  True physics is quantum-mechanical, and this implies a different idea of matter and action from Turing's purely classical picture. It is perhaps odd that Turing did not point this out in this period, since he was well versed in quantum physics. Instead, the analysis and practical development of quantum computing was left to the 1980s. Quantum computation, using the evolution of wave-functions rather than classical machine states, is the most important way in which Turing machine model has been challenged. The standard formulation of quantum computing (Deutsch 1985, following Feynman 1982) does not predict anything beyond computable effects, although within the realm of the computable, quantum computations may be very much more efficient than classical computations. It is possible that a deeper understanding of quantum mechanical physics may further change the picture of what can be physically ‘done.’