Complete physical simulation of the entangling-probe attack on the Bennett-Brassard 1984 protocol

We have used deterministic single-photon two-qubit quantum logic to implement the most powerful individual-photon attack against the Bennett-Brassard 1984 (BB84) quantum key distribution protocol. Our measurement results, including physical source and gate errors, are in good agreement with theoretical predictions for the Rényi information obtained by Eve as a function of the errors she imparts to Alice and Bob’s sifted key bits. The current experiment is a physical simulation of a true attack, because Eve has access to Bob’s physical receiver module. Nevertheless, the physical simulation allows investigation of the fundamental security limit of the BB84 protocol against eavesdropping in the presence of realistic physical errors, and it affords the opportunity to study the effectiveness of error correction and privacy amplification when the BB84 protocol is attacked.