Nonexistence of a universal quantum machine to examine the precision of unknown quantum states
Shengshi Pang, Shengjun Wu, Zeng‐Bing Chen · Physical Review A · 2011
In this work, we reveal a type of impossibility discovered in our recent research which forbids comparing the closeness of multiple unknown quantum states with any nontrivial threshold in a perfect or unambiguous way. This impossibility is distinct from the existing impossibilities in that it is a ``collective'' impossibility on multiple quantum states; most other ``no-go'' theorems are concerned with only one single state each time, i.e., it is an impossibility on a nonlocal quantum operation. This impossibility may provide new insight into the nature of quantum mechanics, and it implies more limitations on quantum information tasks than the existing no-go theorems.