Physicists have attempted to solve the problem by sending photons through a shared fibre along a ‘quantum channel’ at one characteristic wavelength. The trouble is that the fibre scatters light from the normal data traffic into that wavelength, polluting the quantum channel with stray photons. Andrew Shields, a physicist at the Toshiba Cambridge Research Laboratory, UK, and his colleagues have now developed a detector that picks out photons from this channel only if they strike it at a precise instant, calculated on the basis of when the encoded photons were sent. The team publishes its results in Physics Review X.
Still, 90 kilometres is a “world record that is a big step forward in demonstrating the applicability of quantum cryptography in real-world telecommunications infrastructures”, says Vicente Martín, a physicist at the Technical University of Madrid.