Secret key exchange in ultralong lasers by radiofrequency spectrum coding

A new scheme for secret key exchange involving varying the cavity length of an ultralong fibre laser has been experimentally demonstrated. Unlike schemes that rely on random changes to the cavity losses, the proposed scheme employs random varations to the free spectral range of the laser cavity, ensuring above-threshold operation. It is based on the realization that the free spectral range of laser cavity can be used as an information carrier. The researchers, who are based in France, the UK and Spain, demonstrate the scheme experimentally using a 50-km-long fibre laser to link two users, both of whom can randomly add an extra 1-km-long fibre segment. While the classical scheme does not ensure the same security as quantum key distribution, it provides enhanced security beyond standard software protocols and can be implemented using standard components.