I was supposed to post this link https://bitcointalk.org/index.php?topic=5157696.0 but mk4 beat me to it. Anyway, that thread, which is only at least a year old, has a rich discussion on quantum computing and how it is possibly not a threat to Bitcoin.

But please be warned in advance that the discussion is highly technical and with long, albeit very information-rich, posts which left me more confused than before. But you will certainly gain a lot from it if you have dead serious questions and anxieties on quantum computers possibly a threat to Bitcoin and cryptocurrency in general.