Quantum key distribution does this by encoding the information in the polarisation states of individual photons, the particles of light, which are sent from Alice to Bob over an optical fibre.

ECONOMIST: Secure cryptography is only as safe as its weakest link

Sending large amounts of information down an optical fibre is done by encoding many separate streams of data as pulses of light of different colours, so that they can travel along the fibre without interfering with each other.

ECONOMIST: Optical fibres could carry more information in the future