  Quantum key distribution (QKD) has matured in recent years from laboratory
proof-of-principle demonstrations to commercially available systems. One of the
major bottlenecks is the limited communication distance in fiber networks due
to the exponential signal damping. To bridge intercontinental distances, low
Earth orbit satellites transmitting the quantum signals over the atmosphere can
be used. These free-space links, however, can only operate during the night, as
the sunlight otherwise saturates the detectors used to measure the quantum
states. For applying QKD in a global quantum internet with continuous
availability and high data rates, operation during daylight is required. In
this work, we model a satellite-to-ground quantum channel for different quantum
light sources to identify the optimal wavelength for free-space QKD in ambient
conditions. Daylight quantum communication is possible within the Fraunhofer
lines or in the near-infrared spectrum, where the intrinsic background from the
sun is comparably low. The highest annual secret key length considering the
finite key effect is achievable at the H\textalpha\ Fraunhofer line. More
importantly, we provide the full model that can be adapted in general to any
other specific link scenario. We also propose a true single-photon source based
on a color center in hexagonal boron nitride coupled to a microresonator that
can implement such a scheme. Our results can also be applied in roof-to-roof
scenarios and are therefore relevant for near-future quantum networks.
