Postselection is a concept primarily used in quantum mechanics and quantum information theory. It refers to the process of selecting certain outcomes from a quantum experiment after measurement has taken place, effectively discarding other outcomes that do not meet specific criteria. In quantum systems, measurements can yield a range of possible results due to the probabilistic nature of quantum mechanics. Postselection involves analyzing the outcomes and only retaining those results that align with a predetermined condition.
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Postselection conditions a quantum measurement on a specified outcome. If its branch acts by a linear operator on a normalized input , its probability is , and its normalized output is . This conditional state is defined only when the probability is nonzero. Physically unsuccessful trials are discarded; postselection is not a deterministic implementation of the nonlinear normalization map.