Department of Physics and Center for Astroparticle Physics and Space Science, Bose Institute, Block EN, Sector V, Salt Lake, Kolkata 700091, India.
Phys Rev Lett. 2011 May 27;106(21):210402. doi: 10.1103/PhysRevLett.106.210402. Epub 2011 May 25.
Quantum information is nonlocal in the sense that local measurements on a composite quantum system, prepared in one of many mutually orthogonal states, may not reveal in which state the system was prepared. It is shown that in the many copy limit this kind of nonlocality is fundamentally different for pure and mixed quantum states. In particular, orthogonal mixed states may not be distinguishable by local operations and classical communication, no matter how many copies are supplied, whereas any set of N orthogonal pure states can be perfectly discriminated with m copies, where m<N. Thus mixed quantum states can exhibit a new kind of nonlocality absent in pure states. We also argue that a set of orthogonal quantum states may be said to be maximally indistinguishable iff the set is not conclusively locally distinguishable with multiple copies.
量子信息在某种意义上是非局部的，即对于复合量子系统的局部测量，如果系统是在许多相互正交的状态中的一个状态下制备的，那么可能无法揭示系统是在哪个状态下制备的。结果表明，在多副本极限下，这种非局部性对于纯态和混合态有着根本的不同。具体来说，无论提供多少个副本，正交混合态都可能无法通过局部操作和经典通信来区分，而任何一组 N 个正交纯态都可以通过 m 个副本进行完美区分，其中 m<N。因此，混合量子态可以表现出一种在纯态中不存在的新型非局部性。我们还认为，如果一组正交量子态在多个副本下不能被确定地局部区分，则可以说该集合是最大程度不可区分的。
