  Random numbers represent a fundamental ingredient for numerical simulation,
games, informa- tion science and secure communication. Algorithmic and
deterministic generators are affected by insufficient information entropy. On
the other hand, suitable physical processes manifest intrinsic unpredictability
that may be exploited for generating genuine random numbers with an entropy
reaching the ideal limit. In this work, we present a method to extract genuine
random bits by using the atmospheric turbulence: by sending a laser beam along
a 143Km free-space link, we took advantage of the chaotic behavior of air
refractive index in the optical propagation. Random numbers are then obtained
by converting in digital units the aberrations and distortions of the received
laser wave-front. The generated numbers, obtained without any post-processing,
pass the most selective randomness tests. The core of our extracting algorithm
can be easily generalized for other physical processes.
