Single photon per bit optical communication between ground stations and low earth orbit satellites will enable the secure global distribution of cryptographic keys.
The communications satellite AO-40 contains an experimental infra-red payload consisting of a low power 835 nm laser diode transmitter coupled to a small fixed orientation telescope transmitting 400 baud Manchester encoded data . The received signal levels are of the order of 10 photons/bit for an effective telescope aperture of diameter 100 mm . They therefore offer a means of validating models of low photon number OOK signal propagation and detection in the presence of atmospheric turbulence, an issue relevant to free-space quantum communications. This paper describes the design, testing and implementation of a photon counting receiver for AO-40 signals using a Labview software platform to display received data, error statistics, detection and correction (using cyclic redundancy check code CRCC 32), and clock synchronization . Signals are acquired with a 300 mm f/6.3 Schmidt-Cassegrain tracking telescope coupled by 100 micron multimode fibre to a silicon APD photon counting module.
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