Abstract: A method of detecting eavesdropping, i.e. a physical layer attack, on an optical communications channel. The method comprises sending a first, classical message over an optical channel by encoding the message onto an optical carrier using a classical communications technique and sending a second, quantum message over the optical channel using a quantum cryptographic technique, e.g. a Continuous Variable Quantum Key Distribution (CV-QCD) technique; and detecting eavesdropping of the classical message on the optical channel by detecting the eavesdropping of the quantum message. In implementations the classical and quantum signals may be substantially indistinguishable to hinder an adversary from eavesdropping on the first signal without influence the second signal.