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

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