Can One Machine Do It All?
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See chronicle·See most recent related ideasA quantum computer could factor numbers hundreds of digits long in hours, by finding how often the remainders of a number’s powers repeat. That would break the codes that protect most of the internet’s secrets, which rely on factoring being too hard.
A quantum computer could never run long enough. Its quantities vary continuously, like those of an old analog computer, so small errors pile up and can’t be corrected the way a classical computer corrects a slipped bit: you can’t even look at a quantum state to check it without disturbing it.
Quantum errors can be corrected without looking at the data. Spread one qubit over nine, tied together so that checking how they relate to each other shows whether one of them went wrong, and how to undo it, without revealing what the qubit holds.
Error correction can’t save a quantum computer in practice. It needs many more qubits and steps, each of which errs too, and the large entangled states it relies on are extremely fragile, so it would add errors faster than it removes them.