Programmable quantum multimeters and their optical implementations

Programmable quantum multimeters are devices that can realize any generalized quantum measurement from a chosen set (either exactly or approximately). Their main feature is that the desired positive operator valued measure (POVM) is selected by the quantum state of a program register. In particular, programmable quantum devices that can accomplish any projective measurement on a single qubit are described. There are two limit cases: Deterministic devices giving an erroneous result from time to time and probabilistic devices that operate error-free but sometimes lead to an inconclusive result. The intermediate cases are also discussed as well as the optimality of such multimeters. Another example of a quantum multimeter is a programmable device for unambiguous discrimination of pairs of non-orthogonal states. Simple optical implementations of some of these devices are proposed and experimental results are presented.