标题： 量子非定域性和不可分性 显示英文标题

标题： Quantum nonlocality and inseparability

摘要： 一个由两个子系统组成的量子系统，如果其密度矩阵可以写成$\rho=\sum w_K \rho_K'\otimes \rho_K''$，其中$\rho_K'$和$\rho_K''$是两个子系统的密度矩阵，正权重$w_K$满足$\sum w_K=1$，则该系统是可分离的。 可分离性的必要条件被推导出来，并被证明在检测量子不可分性方面比贝尔不等式更敏感。 此外，对贝尔不等式的集体测试（即同时涉及多个复合系统的测试）有时会导致贝尔不等式的违反，即使当每个复合系统单独测试时后者是满足的。 显示英文摘要

摘要： A quantum system consisting of two subsystems is separable if its density matrix can be written as $\rho=\sum w_K \rho_K'\otimes \rho_K''$, where $\rho_K'$ and $\rho_K''$ are density matrices for the two subsytems, and the positive weights $w_K$ satisfy $\sum w_K=1$. A necessary condition for separability is derived and is shown to be more sensitive than Bell's inequality for detecting quantum inseparability. Moreover, collective tests of Bell's inequality (namely, tests that involve several composite systems simultaneously) may sometimes lead to a violation of Bell's inequality, even if the latter is satisfied when each composite system is tested separately.