A quantum system always evolves through coherent superposition of states—an initial state develops into a superposition of states, with the relative weight factors changing with time in a continuous way. The probability of finding a system in the initial state after it is allowed to evolve for a certain period of time is known as survival probability of the evolving system. If we compute the survival probability as a function of the frequency of intermediate measurements (to determine whether the system is in its original state or not), we find that survival probability increases with the frequency of measurements and it tends to unity in the limit of a continuous series of measurements. This means that the dynamical evolution of an isolated quantum system is significantly modified due to intervening measurements. In particular the evolution is inhibited by repeated frequent measurements even if these measurements are apparently nondisturbing; that is, they do not entail a direct exchange of energy or momentum (i.e., the measurements may be of the negative-result type).
KeywordsSurvival Probability Beam Splitter Decay Product Optical Pulse Local Realism
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