Quantum multimeters: A programmable state discriminator

We discuss a possibility to build a programmable quantum measurement device (a “quantum multimeter”). That is, a device that would be able to perform various desired generalized, positive operator value measure (POVM) measurements depending on a quantum state of a “program reg-ister”. As an example, we present a “universal state discriminator”. It serves for the unambiguous discrimination of a pair of known non-orthogonal states (from a certain set). If the two states are changed the apparatus can be switched via the choice of the program register to discriminate the new pair of states unambiguously. The proper POVM is determined by the state of an auxiliary quantum system. The probability of successful discrimination is not optimal for all pairs of non-orthogonal states from the given set. However, for some subsets it can be very close to the optimal value.