If quantum computers were actually available (at least to someone) and were effectively able to successfully attack cryptographic algorhitms, there would be a lot more profitable ways to use them; and, if one really wanted to use them against the Bitcoin ecosystem, cracking private keys would be a quite more useful target than mining.
The profitable way to use these kinds of technology would be to simply mine altcoins on a multipool, trying to do anything else would likely cost more then it would generate in revenue.