It seems to me that there are quite low odds of 4000-qubit computers being deployed without proper preparations? There are very strong incentives for cryptography-using organizations of almost any stripe to transition to post-quantum encryption algorithms as soon as they expect such algorithms to become necessary in the near future, for instance as soon as they catch wind of 200- and 500- and 1000- bit quantum computers. Given that post-quantum algorithms already exist, it does not take much time from worrying about better quantum computers to protecting against them.
In particular, it seems like the only plausible route by which many current or recent communications are decrypted using large quantum computers is one in which a large amount of quantum computation is suddenly directed towards these goals without prior warning. This seems to require both (1) an incredible series of both theoretical and engineering accomplishments produced entirely in secret, perhaps on the scale of the Manhattan project and (2) that this work be done by an organization which is either malicious in its own right or distributes the machines publicly to other such actors.
(1) is not inconceivable (the Manhattan project did happen*), but (2) seems less likely; in particular, the most malicious organizations I can think of with the resources to pull off (1) are something like the NSA, and I think there is a pretty hard upper bound on how bad their actions can be (in particular, “global financial collapse from bank fraud” doesn’t seem like a possibility). Also, the NSA has already broken various cryptographic schemes in secret and the results seem to have been far from catastrophic.
I don’t see a route by which generic actors could acquire RSA-breaking quantum tech and where the users of RSA wouldn’t also be able to recognize this event coming months if not years in advance.
*Though note that there were no corporations working to develop nuclear bombs, while there are various tech giants looking at ways of developing quantum computers, so the competition is greater.
Thanks! This is reassuring. I met someone last week who does his PhD in post-quantum cryptography and he did tell me about an ongoing competition to set the standards of such a cryptography. The transition seems on its way!