A method for providing eavesdropping detection of an optic fiber communication between two users includes the steps of exchanging both data and probe signals through at least two channels (400, 500) between the users, exchanging probe signals (143) on one channel (500 or 400) between quantum probe signal terminals, extracting a key for authentication from the probe signals, and exchanging data signals (142) between transmission units on another channel (400 or 500). A first portion of the key generated by the quantum probe signal terminals is used to authenticate the terminals, wherein a second portion of the key is dedicated to define commutation occurrences of commutation devices adapted to commutate the use of the channels (400, 500) for data (142) and probe (143) signals, thus detecting an eavesdropping event (300) which triggers an alarm (750). A further portion of the key can be used to encrypt the messages.