Volume Bragg gratings for wavelength division multiplexing of entanglement-based quantum key distribution in free space

Wavelength division multiplexing (WDM) improves key generation rates in quantum key distribution (QKD) and enables multi-user networks using shared quantum resources. The last years have seen a flurry of research on the implementation of WDM in fiber-based QKD where low-loss telecom components are widely available. However, WDM has not yet been demonstrated in free-space and satellite-based quantum communication in the wavelength range detectable by space suitable silicon single photon avalanche diodes. Here, we report a low-loss WDM at around 810 nm based on volume Bragg gratings. The total insertion loss for four wavelength channels matches state-of-the-art telecommunication WDM in the C-band. We characterize the performance in conjunction with a polarization-entangled photon source and report on preliminary assessments of background noise for a terrestrial free-space link in broad daylight.