Quantum key distribution with active sagnac interferometer

It is claimed a source of polarization entangled photon pairs for Quantum Key Distribution (QKD), comprising a laser, a circulator, a fiber based Sagnac loop and a wavelength division multiplexing (WDM) means, whereby the fiber based Sagnac loop comprises a polarizing beam splitter (PBS) and a polarization maintaining fiber, a Second-Harmonic-Generation (SHG) means, and a down conversion (DC) means, and whereby the polarizing beam splitter (PBS), the polarization maintaining fiber, the Second-Harmonic-Generation (SHG) means, and the down conversion (DC) means are arranged in a Sagnac configuration in order to generate polarization entangled photon pairs, and whereby the wavelength division multiplexing (WDM) means comprises two or more WDM-output ports. According to the intention the source comprises a first control means arranged in the fiber based Sagnac loop and a second control means arranged behind one or more WDM-output ports, and the first and/or second control means control the state of the polarization entangled photon pairs.