To compute or not to compute?

摘要

Is it possible to get a sensible answer from a computer without even running a program? Yes indeed, report Hosten et al. on page 949 of this issue— and they have the experimental proof. The thread that leads thus far has its origin in the early 1990s, with the introduction of a quantum paradox known as the Elitzur-Vaidman bomb thought-experiment. They showed that by sending a photon into a simple optical interferometer (Box 1), one can sometimes detect the presence of a light-triggered bomb without setting it off — in other words, without light interacting with it at all. 'Sometimes' — there's the rub. At other times, you set the bomb off; at still other times, you get no information about the existence of the bomb at all. But then that's the probabilistic nature of quantum mechanics. It might seem to defy common sense, but the result is a straightforward application of the quantum theory of light.