QCs won't appear overnight. We will see someone demonstrate a small-scale QC that is too small to usefully break real world crypto. As soon as that happens, the

QCs won't appear overnight. We will see someone demonstrate a small-scale QC that is too small to usefully break real world crypto. As soon as that happens, there will be a rush to migrate everything to post-quantum crypto. Anyone not doing this with their coins as soon as they can is a fool. It's possible that bigger QCs will appear in the "black" world before they are publicly known, but in that case they're not likely to be used to steal bitcoin since doing so would indirectly reveal their existence. If too many BTC wallets get silently snarfed, people will suspect a break. Anyone with such a QC would be better off sitting and passively monitoring for as long as they can.