Postselection
Introduction In probability theory, to '''postselect''' is to condition a probability space upon the occurrence of a given event. In symbols, once we postselect for an event , the probability of some other event  changes from  to the conditional probability .For a discrete probability space, , and thus we require that  be strictly positive in order for the postselection to be well-defined.See also PostBQP, a complexity class defined with postselection. Using postselection it seems quantum Turing machines are much more powerful: Scott Aaronson proved '''PostBQP''' is equal to '''PP'''.Some quantum experiments use post-selection after the experiment as a replacement for communication during the experiment, by post-selecting the communicated value into a constant.
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