Quantum-controlled measurement device for quantum-state discrimination

We propose a “programmable” quantum device that is able to perform a specific generalized measurement from a certain set of measurements depending on a quantum state of a “program register.” In particular, we study a situation when the programmable measurement device serves for the unambiguous discrimination between nonorthogonal states. The particular pair of states that can be unambiguously discriminated is specified by the state of a program qubit. The probability of successful discrimination is not optimal for all admissible pairs. However, for some subsets it can be very close to the optimal value.