Programmable quantum measurement device that approximates all projective measurements on a qubit

摘要

A "universal" quantum measurement device is proposed. It can approximate any projective measurement on a qubit. The measurement basis is selected by the quantum state of a "program register". The device is optimized with respect to maximal average fidelity. A contra-intuitive result is that if two qubits in the same state are used as a program the average fidelity is higher than if the second program qubit is in the respective orthogonal state.