If quantum computers ever work, then by using Groover's algorithm mining difficulty would be square root of the usual one.

Let's say that mining difficulty is 80 bits. Then QC difficulty would be equivalent to 40 bits. Usual miners would do 2⁸⁰ operations, while QC would do 2⁴⁰ quantum operations.

But, IMO, unfortunately QC wouldn't work for any task other than generating enormous amounts of noise.