Quantum Cryptography with Femtosecond Parametric down Conversion

We experimentally demonstrate a quantum cryptography system using two-photon entangled (EPR) states generated via the nonlinear process of spontaneous parametric down conversion pumped by a femtosecond laser. There are two major approaches in quantum cryptography which historically appeared almost simultaneously. One uses the quantum features of single photon states produced by signi cant attenuation of original light in a coherent state. The other is based on the quantum nonlocal character of two-photon entangled EPR states. The applicability of the latter one was strongly limited because of low visibility and poor stability of the systems which require synchronous manipulation of two Mach-Zehnder interferometers well separated in space. We developed a new scheme for quantum cryptography which is based on the use of a distributed polarization quantum intensity interferometer. This technique utilizes a double-entangled EPR quantum states generated in the nonlinear process of type-II spontaneous parametric down conversion (SPDC). The high contrast and stability of quantum interference demonstrated in our preliminary experiments promises to bring the performance of this system above the level of the best single-photon polarization techniques, and to do so without their speci c limitations. The use of a high-repetition rate femtosecond pulses as a pump source enhances signi cantly the ux of entangled photon pairs available for the reliable and secure key distribution.