ON-LINE QLunch: Martina Niggli

The task of quantum state discrimination is considered in two different setups: First, we analyze the setup where one party is provided with a quantum system that was prepared in one of two states at random. We review the optimal measurement to distinguish between the two states, the Helstrom measurement, and we provide a theorem that characterizes the set of all optimal measurements. Second, we look at the setup where each of two spatially separated parties is provided with one part of a composite system, which was prepared in one of two possible states. The two parties are restricted to one-way LOCC strategies in order to discriminate the state of their shared system. We review a known optimal strategy for pure states, and we provide a new strategy that can be applied for arbitrary states. The success probability of this new strategy is compared to that of a global Helstrom measurement for different types of states, and we characterize conditions under which the new strategy performs optimally.