Correcting Polarization Degradation in Free-Space QKD Systems

Abstract

Prepare-and-measure quantum key distribution (QKD) schemes with polarization encoding require the minimization of polarization degradation, i.e., undesired variations in the state of polarization, in the transmitter and receiver designs. In particular, the polarization extinction ratio (PER) should be maintained as high as possible to reduce the quantum bit error rate (QBER). In this paper, we analyze the polarization degradation and its correction in the design of free-space QKD systems. The polarization of a wavefront can be degraded in two different ways: uniformly, or non-uniformly; the latter known as polarization aberrations. While uniform polarization degradation of the beam wavefront can be easily corrected with standard retardation plates, polarization aberrations are more complex and require a different correction approach. BB84 diagonal linearly-polarized states are especially affected by polarization degradation as they are reflected off certain optical elements, and its correction is essential to achieve a low QBER. We have experimentally characterized the polarization degradation in a QKD transmitter and its impact in the QBER and achieved an almost perfect correction of the diagonal linearly polarized states when choosing a specific optical design.