In theory, a quantum computer could exploit the principles of quantum mechanics to achieve massively parallel processing. Quantum laws allow for the bizarre phenomenon of “counterfactuality”: one can glean information about a quantum event that did not actually take place.

Two British researchers have described a hypothetical scheme that could achieve just that. It would allow for probing all the possible states of a quantum computer, including that in which all the “switches” are “off” — that is, in which the computer is not even turned on.