Onur Hosten of the University of Illinois at Urbana-Champaign and colleagues have created a quantum computer, which uses the laws of quantum mechanics to perform multiple calculations simultaneously. The quantum computer uses lasers, mirrors and light detectors, to encode information in the quantum states of photons. Quantum objects such as atoms and photons can be put in different mixtures of states, called ’superposition’ states. Each photon is placed in a superposition where it both is and isn’t fed into a ‘black box’ (which processes the quantum states through an algorithm). This allows the computer to work both when it’s on and off. "Some people like to think of this as two different universes", says computer scientist Richard Josza of Bristol University in England. Because a not-running computer is working in a parallel universe, "you wouldn’t be charged for the cost of running it."

The downside is that quantum processes give probabilities rather than exact answers. Eight years ago, researchers thought that the success rate of a quantum computer would be little better than guessing. But Hosten and co. proved them wrong by incorporating a phenomenon known as the quantum Zeno effect, named after the 4th century Greek philosopher famous for his paradoxes. Also known as "the watched pot never boils", the effect is that you can change the probability of a particular outcome simply by looking for it. So, by looking at the photons in the not-running state (those that didn’t pass through the black box), it was possible to significantly increase the probability of the computer finding the right answer. "It’s a very beautiful experiment that probes the strangeness of quantum theory," says Josza.

The computer that works when it’s idle, Nature.com