Hello there, I have a question that annoying me last days.

Can anyone explain me please, what happen if Some organization (corporation or country or maybe even some group of people) create Quantum Computer?
How it could affect on Bitcoin if this (let say group of people) want to destroy the whole system.
As I understand with the Power of such computer you could easily select private keys or create a 51% attack?
And probably give advice what to do in that case, I mean it would be enough to store my Bitcoin on cold wallet(It would be safe?), or panic sell in physical gold))

Thank you in advance.