Photon Detection Rate
The photon detection rate in a quantum-noise limited heterodyne receiver is given by:
eta.Pr Nph = -------- hf
eta = quantum efficiency (0.5),
We have already stated that the 1 kW SETI transmitter at a distance of 10 light years produces a received signal Pr = 1.60 x 10-15 W (-118 dBm). Using the values given above, we find that:
Nph = 2640 photons/second
In digital systems we often measure the sensitivity of an optical receiver in terms of the number of photons required to be detected for each bit of error-free information. While clever digital encoding and modulation schemes can lower the number of photons required per second, present coherent fiber-optic systems under development can achieve values of about 10 photons/bit for Bit Error Rates (BER) 10-9. Such systems have bandwidths of several hundred MHz to several GHz, so the optical power received is much greater than we are discussing here for this SETI application. The difference in received power is about ten orders of magnitude!
One could consider the "ideal" photonics detection system to be one in which only one or two photons are required to be detected for each bit of information.