Postselection
Concept in probability theory
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 
![{\textstyle \operatorname {Pr} [F]](https://wikimedia.org/api/rest_v1/media/math/render/svg/1e0de6b26fd6267317c401229f254b6c89d66278)
![\operatorname {Pr} [F\,|\,E]](https://wikimedia.org/api/rest_v1/media/math/render/svg/b22ed8aff3c909e40b92b4ac14de7ddc269c8abc)
For a discrete probability space, ![{\textstyle \operatorname {Pr} [F\,|\,E]={\frac {\operatorname {Pr} [F\,\cap \,E]}{\operatorname {Pr} [E]}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/cefb4f6c5d2bf963f709c8b1c333bb42760fb68c)
![{\textstyle \operatorname {Pr} [E]](https://wikimedia.org/api/rest_v1/media/math/render/svg/d3524bb493868a439651569e9a732371699d9fd8)
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.
Sources and credits
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