  We analyze the significance for quantum key distribution (QKD) of free-space
quantum communications results reported in a recent paper (J. C. Bienfang et
al., quant-ph/0405097, hereafter referred to as "Bienfang et al."), who
contrast the quantum communications rate of their partial QKD implementation
(which does not produce cryptographically useful shared, secret keys) over a
short transmission distance, with the secret bit rates of previous full QKD
implementations over much longer distances. We show that when a
cryptographically relevant comparison with previous results is made, the system
described by Bienfang et al. would offer no advantages for QKD, contrary to
assertions in their paper and in spite of its high clock rate. Further, we show
that the claim made by Bienfang et al. that "high transmission rates serve ...
to extend the distance over which a QKD system can operate" is incorrect. Our
analysis illustrates an important aspect of QKD that is too often overlooked in
experiments: the sifted bit rate can be a highly misleading indicator of the
performance of a QKD system.
