Dawn and dusk satellite quantum key distribution using time and phase based encoding and polarization filtering

Free-space optical communication links promise better security and increase bandwidths but can suffer from noise in daylight. This is particularly detrimental in quantum communications where current mitigation techniques, such as spectral, temporal and spatial filtering, are not yet sufficient to reduce daylight to levels tolerable for satellite quantum key distribution (SatQKD). As all current SatQKD systems are polarization encoded, polarization filtering has not been investigated. However, by using time- and phase-encoded SatQKD, it is possible to filter the polarization domain and existing domains. Scattered daylight can exceed 90% degree of polarization in the visible band, yielding a reduction in detected daylight of between 3dB and 13dB. Polarization filtering can reduce the brightness of 780nm daylight to below its unfiltered equivalent at 1550nm. Through simulation, it is demonstrated that this extra domain of filtering can allow an extension in the time-window available to perform SatQKD to dawn, and thus dusk as well as an increase to the secure key rate. This could open the way for successful daylight SatQKD utilizing shorter near-infrared wavelengths and retaining their benefits.