Quantum cryptography done on standard broadband fibre

Thousands of kilometres of existing fibre may be used
 to carry quantum codes

The "uncrackable codes" made by exploiting the branch of physics called quantum mechanics have been sent down kilometres of standard broadband fibre.

This "quantum key distribution" has until now needed a dedicated fibre separate from that used to carry data.

The technique, reported in Physical Review X, shows how to unpick normal data streams from the much fainter, more delicate quantum signal.

It may see the current best encryption used in many businesses and even homes.

The quantum key distribution or QKD idea is based on the sharing of a key between two parties - a small string of data that can be used as the basis for encoding much larger amounts.


Tiny, faint pulses of laser light are used in a bid to make single photons - the fundamental units of light - with a given alignment, or polarisation. Two different polarisations can act like the 0s and 1s of normal digital data, forming a means to share a cryptographic key.

What makes it secure is that once single photons have been observed, they are irrevocably changed. An eavesdropper trying to intercept the key would be found out.

Sending these faint, delicate quantum keys has until now been done on dedicated, so-called "dark fibres", with no other light signals present.

That is an inherently costly prospect for users who have to install or lease a separate fibre.

So researchers have been trying to work out how to pull off the trick using standard, "lit" fibres racing with data pulses of millions of photons.

- BBC.co.uk