An optical data transmission system and method for, at an optical transmitter, converting to a second wavelength, an optical data signal at a first wavelength; transmitting at the second wavelength, the optical data signal to an optical receiver over an optical path; and transmitting at the first wavelength, to the receiver over the path a single-photon signal comprising a stream of single photons. The optical path is configured to carry optical signals at different wavelengths and the optical path attenuates signals at the first wavelength less than the optical path attenuates signals at the second wavelength. The optical data transmission system and method for, at the receiver, receiving the single-photon signal at the first wavelength and receiving the optical data signal at the second wavelength; and converting the optical data signal to the first wavelength for detection.