两比特纯态纠缠的直接测量

Direct Measurement of Concurrence for Two-Qubit Pure States

研究了如何在不需要量子态重构的前提下直接测量一个两比特任意纯态的纠缠concurrence。设计了一个concurrence的直接测量量子线路,即通过对待测纠缠粒子对的两个全同拷贝实施σz操作、单比特R旋转操作以及奇偶宇称校验检测,使得concurrence被编码在最后获得某些状态的概率中,从而实现对concurrence的直接测量。在给出concurrence直接测量的量子线路的基础上,基于光学系统设计了一个直接测量光子偏振纠缠concurrence的物理方案,实现两比特耦合的奇偶宇称校验检测操作只出现一次,大大降低了方案的复杂度。另外,该方案可实现远程纠缠态的纠缠直接测量。

How to directly measure the concurrence of two-qubit pure states without quantum state tomography is studied. A quantum circuit is designed which can encode the concurrence of a general two-qubit pure state into the probability of obtaining some specific states at the detection stage. Here, two copies of the initial state are needed in each measurement round and several operations, such as σz operation, simple qubit rotation R and parity-check measurement （PCM）, are carried out on them. An optical implementation scheme for the direct measurement of the concurrence for polarization entangled pure state is proposed. Only one PCM for coupling two qubits is needed, which greatly reduces the complexity of the scheme. In addition, this scheme can directly measure the concurrence of remote entanglement.