I don't quite understand how is a quantum computer any different from any other computer. Of course it is more powerful but that doesn't make it any better for hacking Bitcoin.
Blockchain simply can't be hacked, because what written in the chain, stays in the chain. You can't modify the past blocks. The only way to hack Bitcoin is to get someone's private key which means that you install virus to his computer or you get access some other way. But quantum computer doesn't help you with that.