Houjun Liu


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.

  1. Conservation of energy: a perpetual machine is *impossible
  2. Second law: its impossible to convert all heat into useful work
  3. Heisenberg’s uncertainty: its impossible to copy reliable all states of a qubit

With the impossibles, we can make the possible.