A fundamental threat to quantum cryptography: gravitational attacks

An attack on the “Bennett-Brassard 84” (BB84) quantum key-exchange protocol in which Eve exploits the action of gravitation to infer information about the quantum-mechanical state of the qubit exchanged between Alice and Bob, is described. It is demonstrated that the known laws of physics do not allow to describe the attack. Without making assumptions that are not based on broad consensus, the laws of quantum gravity, unknown up to now, would be needed even for an approximate treatment. Therefore, it is currently not possible to predict with any confidence if information gained in this attack will allow to break BB84. Contrary to previous belief, a proof of the perfect security of BB84 cannot be based on the assumption that the known laws of physics are strictly correct, yet. A speculative parameterization that characterizes the time-evolution operator of quantum gravity for the gravitational attack is presented. It allows to evaluate the results of gravitational attacks on BB84 quantitatively. It is proposed to perform state-of-the-art gravitational attacks, both for a complete security assurance of BB84 and as an unconventional search for experimental effects of quantum gravity.