Secret key exchange in ultralong lasers by radiofrequency spectrum coding

We propose a new approach to the generation of an alphabet for secret key exchange relying on small variations in the cavity length of an ultralong fiber laser. This new concept is supported by experimental results showing how the radiofrequency spectrum of the laser can be exploited as a carrier to exchange information. The test bench for our proof of principle is a 50-km-long fiber laser linking two users, Alice and Bob, where each user can randomly add an extra 1-km-long segment of fiber. The choice of laser length is driven by two independent random binary values, which makes such length become itself a random variable. The security of key exchange is ensured whenever the two independent random choices lead to the same laser length and, hence, to the same free spectral range. Optical encryption: Scheme based on varying cavity length 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.