Earlier quoted context omitted.
>This is called Laplace's Demon. Pretty close! Laplace's Demon also talks about seeing the past - which my hypothetical does not. Nor am I concerned with calculating the entire universe, only the portions that would effect human cognition - just that which will be observable. Much of the universe is beyond our light cone, and unless we are in the center of the universe, almost certainly the vast majority is, and none…
Lets say you have that 300 qubit quantum computer, and you only need to simulate your own light cone. Trouble is, that quantum computer is also in your light cone. So to accurately simulate your light cone, your simulation has to include itself, recursively. Hence: not going to happen.
It might not work for telling the future, but if you limit the computation to see if it accurately models what is now history, you don't need it to simulate itself.