Quantum cryptography using femtosecond-pulsed parametric down-conversion

A new scheme for quantum cryptography, based on a distributed polarization quantum intensity interferometer, is presented. Two-photon entangled states generated via the optical nonlinear process of type-II phase-matched spont aneous parametric down-conversion are used to securely distribute secret cr yptographic keys. The high contrast and stability of the quantum interferen ce pattern obtained by using this design renders it superior to the best ex isting single-photon polarization technique. In addition, the use of high-r epetition-rate femtosecond pump pulses for down-conversion significantly en hances the production rate of entangled photon pairs for key distribution. [S1050-2947(99)50910-4].