Title: Day and Night QKD over an atmospheric free space channel with passive polarization bases compensation

Authors: Saumya Ranjan Behera 1, Melvee George 1, Urbasi Sinha 1

1 Light and Matter Physics Group, Raman Research Institute, Bengaluru, 560080, India

Quantum Communication (QC) represents a future technology, revolutionising secure communication. Photon based Quantum Key Distribution (QKD) is a widely used approach to perform Quantum Key Distribution using the polarisation degree of freedom of photons. However, the photon-polarization gets affected due to the birefringence of fibres in fibre-based implementations, or variation of reference frames due to satellite movement in long distance demonstrations.

We investigate and mitigate the challenges posed by fibre birefringence and atmospheric effects on QKD, by using passive polarisation bases compensation instead conventional active feedback mechanism to track polarisation fluctuation of photons. Following upon our earlier work, we have implemented Day and Night entanglement based QKD over this channel using a combined fibre based and 50 m free space optical link. To overcome daylight QKD challenges, we employ a range of filtering techniques to enhance signal quality and security. Our approach of polarization correction by performing measurement along suitably rotated (optimized) projection bases, can potentially serve as a useful technique for mitigating polarization fluctuations in both free-space and fibre-based QKD implementations.