E91 Protocol
Ekert QKD · entanglement-based · Bell test security
Physics: E91 (Ekert 1991): QKD using entangled photon pairs. Alice and Bob each choose from 3 measurement angles independently. Correlated results (same-angle subsets) → raw key. Uncorrelated subsets → Bell test. Security: any eavesdropping disturbs entanglement → CHSH value S drops below 2√2. Device-independent QKD: security based on Bell inequality violation alone, no device trust required. Loophole-free Bell test needed for full DI-QKD. Micius satellite (2017): 7600 km secure key distribution. BB84 uses prepare-and-measure; E91 uses entanglement. Key rate ∝ (1 − h(QBER)) where h is binary entropy and QBER is quantum bit error rate.