Hopefully someone much more familiar with quantum computing than I will be able to provide the best answer but I suspect the answer is yes, security as we know will be fundamentally changed. Quantum computing breaks the Moores Law @hilarski mentioned above by immediately going to an almost infinite speed where all machine instructions are executed in a parallel pipeline in virtually zero time. So, if using today’s super computers would take millions of years (some people calculate many billions) to break the security of bitcoin, then a quantum computer could do it in a few seconds….providing there’s a quantum computer, that it has a programming language that works, that memory has been invented that can process that fast, and that the channel architecture of the computer has a fast enough bus….