The network connects six locations across Vienna and in the nearby town of St Poelten, using 200 km of standard commercial fibre optic cables.

Quantum cryptography is completely different from the kinds of security schemes used on computer networks today.

These are typically based on complex mathematical procedures which are extremely hard for outsiders to crack but not impossible given sufficient computing resources or time.

But quantum systems use the laws of quantum theory, which have been shown to be inherently unbreakable.

The basic idea of quantum cryptography was worked out 25 years ago by Charles Bennett of IBM and Gilles Brassard of Montreal University, who was in Vienna to see the network in action.

“All quantum security schemes are based on the Heisenberg Uncertainty Principle, on the fact that you cannot measure quantum information without disturbing it,” he explained.

“Because of that, one can have a communications channel between two users on which it’s impossible to eavesdrop without creating a disturbance. An eavesdropper would create a mark on it. That was the key idea.”

In practice this means using the ultimate quantum objects: photons, the “atoms of light”. Incredibly faint beams of light equating to single photons fired a million times a second raced between the nodes in the Vienna network.

Each node, housed in a different Siemens office (Siemens has provided the fibre links), contains a small rack of electronics – boxes about the size of a PC – and a handful of sensitive light detectors.

Quantum cryptography is a surprising outgrowth of recondite arguments that bounced around for decades about the meaning of quantum mechanics.

Albert Einstein, who discovered the quantum properties of photons of light – indeed, discovered the very concept of the photon – always resisted quantum theory’s spooky behaviour, “God does not play dice”, being among his oft-quoted objections.

But experiments eventually proved that he apparently does, and also laid the technical foundations for today’s quantum information revolution – cryptography, teleportation, and computation.

One of the grandees of quantum science, Vienna University’s Anton Zeilinger, used the occasion to argue for continued funding of fundamental science in these increasingly application-focused days.

“Real breakthroughs are not found because you want to develop some new technology, but because you are curious and want to find out how the world is,” Dr Zeilinger said.

“It may not have surprised the founding fathers of quantum science that technology has advanced so that you can play with individual quantum systems, in great detail.

“Maybe this would not surprise, but what could surprise them is that people are thinking and doing practical applications.”