Quantum Fisher Information and Phase Sensitivity in Various Initial States

I investigate the dependence of phase sensitivity on the initial input states. By adopting frozen-spin approximation, I first derive the analytical expresses of the angular momentum operators and then of quantum Fisher information, the phase sensitivity. It is shown that the initial input state with smaller value of angular momentum has the better degree of entanglement and phase sensitivity beats the Heisenberg limit.     

纠缠压缩真空态的纠缠转移

通过分析光学分束器对压缩真空态光场的作用,发现如果分束器的输入光是两束具有同样振幅和相位的单模压缩真空态光场,则输出光为双模压缩真空态光场;若分束器的输入光是两束具有同样振幅但有π相位差的单模压缩真空态光场,则输出光仍为两束单模压缩真空态光场.对于双模压缩真空态光场,每个模中容纳的光子数可以是基数或偶数.而对于单模压缩真空态光场,每个模中只能包含偶数个光子.根据这些结果,提出了一个纠缠转移的方案.在这个方案中,两个纠缠压缩真空态光场被用作量子信道,通过利用光学分束器作用和光子数探测的方法,并在经典通讯的帮助下,实现了三个通讯伙伴之间的纠缠转移.     

Entanglement and Phase-Sensitivity of a Mach-Zehnder Interferometer for the Coherent Spin State Input

We study quantum entanglement and phase-sensitivity of a Mach-Zehnder interferometer for the coherent spin state input. It’s shown that entanglement and the Heisenberg limit of phase-sensitivity can be obtained adjusting the phase shift and increasing the total photons’ number.     

Entanglement behaviors of two-mode squeezed thermal states in two different environments

We study entanglement properties of two-mode squeezed thermal states subjected to two sources of decoherence: the common reservoirs and the bosonic memory Gaussian channel. For the former one, we find that there exist three different behaviors: no-sudden death, sudden death, and no-creation of entanglement. The range of parameters characterizing these processes is obtained. For the latter one, we obtain a threshold in the degree of squeezing above which the input states remain always entangled. Otherwise, no entanglement is allowed in bosonic Gaussian channel with memory effect. We show that a degree of memory for quantum channel can be help to increase the initial entanglement, while the mean number of added thermal photons is to fasten the decoherence process.     

Entanglement Swapping of Pair Coherent States with Phase Decoherence

We investigate the entanglement swapping of continuous state and the two-mode squeezed vacuum which is exposed variable using the pair coherent state as the input in a phase decoherence environment as the quantum channel. By adopting the log-negativity as the measure of entanglement, we analyze how entanglement of the two initial states and the phase decoherence environment affect the entanglement swapping quality.     

Entanglement evolution in multipartite cavity-reservoir systems under local unitary operations

We analyze the entanglement evolution of two cavity photons being affected by the dissipation of two individual reservoirs. Under an arbitrary local unitary operation on the initial state, it is shown that there is only one parameter which changes the entanglement dynamics. For the bipartite subsystems, we show that the entanglement of the cavity photons is correlated with that of the reservoirs, although the local operation can delay the time at which the photon entanglement disappears and advance the time at which the reservoir entanglement appears. Furthermore, via a new defined four-qubit entanglement measure and two three-qubit entanglement measures, we study the multipartite entanglement evolution in the composite system, which allows us to analyze quantitatively both bipartite and multipartite entanglement within a unified framework. In addition, we also discuss the entanglement evolution with an arbitrary initial state.     

Entangling single photons on a beamsplitter

We report on a scheme for the creation of time-bin entangled states out of two subsequent single photons. Both photons arrive on the same input port of a beamsplitter and the situation in which the photons leave the beamsplitter on different output ports is post-selected. We derive a full quantum mechanical analysis of such time-bin entanglement for emitters subject to uncorrelated dephasing processes and apply this model to sequential single photons emerging from a single semiconductor quantum dot. Our results indicate that the visibility of entanglement is degraded by decoherence effects in the quantum dot, but can be restored by use of CQED effects, namely the Purcell effect.     

Entanglement evolution of three-qubit mixed states in multipartite cavity—reservoir systems

We analyze the multipartite entanglement evolution of three-qubit mixed states composed of a GHZ state and a W state.For a composite system consisting of three cavities interacting with independent reservoirs,it is shown that the entanglement evolution is restricted by a set of monogamy relations.Furthermore,as quantified by the negativity,the entanglement dynamical property of the mixed entangled state of cavity photons is investigated.It is found that the three cavity photons can exhibit the phenomenon of entanglement sudden death(ESD).However,compared with the evolution of a generalized three-qubit GHZ state which has the equal initial entanglement,the ESD time of mixed states is later than that of the pure state.Finally,we discuss the entanglement distribution in the multipartite system,and point out the intrinsic relation between the ESD of cavity photons and the entanglement sudden birth of reservoirs.     

Phase Sensitivity of a Multi-mode Nonlinear Interferometer with Single Photons

We propose a multi-mode nonlinear interferometer and study its phase sensitivity for two, three and four modes with single photons as inputs. We find that the phase sensitivity of the nonlinear interferometers can be improved by increasing the nonlinear index k. And when the total number of the input photons is fixed, increasing the number of modes is detrimental to the phase sensitivity.     

How a single photon can mediate entanglement between two others

We describe a novel quantum information protocol, which probabilistically entangles two distant photons that have never interacted. Different from the entanglement swapping protocol, which requires two pairs of maximally entangled photons as the input states, as well as a Bell-state measurement (BSM), the present scheme only requires three photons: two to be entangled and another to mediate the correlation, and no BSM, in a process that we call “entanglement mediation”. Furthermore, in analyzing the paths of the photons in our arrangement, we conclude that one of them, the mediator, exchanges information with the two others simultaneously, which seems to be a new quantum-mechanical feature.     

Time evolution of pairwise entanglement between two atoms

We investigate dynamics of pairwise entanglement between two atoms for various entangled initial states. The initial entanglement shows an interesting time evolution, including entanglement sudden death. The initial portion of the excited state is responsible for the sudden death of the entanglement. The initial portion of excited–ground–ground state or the initial photons in the system can induce pairwise entanglement. Meanwhile, we find that our model gives a concrete example to support the two theorems in [F. Verstraete, K. Audenaert, J. Dehaene and B. De Moor, J. Phys. A: Math. Gen. 34 (2001) 10327].     

第二红边带激发时强度涨落对两囚禁离子纠缠度的影响

利用双光子T-C模型研究了两囚禁离子与第二红边带经典激光场相互作用时的纠缠动力学。采用共生纠缠度标准,分析了两离子初态情况,激光强度涨落等主要因素对两离子纠缠动力学的影响,并与单光子J-C模型时情形作了适当对比。结果显示：1) 激光强度涨落产生的噪声使两离子纠缠度随时间呈现类似指数衰减;2) 两离子初态为混合态时,两离子纠缠度出现整体急剧下降;3) 双光子和单光子激发时两离子纠缠度随时间的演化存在显著差别。     

Interaction of a Three-Level Atom and Field in a Squeezed Vacuum State with Added Photons: Quantum Phase and Nonclassical Properties

We present a detail study of the evolution of nonlocal correlations of an interacting quantum system comprising a three-level atom and a field mode initially prepared in a squeezed vacuum state with added photons. We compare the dynamical behavior of the quantum phase and entanglement by varying the number of photons added to the squeezed vacuum state. Furthermore, we examine the influence of the added-photon number and the squeeze parameter on the dynamical behavior of entanglement, quantum phase, and nonclassical properties of the field. Moreover, we explore the link between the quantum phase and the nonlocal correlation. Finally, we introduce an effective method to generate and maintain a high level of entanglement for this quantum system based on precise parameter ranges.     

Entanglement swapping in two independent Jaynes-Cummings models

We consider two independent Jaynes-Cummings models which consist of an atom interacts with electromagnetic field modes. The generation of atom-atom entanglement is investigated. No direct interaction between the two atoms exists. Entanglement can be created using entanglement swapping method by an interference measurement performed on photons. We discuss the influence of off-resonance and the initial state of the atom on the atom-atom entanglement. It is shown that the maximal atom-atom entanglement can be obtained and the atom-atom entanglement is sensitive to the off-resonance and the initial state of the atom.     

How much quantum noise of amplifiers is detrimental to entanglement

We analyze the effect of the quantum noise of an amplifier on the entanglement properties of an input state. We consider both phase insensitive and phase sensitive amplification and specialize to Gaussian states for which entanglement measures are well developed. In the case of phase insensitive amplification in which both the modes are symmetrically amplified, we find that the entanglement in the output state vanishes if the intensity gain exceeds a limiting value 2/(1+exp[-E_N]) where E_N is the logarithmic negativity of the input state which quantifies the initial entanglement between the two modes. The entanglement between the two modes at the output is found to be more robust if only one mode is amplified.     

Sudden birth of entanglement between two atoms in a double JC model

Sudden birth of entanglement between two initially separate atoms interacting with two entangled photons in a double JC model is investigated, and the influences of different atomic initial states on entanglement among atoms are discussed. The results show that sudden birth of entanglement can occur when the two atoms are initially in excited states.     

Quantum Entanglement of Monochromatic and Non-Monochromatic Photons on a Waveguide Beam Splitter

It is well known that the waveguide beam splitter can be used as a source for the quantum entanglement of photons. The analysis of such quantum entanglement is a difficult problem even for monochromatic photons, since the system under study is multiparametric. This paper will show that quantum entanglement can be represented in a simple form not only for monochromatic photons but also for non-monochromatic ones. It will be shown that quantum entanglement for non-monochromatic photons can be very different from monochromatic photons, which can be used to create large quantum entanglement.     

Properties on the distant distribution of entanglement for arbitrary two-qubit pure states via noisy quantum channels

This paper investigates the change of entanglement for transmitting an arbitrarily entangled two-qubit pure state via one of three typical kinds of noisy quantum channels: amplitude damping quantum channel, phase damping quantum channel and depolarizing quantum channel. It finds, in all these three cases, that the output distant entanglement (measured by concurrence) reduces proportionately with respect to its initial amount, and the decaying ratio is determined only by the noisy characteristics of quantum channels and independent of the form of initial input state.     

高斯型耦合多光子Tavis-Cummings模型中场与原子的纠缠特性

利用全量子理论,研究了多光子跃迁过程高斯型耦合Tavis-Cummings模型中场与原子的纠缠特性。讨论了多光子原子运动速度、光场初态的平均光子数及跃迁光子数对场与原子之间纠缠特性的影响。结果表明:原子运动速度的变化改变了场熵演化曲线的振荡时域;光场初态影响场熵演化曲线的周期性;跃迁光子数的增加使场熵演化曲线的振荡频率和场与原子之间的平均纠缠度变大。     

Entanglements induced by cavity interacting with two-coupled atoms

We consider a quantum optics model where the cavity interacts with two-coupled atoms. The atom-atom entanglement, atoms-cavity entanglement and the mixture for the two atoms are investigated, and discuss the effects of the initial conditions, atom-atom coupling and the mean number of photons on the entanglements and mixture. We find that atom-atom coupling plays an important role in the entanglement and mixture. Numerical results show that under some conditions the phenomena of “entanglement sudden death” and “entanglement collapse and revival” emerge.