All present-day computers, whatever quantum-mechanical processes they may exploit, are merely different technological implementations of the same classical idea, that of the universal Turing machine. That is why the repertoire of computations available to all existing computers is essentially the same: they differ only in their speed, memory capacity and input–output devices. That is to say, even the lowliest of today’s home computers can be programmed to solve any problem, or render any environment, that our most powerful computers can, provided only that it is given additional memory,
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