The value of quantum cryptography in telecommunication networks has now been proven, Swiss scientists and engineers claim, by demonstrating its long-term effectiveness in a real-time network.

Their international encrypted network (created in collaboration with ID Quantique) was installed in the Geneva metropolitan area and connects to CERN in France. The network ran for 21 months — the longest deployment of a quantum key distribution (QKD) network, and a demonstration of its robustness and reliability when coupled with a real-time telecommunications network, the researchers said.

QKD is a process that enables two parties to share a secret key before using that key, allowing them to protect data they want to send over a network. The key that the two parties share is built up from a stream of photons.

How to create a secret message

Here’s a theoretical scenario where Alice and Bob want to protect a piece of information using a quantum key: Alice would send a stream of photons to Bob with each one having a specific polarization (photons can spin vertically, horizontally, and diagonally).

Bob would then attempt to measure the photons coming in by randomly choosing which direction to measure them in. Sometimes he will choose the correct orientation, other times he won’t. Alice and Bob would then share the measurements using classical communication methods, simply stating if Bob was right or wrong, but not mentioning the actual direction the photons were spinning in.

Alice can then discard all of Bob’s wrong measurements and use the correct ones to encrypt their secret data. The beauty of QKD is that if a potential eavesdropper wanted to get hold of this key, they would actually destroy the photons when trying to measure them. As a result, they would need to send their own stream of photons on to Bob to cover their tracks, but this would introduce errors and be discarded during key distillation.

Already in commercial and government use

QKD has already been used for a number of applications, notably by ID Quantique to protect the votes in Geneva’s elections and in other commercial installations where high security is needed.

For QKD to become more widespread in the commercial world, its reliability needed to be thoroughly tested, as these networks run constantly. Furthermore, the robustness of the network needed to be demonstrated, as the systems are moving from safeguarded laboratories into more demanding environments.

“This experiment is a big step in the direction of a wider deployment of QKD in telecommunications networks,” said study co-author Dr. Damien Stucki. “From a scientific point of view, the deployment of the quantum layer over a duration of 21 months with high reliability is very significant. The SwissQuantum network was very reliable, with the only interruptions coming from external problems, such as power cuts and air conditioning problems, not the QKD layer.”

Ref.: D Stucki et al., Long-term performance of the SwissQuantum quantum key distribution network in a field environment, New Journal of Physics, 2011 [doi:10.1088/1367-2630/13/12/123001]