ATTEMPTS to build quantum computers could run up against a fundamental limit on how long useful information can persist inside them. Exceed the limit and information could just leak away, making computation impossible.

A quantum computer manipulates stored information in quantum bits, or qubits. Because a qubit can be in twostates at the same time, and can be entangled with other qubits, a quantum computer can carry out multiple calculations simultaneously. But the entire system is delicate: during a computation the qubits have to be isolated from their environment, because any outside disturbance can cause "decoherence" and spoil the calculations.

Coherence is harder to maintain in larger qubits containing more particles, because there is more potential for interaction with the surroundings. To try and limit this effect, researchers are pursuing ways of making microscopic qubits. These can be made using superconducting circuits on silicon chips or with quantum dots, which are essentially droplets of semiconducting materials that contain free electrons. In principle, qubits can be made out of individual electrons and photons.

But physicists Jasper van Wezel, Jeroen van den Brink and Jan Zaanen of Leiden University in the Netherlands have shown that efforts to engineer quantum computers around ever-smaller qubits may face significant obstacles. "We have proven that there is a universal decoherence rate for qubits," says van den Brink. This means that quantum information will inevitably be lost after a certain time, even without any external disturbance. Rather than remaining in a superposition of two states, a qubit will spontaneously collapse into one state or another (Physical Review Letters, vol 94, p 230401). "When we discovered this we were stunned," says van den Brink.

Worryingly, the time limit for decoherence seems to grow shorter as systems get smaller. Zaanen says that for some of the most promising qubit technologies the limit would be about 1 second. It's not a problem at the moment, he says, because researchers are fighting to get coherence times up to around a microsecond. "But this fundamental limit is getting within reach."

"This is very interesting," says physicist Peter Zoller of the University of Innsbruck in Austria. But the real implications for quantum computing will only become clear with further work, especially experiments that would measure these effects, he says.