  Despite quantum networking concepts, designs, and hardware becoming
increasingly mature, there is no consensus on the optimal wavelength for
free-space systems. We present an in-depth analysis of a daytime free-space
quantum channel as a function of wavelength and atmospheric spatial coherence
(Fried coherence length). We choose quantum key distribution bit yield as a
performance metric in order to reveal the ideal wavelength choice for an actual
qubit-based protocol under realistic atmospheric conditions. Furthermore, our
analysis represents a rigorous framework to analyze requirements for spatial,
spectral, and temporal filtering. These results will help guide the development
of free-space quantum communication and networking systems. In particular, our
results suggest that shorter wavelengths should be considered for free-space
quantum communication systems. Our results are also interpreted in the context
of atmospheric compensation by higher-order adaptive optics.
