= Physical Church-Turing principle
The <physical Church-Turing principle> asserts that a universal physical computing machine can simulate every finitely realizable physical system. It is a claim about physical laws and possible apparatus, stronger in subject matter than a mathematical account of effective symbolic calculation. Its exact force depends on what simulation accuracy and resource use are required.
<David Deutsch>'s 1985 paper formulated a strong version in terms of perfect simulation by finite means and investigated a universal quantum model.<Quantum theory, the Church-Turing principle and the universal quantum computer by David Deutsch>[Abstract and §2, statement of the physical principle]{ref} For example, describing one <qubit> requires specifying its <quantum state> and predicting <quantum measurement> probabilities; copying a short list of observed outcomes does not simulate all its permitted subsequent operations. Approximation to a chosen accuracy and efficient simulation are further, distinct requirements. The principle motivates <universal quantum computation> without itself being an experimental proof of every proposed universality claim.
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