The quantum computer is colder than space, and it forgets

Superconducting quantum bits run near 15 millikelvin, far colder than the cosmic microwave background. They lose their state in microseconds to milliseconds. Error correction, not the cold, is the unsolved part.

A superconducting qubit is a small circuit that can sit in a superposition of two electrical states, if it is cold enough that thermal noise does not choose for it, and if it is quiet enough that no stray photon measures it by accident. Dilution refrigerators hold these circuits near 15 millikelvin. That is about two hundred times colder than the 2.7-kelvin afterglow of the Big Bang. Even so, the state leaks. A computation has to finish, or be corrected, before the leak wins. The correction itself needs many physical qubits to make one reliable one, and that overhead is the centre of the engineering problem.

Experiments have run particular circuits — sampling tasks, small simulations — that are awkward for ordinary machines. Those results are real and they are easy to overread. They are not a general computer that outruns the one on a desk, and they are not, today, a way through the public-key cryptography that moves money. A large error-corrected machine would change the second of those sentences. Building it is the part that is still ahead.