Frankfurt 2006 – scientific programme

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SYSA: Quantum state analysis and estimation

SYSA 2: Quantum state analysis and estimation II

SYSA 2.1: Invited Talk

Tuesday, March 14, 2006, 14:00–14:30, HVI

Optimal quantum measurements: From minimum error to maximum confidence — •Stephen Barnett — Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK

The superposition principle carries with it the existence of quantum states that are similar in the sense that no measurement strategy can distinguish between them with certainty. Under such circumstances it is both interesting and of practical importance to ask what is the best that can be done.

In my talk I will concentrate of quantum communications problems in which prior information about the set of possible states and their associated probabilities is known. Within this I will discuss state detection with minimum error and unambiguous, or error-free state discrimination, which works by including the possibility of an inconclusive result. I will also describe a new strategy, the maximum confidence measurement, which is optimised so that we can be as confident of our state identification as is possible within the rules of quantum theory. I will show how such measurements can be performed by reference to experiments performed using optical polarisation.

If we have two quantum systems then the range of possible observations and conclusions is greatly enhanced by the possibility of using entangled states. As an example, we seek to determine whether or not the two systems have been prepared in the same state, without the need to identify the state itself. I will conclude with some examples of such No-dqstate comparisonNo-dq measurements.