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David Deutsch proposed a universal quantum model of computation in 1985 and formulated a physical version of the physical Church-Turing principle. His argument treated the laws governing a computing device as part of the theory of computation, rather than as an implementation detail.^[ref]

A quantum device evolves amplitudes through permitted quantum operations, which can produce behaviour unavailable to the corresponding classical restriction. This motivates universal quantum computation; it does not mean that reading one quantum state returns every amplitude or that every computational problem automatically becomes easy.

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.^[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.