Polarization insensitive phase modulator for quantum cryptosystems

Abstract

In this paper, we propose a polarization-insensitive phase modulation scheme based on frequency modulation of light waves using either one or a pair of acousto-optic modulators. A stable Sagnac quantum key distribution (QKD) system employing this technique is also proposed. The interference visibility for a 40km and a 10km fiber loop is 96% and 99% respectively, at single-photon level. We ran standard BB84 QKD protocol in a simplified Sagnac setup (40km fiber loop) continuously for one hour and the measured quantum bit error rate stayed within 2%-5% range.