基于操作腔外原子比特控制腔内原子比特相干性时间演化  

Controllable Time Evolution of Coherence of an Atom Bit inside Cavity by Manipulating the Atom Bit Outside Cavity

作　　者：周并举[1] 彭朝晖[1] 刘小娟[1] 

机构地区：[1]湖南科技大学物理与电子科学学院,湖南湘潭411201

出　　处：《光子学报》2014年第8期187-192,共6页Acta Photonica Sinica

基　　金：国家自然科学基金(No.11074072);湖南省自然科学湘潭联合基金(No.10JJ9002)资助

摘　　要：考虑初始处于两类Bell态的两原子比特,将其中一个注入真空态腔中发生共振相互作用的情况,研究基于操作腔外原子比特控制腔内原子比特相干性时间演化.考察对腔外原子比特施Hadamard(H)门,H类门,Y门及位相门四种逻辑门操作及态选择测量前后,腔内原子比特约化密度矩阵非对角元时间演化曲线,讨论两类Bell态之间的关联性.结果表明,对腔外原子比特操作前,腔内原子比特密度矩阵非对角元在任意时刻都为零,总是处于混合态或经典态,因此始终退相干.而对腔外原子比特施H门和H类门操作及基态测量后,腔内原子比特密度矩阵非对角元呈现周期为2π的时间演化,除(2n+1)π/2时刻外,其相干性得到恢复;在nπ时刻,其最大相干叠加态和一般相干叠加态被制备;两类Bell态之间存在R∧(π/2)旋转关联性.而对腔外原子比特施Y门及位相门操作后,腔内原子比特的相干性得不到恢复.发现腔内原子比特的相干性得到恢复的必要条件是,逻辑门操作能够调节腔内原子比特与环境的相互作用,并使两原子比特与场分离的同时,它们之间形成强相干性的纠缠态分布.Abstract： Considering two atom bits in Bell state initially, one bit was injected into a vacuum cavity to produce resonance while leaving the other outside. The controllable coherence of an atom bit inside the cavity was investigated by manipulating the atom bit outside the cavity. For the cases of before and after the Hadamard, Hadamard-like, Y, and phase logic gate manipulations on the atom bit outside the cavity, the correlation of two kinds of initial Bell state was discussed by numerical calculation. The time evolution curves of off-diagonal elements in the reduced density matrix for the atom bit inside the cavity was studied. The results show that the off-diagonal elements of the density matrix for the atom bit inside the cavity are zero at any moment before operating the atom bit outside the cavity. That is, the atom bit inside the cavity is always in a mixed state or classical states, and its coherence is always suppressed. However, after Hadamard gate and Hadamard-like gate operations and ground state measurements, offdiagonal elements in the reduced density matrix of the atom bit inside the cavity show the 2π period time evolution, and the coherence can be restored except （2n＋ 1）π/2. At nn the maximal coherence superposition state and general coherence superposition state were prepared, and a R （π/2） rotationcorrelation existed between the two kinds of Bell states. In addition, performing Y gate and phase gateoperations on the outside atom bit, the coherence for the inside atom bit cannot restore. The necessary condition for the restoration of the coherence of the inside atom bit was found out as following： while logic gate operations manipulate the interactions between the atom bit inside the cavity and its environment, and the two atom bits are separated from the field, a strongly coherent entanglement is necessary between the two atom bits.

关 键 词：逻辑操作 基态测量 原子比特 密度矩阵 相干性 量子计算 纠缠态分布 

分 类 号：O431.2[机械工程—光学工程]