How Phase Kickback Saves BB84

BB84 is perhaps the most famous quantum key distribution (QKD) protocol, the first demonstration that quantum principles could be applied to key distribution, an critical part of information security. As quantum computing progresses, classical methods for key security are vulnerable to attacks from quantum computers. In this presentation, we prove the security of the BB84 QKD protocol against a repetition attack built with CNOT gates. While seemingly compromising, the attack is undermined by phase kickback, a fundamental but sometimes overlooked property in quantum computation.