I don't understand this. I may never understand this.
Uncertain Principles :You Can't Get There From Here: Category: Quantum Optics: Buried beneath some unseemly but justified squee-ing, Scalzi links to an article about "counterfactal computation", an experiment in which the group of Paul Kwiat group at Illinois managed to find the results of a quantum computation without running the computer at all. Really, there's not much to say to that other than "Whoa." The article describing the experiment is slated to be published in Nature, so I don't have access to it yet, but I'll try to put together an explanation when I get a copy. The experiment involves a phenomenon know as the "Quantum Zeno Effect".... [M]ake the measurement a much shorter time after the excitation-- a tenth of a second, say. The probability that the atom has already decayed is really, really small-- 0.002%-- so you're really likely to find it in the excited state, after which the atom is entirely in the excited state again, and the decay clock starts over.... If you keep making measurements at short intervals, you can keep the atom in the excited state basically forever.
The cool thing is, you can do this sort of thing with passive measurements. You don't have to bounce a photon off the atom to prove that it's in the excited state-- instead, you can send in a photon that will only be absorbed by a ground-state atom, and see what happens. If it isn't absorbed (and it most likely won't be), that's just as effective at keeping the atom in the excited state as if you'd done something more active to detect the excited-state atom.... If you're really clever about it (and Paul Kwiat is a really clever guy)... computing without running a computer should come as no surprise...
