  The statistical fluctuations in free-space links in the turbulent atmosphere
are important for the distribution of quantum signals. To that end, we first
study statistics generated by the turbulent atmosphere in an entanglement based
free-space quantum key distribution (QKD) system. Using the insights gained
from this analysis, we study the effect of link fluctuations on the security
and key generation rate of decoy state QKD concluding that it has minimal
effect in the typical operating regimes. We then investigate the novel idea of
using these turbulent fluctuations to our advantage in QKD experiments. We
implement a signal-to-noise ratio filter (SNRF) in our QKD system which rejects
measurements during periods of low transmission efficiency, where the measured
quantum bit error rate (QBER) is temporarily elevated. Using this, we increase
the total secret key generated by the system from 78,009 bits to 97,678 bits,
representing an increase of 25.2% in the final secure key rate, generated from
the same raw signals. Lastly, we present simulations of a QKD exchange with an
orbiting LEO satellite and show that an SNRF will be extremely useful in such a
situation, allowing many more passes to extract a secret key than would
otherwise be possible.
