Quantumkeydistribution has been demonstrated successfully in fibres, but it suffers from one major drawback: it requires a dedicated link, and so cannot be implemented in a network.
Quantumkeydistribution 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.
In practice, quantum-key-distribution systems rely on sophisticated optical equipment to prepare, transmit and detect the individual polarised photons that make up the key.