SCOTUS has already given POTUS immunity in any form other than impeachment followed by a conviction. The problem with that is that just removes POTUS from office. It does nothing to punish for those crimes that were deemed worthy enough of being impeached/convicted. SCOTUS said that POTUS cannot be held accountable for things done as official acts of office. So Congress cannot hold POTUS criminally accountable, but removed from office to stop the criminal acts. Once POTUS becomes a citizen they are free. At this point, I can only see where the newly sworn POTUS would use their new pardon power to end the question as well.
However, all of this is very far away from the legality of quantum computing
> SCOTUS has already given POTUS immunity in any form other than impeachment followed by a conviction.
That's not exactly accurate and that nuanced difference may be the key to holding the executive branch accountable, now that we're in this disastrous state of the world.
Specifically, POTUS has absolute immunity from criminal prosecution for actions that are within their constitutional authority, presumptive immunity from all official acts (but not absolute), and no immunity from unofficial, private acts committed by the president.
Congress could also pass legislature or constitutional amendments to revert the effect of that tragic decision, though of course those also come with their own sets of challenges.
Let's not forget the complacency of this Congress. POTUS could not get away with this nonsense if Congress would do its job. The right has been working their way to this perfect scenario for decades with gerrymandering at the state level, Congress refusing to accept a SCOTUS nominee from POTUS holding out that the next POTUS would be their guy, and all of the other nonsense that has happened to get us to this spot.
However, all of this is very far away from the legality of quantum computing