Experimental evaluation of geometric constellation for m-ary quantum polsk in turbulent free-space laser communications

Abstract

As urban Free-Space Optical (FSO) networks evolve toward quantum-secure communications, physical layers must withstand atmospheric turbulence without relying on the high optical power of classical detectors. While intensity-based modulation is vulnerable to scintillation-induced threshold fading, Polarization Shift Keying (PolSK) may offer a robust, self-referencing alternative encoded in the ratio of orthogonal states. This study experimentally characterizes the performance of photon-counting PolSK, focusing on the quaternary (M=4) regime with extensions to M-ary protocols, across simulation and in-laboratory experiments by using a polarizing beam splitter and two Single-Photon Avalanche Diode (SPAD) detectors. We investigate the impact of the integration window on error rates, aiming to identify the performance requirements and assessing the limitations of standard linear angle spacing due to the non-linearity of direct detection. We propose and evaluate a geometric constellation based on equal-probability spacing, and dynamic thresholds designed to equalize noise margins, providing a scalable engineering blueprint for resilient, rate-adaptive quantum receivers.