From: <www.newscientist.com/channel/info-tech/mg18625054.000> 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...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...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 plus the "no-cloning theorem" means that if a quantum computer cannot factor an RSA modulus in under a second, RSA will remain unbreakable. (I'm not a quantum physicist or quantum computer programmer so I don't even know if the no-cloning theorem, which states qubits of unknown states cannot be copied, applies.) -Michael Heyman --------------------------------------------------------------------- The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to [EMAIL PROTECTED]