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,” though, which deserves a post of its own, below the fold.

The “Quantum Zeno Effect” takes its name from Zeno’s paradoxes, which argue that motion ought to be impossible, because to cover any given distance requires you to first move half the distance, then half the remaining distance, then half again, and so on *ad infinitum*. Each of those distances should require a finite time to move, and there are infinitely many steps, so you should never get anywhere.

The paradox fails, of course, but in the quantum world, it can be made surprisingly real, thanks to the nature of quantum measurement. Consider the case of an atom placed into an excited state with a finite lifetime. After some period of time, say one second, there is a 50% probability that the atom has spontaneously decayed to the ground state.

If you do a measurement that determines the state of the atom, you have a 50% chance of finding it in the excited state, and a 50% chance of finding it in the ground state. “Big deal,” you say, but here’s the key: after you make that measurement, the atom is 100% in whichever state you measured. A second measurement a short time later is guaranteed to find the same result as the first.

So, imagine a different experiment– rather than waiting until the results are 50/50, make 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. Then mesaure it again, and again, and again, waiting a tenth of a second each time. After ten measurements, you’re one second past the original excitation, and the probability of finding the particle in the excited state is almost 100% (99.98%, give or take). 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), you can use this basic idea to make lots of different measurements. On their web site, there’s a very nice explanation (there are also some nifty little movies of the interferometer technique they use) of a way to use the quantum Zeno effect to detect the presence of a photo-sensitive bomb without hitting it with a photon and blowing yourself up. And really, if you can do **that**, computing without running a computer should come as no surprise…