A digital optical fiber communication system operating at a wavelength of 1 µm requires a maximum bit error rate of 10^-9. What is the theoretical quantum limit at the receiver in terms of the quantum efficiency of the detector and the energy of an incident photon?

A. $$\frac{{20.7hf}}{\eta }$$

B. $$\frac{{10.7hf}}{\eta }$$

C. $$\frac{{20.7\eta }}{{hf}}$$

D. $$\frac{{10.7\eta }}{{hf}}$$

Answer: Option A