Here is what would make a quantum computer so powerful.
I find it fascinating that based on what we now know, we can’t yet say that it’s impossible to travel in time.
These virtual particles are really particles that simply are created from empty space, particle anti-particle pairs that exist for a time-scale so short that you can’t observe them by any measurement, but they're there.
In cosmology we are at a similar place to where Darwin was in biology.
You want to make this macroscopic object, you want to keep it behaving quantum mechanically which means isolating it very carefully from, within itself, all the interactions and the outside world.