Postselection

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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 E, the probability of some other event F changes from Pr[F] to the conditional probability Pr[F|E].

For a discrete probability space, Pr[F|E] = Pr[F and E]/Pr[E], and thus we require that Pr[E] 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[1][2] PostBQP is equal to PP.

References

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