  Many quantum key distribution (QKD) implementations using a free space
transmission path are restricted to operation at night time in order to
distinguish the signal photons used for a secure key establishment from
background light. Here, we present a lean entanglement-based QKD system
overcoming that imitation. By implementing spectral, spatial and temporal
filtering techniques, we were able to establish a secure key continuously over
several days under varying light and weather conditions.
