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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 , 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[1][2]PostBQP is equal to PP.

Some quantum experiments[3] use post-selection after the experiment as a replacement for communication during the experiment, by post-selecting the communicated value into a constant.


  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 [1]
  2. ^ Aaronson, Scott (2004-01-11). "Complexity Class of the Week: PP". Computational Complexity Weblog. Retrieved . 
  3. ^ Hensen; et al. "Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres". Nature. 526: 682-686. doi:10.1038/nature15759. PMID 26503041. 

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