It might so happen that a government is the first to fund and consequently produce a quantum computer. Considering this, it is unlikely an unprivileged individual would have access to such technology unless they developed or funded the research. All sophistry aside, quantum computers will cause all current forms of cryptography to become antiquated. The good/bad news is that building a functional quantum computer has proven to be a great challenge thus far. The better news is that cryptocurrencies are funding technological advances towards higher efficiency machines. It's only a matter of time until some entity realizes that such computing power translates into real world power.