We address the problem of unambiguous discrimination among a given set of quantum operations. The necessary and sufficient condition for them to be unambiguously distinguishable is derived in the cases of single use and multiple uses, respectively. For the latter case we explicitly construct the input states and corresponding measurements that accomplish the task. It is also found that the introduction of entanglement can improve the discrimination.

• Received 19 December 2005

DOI:https://doi.org/10.1103/PhysRevA.73.042301