Entangled State Quantum Cryptography: Eavesdropping on the Ekert Protocol

TLDR

The near‑perfect correlations of entangled photons enable secret key sharing between two parties. The study proposes a procedure to enhance detection of eavesdroppers in Ekert’s protocol. The protocol monitors Bell‑inequality violations to detect eavesdroppers, evaluates strategies such as pseudo‑quantum‑nondemolition measurements, and introduces a method to improve detection. Using polarization‑entangled photons, Ekert’s protocol was implemented and any eavesdropper was found to be readily detectable.

Abstract

Using polarization-entangled photons from spontaneous parametric down-conversion, we have implemented Ekert's quantum cryptography protocol. The near-perfect correlations of the photons allow the sharing of a secret key between two parties. The presence of an eavesdropper is continually checked by measuring Bell's inequalities. We investigated several possible eavesdropper strategies, including pseudo--quantum-nondemolition measurements. In all cases, the eavesdropper's presence was readily apparent. We discuss a procedure to increase her detectability.