The general impossibility of determining the state of a single quantum system is proved for arbitrary measuring schemes, including a succession of measurements. Some recently proposed methods are critically examined. A scheme for tomographic measurements on a single copy of a radiation field is devised, showing that the system state is perturbed however weak the system-apparatus interaction is, due to the need of preparing the apparatus in a highly ``squeezed'' state.
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D’Ariano et al. (1996) studied this question.
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