(New Scientist) An electronic money transaction has been carried out in at a bank in Austria using entangled photons to create an unbreakable communications code. Although of commercial quantum cryptography products already exist, none of these use entangled photons to guarantee secure communications.

The link was used to transfer money between Vienna City Hall and Bank Austria Creditanstalt on Wednesday. The cryptographic system was developed by Anton Zeilinger and colleagues from the University of Vienna and the Austrian company ARC Seibersdorf Research.

Entangled photons obey the strange principles of quantum physics, whereby disturbing the state of one will instantly disturb the other, no matter how much distance there is in between them.

The pairs of entangled photons used were generated by firing a laser through a crystal to effectively split single photons into two. One photon from each entangled pair was then sent from the bank to the city hall via optic fibre.

Key creation

When these photons arrived at their destination, their state of polarisation was observed. This provided both ends of the link with the same data, either a one or a zero. In this way, it is possible to build a cryptographic key with which to secure the full financial transaction.

Quantum entanglement ensures the security of communications because any attempt to intercept the photons in transit to determine the key would be immediately obvious to those monitoring the state of the other photons in each pair.

And because the resulting key is random it can be used to provide completely secure link even over an unprotected communications channel, provided a new key is used each time.

This system can be guaranteed secure. By contrast, most existing non-quantum cryptographic systems rely on extremely time-consuming mathematical problems to create a code that are impractical – but not impossible – to break.

“If you are talking about large sums of money, people are interested,” Zeilinger told New Scientist. He adds that the system should not be much more expensive to implement than current technology.

Sewer link

The photon-encrypted money transfer saw the Mayor of Vienna transfer a 3000-Euro donation into an account belonging to the University of Vienna team. The two buildings are just 500 metres away from one another, but fibre optics had to be fed through 1.5 kilometres of sewage system to make the link.

Zeilinger says in principle it should be possible to extend this link to 20 kilometres. Beyond this distance it becomes difficult to transmit single photons reliably.

In June 2003 the same team at the University of Vienna transmitted entangled photons through free space across the river Danube.

The commercial quantum cryptographic devices that exist already, made by companies like ID Quantique and MagiQ, use different principles to create a secure key. They also use weak light pulses instead of individual photons.

Because these pulses must be sufficiently weak to guarantee security, more sensitive detectors are required, says Tim Spiller, a quantum communications researcher at Hewlett-Packard’s research laboratory in Bristol, UK. He notes that using pairs of entangled photons would make it easier to guarantee absolute secrecy.