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 faster than they arise, as long as each step errs less often than a certain threshold. Nest codes inside codes, and a quantum computation can run as long as needed, as accurately as wanted.
The threshold theorem assumes errors strike independently, but in real devices they don’t: noise is correlated with the very states being computed. So no quantum device will get far enough below the threshold for error correction to work.
On our Willow chip, logical qubits spread over squares of 3 by 3, 5 by 5, and 7 by 7 qubits failed less often the bigger they were: each step up in size cut the failures by more than half. The errors are far enough below the threshold for error correction to work, and the biggest logical qubit outlived the best physical qubit in it.
Drawing samples from random quantum circuits, Willow did in under five minutes what would take one of today’s fastest supercomputers 10 septillion years. That suggests quantum computation happens in many parallel universes, as David Deutsch first predicted.