[jcranmer]

A high-end desktop is better at simulating quantum computers than any that we've ever built [1]. The best quantum computers achieve about 10 qubits. Furthermore, quantum operations are slower than classical operations. A classical computer can do a 64-bit operation (depending on clock speed) in ~0.25ns, a quantum computer takes about 5ns. Also, quantum computers don't scale like classical computers. 2 classical computers can solve twice as many problems or problems twice as hard, while 2 quantum computers can only solve twice as many problems.

The hard part is hardware, and you need the minimum number of qubits to be of any use to breaking RSA. That number is on the order of O(N^2), where N is the number of bits--so you need qubits on the order of hundreds of thousands if not millions. Where the state of the art is around 10.

[1] D-Wave is approaching this equivalency, for certain definitions of quantum computers.