quantum information theory requires manipulating counterfactual information—not what the current known states are, but what are the next possible states.
Inside physics, there is already a few principles which are counterfactual.
- Conservation of energy: a perpetual machine is *impossible
- Second law: its impossible to convert all heat into useful work
- Heisenberg’s uncertainty: its impossible to copy reliable all states of a qubit
With the impossibles, we can make the possible.