There are a few quantum systems that are out there already, but mainly being used in research and development projects. Bitcoin mining today is very brute-force oriented with a nonce-based algorithm, the quantum computer would probably do an amazing job on boolean SAT solving model. Hashing... not so much, so not really viable.

Or.... you skip the mining and just use it to figure out the prime factorization for SHA256 and get the private key for 3A1KUd5H4hBEHk4bZB4C3hGgvuXuVX7p7t

Lockheed, Google, and USC have one of these systems which has 1000 qubits:

http://www.dwavesys.com/d-wave-two-system