cumming-Paul模型中原子布居数反转的动力学演化.讨论在不同初始状态时,在相互作用过程中原子布居数反转的演化情况.结果表明,无论原子处于相干还是非相干叠加态,只要当输入光场为Fock态时,反转现象明显;当输入光场为相干态时,反转现象存在但不明显;当输入光场为压缩真空态和热态时,几乎无反转现象.     

Estimation of photon counting statistics with imperfect detectors

The study on photon counting statistics is of fundamental importance in quantum optics. We theoretically analyzed the imperfect detection of an arbitrary quantum state. We derived photon counting formulae for six typical quantum states(i.e.,Fock, coherent, squeeze-vacuum, thermal, odd and even coherent states) with finite quantum efficiencies and dark counts based on multiple on/off detector arrays. We applied the formulae to the simulation of multiphoton number detections and obtained both the simulated and ideal photon number distributions of each state. A comparison between the results by using the fidelity and relative entropy was carried out to evaluate the detection scheme and help select detectors for different quantum states.     

The hidden phase of Fock states; quantum non-local effects

We revisit the question of how a definite phase between Bose-Einstein condensates can spontaneously appear under the effect of measurements. We first consider a system that is the juxtaposition of two subsystems in Fock states with high populations, and assume that successive individual position measurements are performed. Initially, the relative phase is totally undefined, and no interference effect takes place in the first position measurement. But, while successive measurements are accumulated, the relative phase becomes better and better defined, and a clear interference pattern emerges. It turns out that all observed results can be interpreted in terms of a pre-existing, but totally unknown, relative phase, which remains exactly constant during the experiment. We then generalize the results to more condensates. We also consider other initial quantum states than pure Fock states, and distinguish between intrinsic phase of a quantum state and phase induced by measurements. Finally, we examine the case of multiple condensates of spin states. We discuss a curious quantum effect, where the measurement of the spin angular momentum of a small number of particles can induce a big angular momentum in a much larger assembly of particles, even at an arbitrary distance. This spin observable can be macroscopic, analogous to the pointer of a measurement apparatus, which illustrates the non-locality of standard quantum mechanics with particular clarity. The effect can be described as the teleportation at arbitrary distances of the continuous classical result of a local experiment. The EPR argument, transposed to this case, takes a particularly convincing form since it does not involve incompatible measurements and deals only with macroscopic variables.     

Characterization of a Two-Photon Quantum Battery: Initial Conditions,Stability and Work Extraction

We consider a quantum battery that is based on a two-level system coupled with a cavity radiation by means of a two-photon interaction. Various figures of merit, such as stored energy, average charging power, energy fluctuations, and extractable work are investigated, considering, as possible initial conditions for the cavity, a Fock state, a coherent state, and a squeezed state. We show that the first state leads to better performances for the battery. However, a coherent state with the same average number of photons, even if it is affected by stronger fluctuations in the stored energy, results in quite interesting performance, in particular since it allows for almost completely extracting the stored energy as usable work at short enough times.     

Engineering multipartite steady entanglement of distant atoms via dissipation

We propose a scheme for generating an entangled state for three atoms trapped in separate optical cavities that are coupled to each other through two optical fibers based on coherent driving and dissipation, which are induced by the classical fields and the decay of non-local bosonic modes, respectively. In our scheme, the interaction time need not be controlled strictly in the overall dynamics process, and the cavity field decay can be changed into a vital resource. The numerical simulation shows that the fidelity of the target state is insensitive to atomic spontaneous emission, and our scheme is good enough to generate the W state of distant atoms with a high fidelity and purity. In addition, the present scheme can also be generalized to prepare the N-partite W state of distant atoms.     

Interference of Two-Component Bose-Einstein Condensates with a Coupling Drive in Presence of Dissipation

The interference of the two-component Bose-Einstein condensates with a coupling drive in the presence of the dissipation is studied. We find that when the two-component Bose-Einstein condensates are initially in the coherent states, for the smaller dissipation parameters compared with that of the rf frequency ωrf, the interference intensity exhibits damply oscillation behavior, whereas when the dissipation parameters are larger than that of the ωry, the interference intensity exhibits a fast attenuation behavior. As a comparison, the interference intensity in the absence of the dissipation is also studied. We conclude that the dissipation of the system can be evaluated by selecting the ωrf experimentally.     

Coherent states of the inverted Caldirola-Kanai oscillator with time-dependent singularities

The coherent states for a system of time-dependent singular potentials coupled to inverted CK (Caldirola-Kanai) oscillator are investigated by employing invariant operator method and Lie algebraic approach. We considered Coulomb potential and inverse quadratic potential as singularities of the system. The spectrum of quantum states is discrete for λ < 0 while continuous for λ ? 0. The probability densities for both Fock state and coherent state are converged to the center as time goes by according to the dissipation of energy. We confirmed that the probability density in the coherent state oscillates back and forth like a classical wave packet.     

Realistic teleportation of coherent states using squeezed vacuum states

This paper presents a realistic scheme for the teleportation of coherent states in which a two-mode squeezed vacuum state serves as the quantum channel and the position-sum and momentum-difference of two local modes serve as the measuring observables. The average fidelity of the teleportation of coherent states is derived for finite squeezing parameters and it turns out that fidelity greater than 1/2 cannot be achieved by using a classical channel alone and the probability distribution of the measurement result is a Gaussian distribution around the unknown parameter of the input coherent state with a width given by the squeezing parameter.     

Effects of Cavity-Field Statistics on Atomic Entanglement in Two-Mode Jaynes-Cummings Model

We study the entanglement properties of a pair of two-level Rydberg atoms passing one after another into a lossless cavity with two modes. The initial joint state of two successive atoms that enter the cavity is unentangled. Interactions mediated by the cavity field results in the final two-atom mixed entangled type state. The entanglement of formation of the joint two-atom state as a function of the Rabi angle gt is calculated for Fock state field, coherent field and thermal field respectively inside the cavity. We present a comparative study of two-atom entanglement corresponding to the different field statistics.     

光场诱导的原子激光的量子相干性

基于伞量子理论,分别研究了几种重要的光场作用下,从原子玻色-爱因斯坦凝聚(BEC)体耦合输出的原子激光的量子相干特性.结果表明,粒子数态光场诱导的原子激光总是反聚束的,相干态光场诱导的原子激光是任意阶相干的,而压缩相干态光场诱导的原子激光总是聚束的.表明用光场诱导产生的原子激光具有与初始光场完全相间的量子相干性质.     

Quantum fidelity for Gaussian states describing the evolution of open systems

Using the expression of the fidelity for the most general Gaussian quantum states, the quantum fidelity is studied for the states of a harmonic oscillator interacting with an environment, in particular with a thermal bath. The time evolution of the considered system is described in the framework of the theory of open systems based on quantum dynamical semigroups. By taking a correlated squeezed Gaussian state as initial state, we calculate the quantum fidelity for both undisplaced and displaced states. The time evolution of the quantum fidelity is analyzed depending on the squeezing and correlation parameters characterizing the initial Gaussian state and on the dissipation constant and temperature of the thermal bath.     

New approach for deriving the exact time evolution of the density operator for a diffusive anharmonic oscillator and its Wigner distribution function

Using thermal entangled state representation,we solve the master equation of a diffusive anharmonic oscillator(AHO) to obtain the exact time evolution formula for the density operator in the infinitive operator-sum representation.We present a new evolution formula of the Wigner function(WF) for any initial state of the diffusive AHO by converting the WF calculation into an overlap between two pure states in an enlarged Fock space.It is found that this formula is very convenient in investigating the WF’s evolution of any known initial state.As applications,this formula is used to obtain the evolution of the WF for a coherent state and the evolution of the photon-number distribution of diffusive AHOs.     

Squeezing in a three-level Jaynes-Cummings model with strong field

Squeezing in three-level Jaynes-Cummings model with the strong initial coherent state is studied numerically. The dependence of the squeezing on the field intensity and time is analysed. The cases that the light field is initially in a Fock state and in a squeezed vacuum state are also discussed.     

相位扩散通道中密度算符相关态的时间演化

利用热纠缠态的性质,对具有代表性的相位扩散主方程进行求解,得到关于密度算符的算符和表示形式,分析不同初始态下的密度算符的时间演化结果,发现在相位扩散通道下当初始态为粒子数态或热态时密度算符保持恒定,而当初始态为相干态时系统在发生相扩散的同时始终保持相干态特性不变.     

Dissipative preparation of entanglement in optical cavities

We propose a novel scheme for the preparation of a maximally entangled state of two atoms in an optical cavity. Starting from an arbitrary initial state, a singlet state is prepared as the unique fixed point of a dissipative quantum dynamical process. In our scheme, cavity decay is no longer undesirable, but plays an integral part in the dynamics. As a result, we get a qualitative improvement in the scaling of the fidelity with the cavity parameters. Our analysis indicates that dissipative state preparation is more than just a new conceptual approach, but can allow for significant improvement as compared to preparation protocols based on coherent unitary dynamics.