Come on Santa, light my fibre!

During the festive season we will not only be lighting up trees and other Christmas decorations but also our optical fibres. Lit fibres, as they are called, are optical fibres that carry large classical data traffic.  In modern telecommunication networks, fibres can carry data in up to 80 individual channels simultaneously with a combined power of about 8 x 10-3 Watts. Imagine now a (dark) fibre exclusively for a single quantum channel, only carrying 1 x 10-11 Watts, a billion times less. Is this not a waste of resources? The possible usage of lit-fibres in quantum communication transmission, called coexistence, is an important feature since it allows to economise on the expensive fibre infrastructure.

It is however not so easy to shield the single photons against “contamination” from the environment, ask the guys next door in the Quantum Computer lab about it. The high-power difference of about 100 dB between classical and quantum channels can quickly become a showstopper when these channels are located too close to each other. Even the 100-nm far wings of stimulated Raman scattering are quickly imposing severe crosstalk to the quantum channel. Nevertheless, researchers have demonstrated that tight optical filtering and/or spectral displacement away from the C-band can alleviate cross-talk issues and allow for a coexisting transmission of quantum and classical signals along the same fibre.