Most modern encryption algorithms make use of a shared secret – the encryption key, known only to the legitimate parties. This encryption key is a number/sequence of bits of varying length that is used by the encryption algorithm to scramble the original content of the message in a way that only other bearers of a relevant key could unscramble. Once all parties have the relevant keys, they can start transferring secure messages back and forth, however, the longer the same key is being used, the higher the risks are that an ill purposed individual might manage to “guess” the right key and gain access to the content of the encrypted messages. To reduce the chances of that ever happening, it’s necessary to generate and exchange encryption keys securely, for long distances and at the fastest possible rates. One of the most studied solution to this is Quantum Key Distribution (QKD) which theoretically offers absolute security; however, its physical implementations require expensive equipment which is often not completely secure and its bit-rate drops exponentially with the distance due to attenuation making it impractical for distances in excess of 150km.
In recent years efforts have been made to find a practical solution to the key distribution problem using methods that involve classical physics. One such method involves stabilizing the lasing dynamics of ultra-long fiber lasers where matching Bragg mirrors on both ends of a fiber cavity alter its spectral transmission profile and measurements of lasing frequency indicate, to each of the legitimate parties, if the other party placed an identical mirror or not and thus allow them to synchronize a secure key bit while an outsider, lacking any knowledge of the mirror selection of both parties, can not gain any insight into the choices of bits.
