Dear Arash,
to have a single photon system you need to set the layout bit rate to 1THz and use NRZ pulses that when modulated with 0dBm laser, the output of the modulator is -68.9dBm.
You could use an APD with some gain to detect the single photon pulses. However, the system will be very sensitive to noise and you should have almost noise free amplifier to be able to detect the NRZ pulses.
Regards,
Ahmad