Entangled state quantum cryptography: Eavesdropping on the Ekert protocol

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