超导能隙的自旋类比求解

超导体中准粒子激发存在能隙是超导的重要特征之一, BCS理论成功地解释了这一特性.虽然BCS理论中对Hamiltonian形式简单,但严格求解相关问题存在困难,通常需作近似处理: 如平均场近似,变分法等.而我们注意到费米对粒子产生与湮灭算符和Pauli自旋升降算符满足相同的代数关系(即sl(2) = {c-k,↓ck,↑,c+k,↑c+-k,↓,nk,↑+n-k,↓-1} = {σ-k, σ+k, σzk}),故假定自旋向上、向下分别代表对态(pair state)的占据和未占据两种状态,从而将BCS对Hamiltonian谱的求解问题对应到自旋直积空间中去研究,尝试求解能隙,得到了有用的结果,并与由BCS理论中能隙方程导出的结果进行了比较.特别,我们讨论了目前实验上量子计算所能达到的量子比特数目的情形,为在目前实验条件下进行量子模拟实验作了理论上的准备.     

Experimental Creation of Entanglement Using Separable States

We experimentally demonstrate that the entanglement can be created on two distant particles using separable states. We show that two working particles can share some entanglement, while one ancilla particle always remains separable from the two working particles during the experimental evolution of the system. Our experiment can be viewed as a benchmark to illustrate the idea that no prior entanglement is necessary to create entanglement.     

费米环境中两个三能级原子系统的纠缠演化

利用负性纠缠度(negativity)研究了两个三能级原子系统在费米环境中的纠缠演化问题-结果表明,两个三能级原子系统的纠缠演化不仅依赖于系统和环境的相互作用强度,而且还依赖于系统所处的具体量子态-通过例子发现,系统和环境相互作用强度越大,纠缠衰减越快;对于纯态,仅当时间趋于无穷时纠缠才被完全破坏;对于混态,则在有限的时间内纠缠即被彻底破坏-通过一般的分析找到了一类免退相干的量子子空间-在这些子空间中,量子态不受环境的影响,故其纠缠不变-研究有助于理解费米环境造成的退相干对玻色系统纠缠的影响- 关键词： 费米环境 纠缠演化 两个三能级原子     

Entanglement Evolution of Three-Qubit States under Local Decoherence

By using negativity as entanglement measure, we have investigated the effect of local decoherence from a non-Markovian environment on the time evolution of entanglement of three-qubit states including the GHZ state, the W state, and the Werner state. From the results, we find that the entanglement dynamics depends not only on the coupling strengths but also on the specific states of concern. Specifically, the entanglement takes different behaviors under weak or strong coupling and it varies with the quantum states under study. The entanglement of the GHZ state and the Werner state can be destroyed completely by the local decoherence, while the entanglement of the W state can survive through the local decoherence partially.     

Voltage-Controlled Quantum Dynamics and Generation Entanglement between Two Separated Quantum-Dot Molecules Embedded in Photonic Crystal Cavities

Voltage-controlled quantum dynamics of two quantum-dot molecules （QDMs） embedded in two separated photonic crystal cavities are theoretically investigated. We show numerically that generation of entangled states and population transfer between the two QDMs can be realized with the same coupling parameters. The effects of parameters deviation and dissipations on generation entangled states be used for realization of new-type of solid state quantum and populations transfer are also discussed. The results may devices and integrated electro-optical devices,     

Effect of Decoherence Induced by a Spin Environment on Quantum Channel Capacity

We investigate the effect of decoherence from a spin environment on the quantum channel capacity.Our results imply that the time evolution of the quantum channel capacity depends on the number of freedom degrees of the environment,the tunneling element,the initial state of the environment,and the system-environment coupling strength.From the analysis,we find that the strong tunneling elements and the weak coupling strength can enhance the quantum channel capacity while the environment with a large number of freedom degrees and the strong coupling strength will shrink it.     

Transmission Characteristics of Waveguide-Coupled Nanocavity Embedded in Two Atoms with Dipole-Dipole Interaction

Photon scattering properties in one-dimensional waveguide side coupled to a nanocavity embedded in two atoms with dipole-dipole interaction （DDI） are investigated theoretically. The analytical expressions of the transmission and reflection amplitudes are deduced by using the real-space Hamiltonian. A method to determine the coupling strength of DDI is proposed. Realization of single photon switching by modulation the DDI is investigated. The influence of dissipations on the performance of the single photon switching are exhibited. An asymmetric Fano-type resonance, which can be controlled by the DDI, appears in the transmission spectrum.     

利用核磁共振产生无耦合自旋之间的纠缠

纠缠是量子信息中重要的资源,因此产生纠缠是实现量子信息处理的一项基本任务.通常,用整体操作和纠缠交换等方法可以产生纠缠.在大多数的物理体系中,量子位之间没有相互作用,但需要产生它们之间的纠缠.在这种情况下,如何产生纠缠?并且,产生纠缠需要预先的纠缠吗?     

Entanglement Dynamics of Quantum States Undergoing Decoherence from a Driven Critical Environment

In this paper,we have investigated the quantum entanglement of quantum states undergoing decoherence from a spin environment which drives a quantum phase transition.From our analysis,we find that the entanglement dynamics depends not only on the coupling strength but also on the external magnetic field and the number of the freedom degrees of the environment.Specially,our results imply that the decay of the entanglement can be enhanced by the quantum phase transition of the environment when the system is coupled to the environment weakly.Additionally,the discussion of the case of the multipartite states with high dimensions is made.