so we do have quantum computers today. however they are no practical threat to the bitcoin network or any other at the moment. these types of machines are really only currently used for problems in optimization. for instance if you want to route all the flights of an airline carrier in the most effective way possible. quantum computers can handle problems with many many variables. like for instance taking all starting point and end points of the destinations of all your passengers, as well as all the gates you have available at all the airports in the country, as well as how many pilots you have on staff, and the amount of airplanes the company has in service... then the computer can spit out what it believes is the most optimum outcome for which people belong on which planes in order to to get everyone to their destination in the most cost effective manner possible. quantum computers are also used in weather forecasting, where again there are a tremendous amount of data and variables and the outcome is not necessarily an exact answer but a probability a specific outcome will occur. these types of computers are EXTREMELY difficult to just write programs for and to date we have not unlocked the real potential for this kind of technology. your bitcoins are safe. you should be more concerned about Bitmain tbh.