On Optimal Local Discrimination Algorithm of Two-Qubit Unitary Operations

It has been known that two different bipartite unitary operations are perfectly distinguishable by local operations and classical communication (LOCC) when a finite number of queries is allowed. However, it is still a longstanding open problem to determine the optimal number of queries needed to distinguish bipartite unitary operations perfectly by LOCC since 2007. In this paper, we show that if two two-qubit unitary operations are perfectly distinguishable using T queries, they are also perfectly distinguishable in the LOCC scenario using \lceil 6 \pi \cdot T \rceil queries. As a result, LOCC operations are as powerful as global operations when using constant times the number of queries for the perfect discrimination of two-qubit unitary operations.