Postselection

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

1. ↑ Aaronson, Scott (2005). "Quantum computing, postselection, and probabilistic polynomial-time". Proceedings of the Royal Society A 461 (2063): 3473–3482. doi:10.1098/rspa.2005.1546. . Preprint available at
2. ↑ Aaronson, Scott (2004-01-11). "Complexity Class of the Week: PP". Computational Complexity Weblog. Retrieved 2008-05-02.