Free-space quantum key distribution links in an urban environment have demanding operating needs such as functioning in daylight and under atmospheric turbulence, which can dramatically impact their performance. Both effects are usually mitigated with a careful design of the field of view of the receiver. However, a trade-off is often required, since a narrow field of view improves background noise rejection but is linked to an increase in turbulence-related losses. We present a high-speed automatic tracking system to overcome these limitations. Both a reduction in the field of view to decrease the background noise and a mitigation of the losses caused by atmospheric turbulence are addressed. Two different designs are presented and discussed, along with technical considerations for the experimental implementation. Finally, preliminary experimental results of beam wander correction are used to estimate the potential improvement of both the quantum bit error rate and secret key rate of a free-space quantum key distribution system.
Acknowledgments
We thank the project TEC2012-35673 from the Ministerio de Economía y Competitividad.
