We study the relationship between the error in the discrimination of two possible evolution paths of a quantum system and its initial state. We consider the case of two-level discrimination when there are no fluctuations of nonquantum nature (the system is in a pure state). It is shown that two evolution paths can be distinguished only when the indeterminacy in the difference of quantum phases ($\sigma _{\phi}$) in the possible evolution paths is greater than unity; the discrimination error is then independent of the average value of the phase difference.
Department of Physics of Oscillations, Faculty of Physics, Moscow State University, Leninskie Gory, Moscow, 119992, Russia
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