A Classical-Light Attack on Energy-Time Entangled Quantum Key Distribution, and Countermeasures

Abstract: Quantum key distribution (QKD) is an application of quantum mechanics that allowstwo parties to communicate with perfect secrecy. Traditional QKD uses polarization of individual photons, but the development of energy-time entanglement could lead to QKD protocols robust against environmental effects. The security proofs of energy-time entangled QKD rely on a violation of the Bell inequality to certify the system as secure. This thesis shows that the Bell violation can be faked in energy-time entangled QKD protocols that involve a postselection step, such as Franson-based setups. Using pulsed and phase-modulated classical light, it is possible to circumvent the Bell test which allows for a local hidden-variable model to give the same predictions as the quantum-mechanical description. We show that this attack works experimentally and also how energy-time-entangled systems can be strengthened to avoid our attack.