In quantum information and quantum computing, the carrier of information is some quantum system and information is encoded in its state. After processing the state in the quantum processor, the information has to be read out. Clearly, this task is equivalent to determining the final state of the system. We begin by briefly reviewing various possible state discrimination strategies that are optimal with respect to some reasonable criteria and report on recent advances in the unambiguous discrimination of mixed quantum states. This strategy has been successfully applied to devise a class of novel probabilistic quantum algorithms and has been demonstrated experimentally, using a linear optical implementation via generalized interferometers. In the second part we present a scheme for communication via completely unknown quantum states. In this context we discuss programmable quantum state discriminators that are universal, i.e. perform optimally on average, independently of the actual states used for the communication scheme. We conclude with a discussion of possible experimental implementations of the proposed device.
In quantum information and quantum computing, the carrier of information is some quantum system and information is encoded in its state. The state, however, is not an observable in quantum mechanics, thus a fundamental problem arises: after processing the information it has to be read out or, in other words, the state of the system must be determined. When the possible target states are orthogonal, this is a relatively simple task, provided the set of possible target states is known. But when the possible target states are not orthogonal their discrimination is far from being trivial even if their set is known. Thus the problem of discriminating among non-orthogonal states is ubiquitous in quantum information and quantum computing, underlying all communication and computing schemes. It is the subject of this talk to review recent theoretical and experimental advances in this field.
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Noise and Information in Nanoelectronics, Sensors, and Standards III