Unambiguous Examining the Orthogonality of Multiple Quantum States
  In this article, we study an opposite problem of universal quantum state comparison, that is unambiguous determining whether multiple unknown quantum states from a Hilbert space are orthogonal or not. We show that no unambiguous quantum measurement can accomplish this task with a non-zero probability. Moreover, we extend the problem to a more general case, that is to compare how orthogonal multiple unknown quantum states are with a threshold, and it turns out that given any threshold this extended task is also impossible by any unambiguous quantum measurement except for a trivial case. It will be seen that the impossibility revealed in our problem is stronger than that in the universal quantum state comparison problem and distinct from those in the existing "no-go" theorems. 
Shengshi Pang and Shengjun Wu
quant-ph