  Previous demonstrations of free-space quantum communication in daylight have
been touted as significant for the development of global-scale quantum
networks. Until now, no one has carefully tuned their atmospheric channel to
reproduce the daytime sky radiance and slant-path turbulence conditions as they
exist between space and Earth. In this article we report a quantum
communication field experiment under conditions representative of daytime
downlinks from space. Higher-order adaptive optics increased quantum channel
efficiencies far beyond those possible with tip/tilt correction alone while
spatial filtering at the diffraction limit rejected optical noise without the
need for an ultra-narrow spectral filter. High signal-to-noise probabilities
and low quantum-bit-error rates were demonstrated over a wide range of channel
radiances and turbulence conditions associated with slant-path propagation in
daytime. The benefits to satellite-based quantum key distribution are
quantified and discussed.
