Tag
#E91
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- E91: The Protocol That Turns Bell's Theorem Into an Eavesdropping Detector
Where BB84 detects eavesdroppers by watching for measurement disturbances on individual photons, E91 — proposed by Artur Ekert in 1991 — detects them by testing whether quantum correlations between entangled photon pairs violate a Bell inequality. If they do, no eavesdropper could have intervened. Here is the physics, the mathematics of the CHSH test, and why E91's entanglement-based approach leads directly to device-independent quantum cryptography.
