BBM92

BBM92 is a quantum key distribution protocol that uses entangled photon pairs to establish a shared secret between two parties, typically called Alice and Bob. It was proposed in 1992 by Charles Bennett, Gilles Brassard, and N. David Mermin as an entanglement-based variant of the BB84 scheme.

In BBM92, a source emits pairs of maximally entangled photons, such as a Bell state. One photon of each pair is sent to Alice and the other to Bob. Each party independently and randomly chooses one of two measurement bases, often corresponding to two polarization bases (rectilinear and diagonal). When both parties measure in the same basis, their results are ideally perfectly correlated (or anti-correlated, depending on the state), producing raw key bits. When the bases differ, the results are discarded for key generation but may be used to test for eavesdropping. The protocol’s security can be connected to the violation of Bell inequalities, providing a link between observed correlations and the presence of an eavesdropper.

Implementation typically relies on polarization-entangled photon pairs generated by spontaneous parametric down-conversion, transmitted over optical fibers or through free space. Practical challenges include channel loss, detector inefficiency, and background noise. After measurement, classical post-processing steps such as error correction and privacy amplification are applied to obtain a secure secret key.

BBM92 is closely related to BB84 and E91, representing a bridge between prepare-and-measure and entanglement-based approaches. It offers the advantages of entanglement-based correlations while sharing the fundamental security principles of quantum key distribution, though it faces technical complexities and rate limitations in real-world deployments.