  We have demonstrated quantum key distribution (QKD) over a 10-km, 1-airmass
atmospheric range during daylight and at night. Secret random bit sequences of
the quality required for the cryptographic keys used to initialize secure
communications devices were transferred at practical rates with realistic
security. By identifying the physical parameters that determine the system's
secrecy efficiency, we infer that free-space QKD will be practical over much
longer ranges under these and other atmospheric and instrumental conditions.
