Someone could code up a "coin wallet" right now that could be easily be cracked with a quantum computer doing quantum computation. It’d be a super weak key (maybe 22-bit RSA or ECC or something), but it would be a demonstration of real quantum computation cracking a real ‘wallet-style’ key using real quantum effects, and provable. Any classical computer could crack that key too, of course, though not by making use of entanglement.