Gavin Andresen - 2011-12-14 23:34:29

I spent some time today looking again at the state of quantum computing: I'm still not worried.

The D-Wave system is not a general-purpose quantum computer; it is pretty specialized for solving certain problems (I'm reasonably certain cracking ECDSA encryption is not one of the problems it would be good at, but I am definitely NOT a quantum crypto expert).

Skimming the research, it looks like you'd need a specially-constructed quantum computer with 515 qbits and over 100million quantum gates, running more than 16 million quantum operations to crack Bitcoin's 256-bit ECDSA private keys using Shor's algorithm.

There's was a good reality-check article in the New York Times just last week:
   http://www.nytimes.com/2011/12/06/science/scott-aaronson-quantum-computing-promises-new-insights.html

Quote
Unfortunately, while small quantum computations have already been demonstrated in the lab, they typically fall apart after only a few dozen operations. That’s why one of the most-celebrated quantum computations to date has been to factor 15 into 3 times 5 — with high statistical confidence! The problem is decoherence: basically, stray interactions that intrude prematurely on the computer’s fragile quantum state, “collapsing” it like a soufflé. In theory, it ought to be possible to reduce decoherence to a level where error-correction techniques could render its remaining effects insignificant. But experimentalists seem nowhere near that critical level yet.

I've said it before:  I'll start to worry when quantum computers can factor 64-bit numbers.