  Quantum Counterfactual Communication is the recently-proposed idea of using
quantum physics to send messages between two parties, without any matter/energy
transfer associated with the bits sent. While this has excited massive
interest, both for potential `unhackable' communication, and insight into the
foundations of quantum mechanics, it has been asked whether this process is
essentially quantum, or could be performed classically. We examine
counterfactual communication, both classical and quantum, and show that the
protocols proposed so far for sending signals that don't involve matter/energy
transfer associated with the bits sent must be quantum, insofar as they require
wave-particle duality.
