Abstract

We demonstrated quantum cryptography, or quantum key distribution (QKD), in free space over an outdoor optical path of ~ 950 m under nighttime conditions. The success of free-space QKD against a high background depends on the transmission and detection of single photons through an optically thick and turbulent medium. It has been shown that a combination of sub-nanosecond timing, narrow filters,¹ spatial filtering, and adaptive optics can render the difficult detection problem tractable. Further, the nonbirefringent nature of the atmosphere at optical wavelengths allows the transmission of the single-photon polarization states used in the free-space QKD protocol.

© 1998 Optical Society of America