Two quantum systems are identically prepared in different locations. An observer’s task is to determine their state. A simple example shows that a pair of measurements of the von Neumann type is less effective than a sequence of nonorthogonal probability-operator measures, alternating between the two quantum systems. However, the most efficient set of operations of that type that we were able to design falls short of a single combined measurement, performed on both system together.

• Received 15 February 1990

DOI:https://doi.org/10.1103/PhysRevLett.66.1119