Munich

One of the most secure methods of quantum cryptography has been used commercially — for a single transaction, at least. On 21 April, Austrian scientists used the technique to transfer a €3,000 (US$3,500) donation to their lab.

Quantum cryptography uses the odd properties of quantum particles to create secure keys for encoding and decoding messages. The very act of observing these particles changes their nature, making it easy to detect any eavesdroppers.

Anton Zeilinger, a quantum physicist at the University of Vienna, and his team carried out their bank transaction by applying a particularly secure technique that uses a pair of entangled photons to create the key.

The properties of these photons depend on each other, even when they are separated by long distances. After entangling the pair, one is sent to the recipient. Upon arrival, both photons are measured by their respective owners. This act of measurement determines the state of the photons, and thus the state of the key.

Before measurement, neither photon carries any useful information that could be stolen by a snoop. “This makes data transmission more secure,” says Zeilinger.

In last week's trial, the entangled photons were created in a branch of the Bank of Austria in Vienna. One was sent to the city hall through a 1,450-metre-long fibreglass cable. The transfer took 90 seconds to complete; the money was then donated to Zeilinger's lab.

Some companies already sell quantum-cryptography hardware. But these systems use single photons to communicate the key (see Nature 418, 270–272; 2002). In such systems, there is a small possibility that the key could be intercepted without anyone noticing. Zeilinger's photon pairs eliminate this potential flaw, he says.

It has taken Zeilinger's group two years to create a commercial prototype of entangled-photon cryptography, with the help of the Austrian company ARC Seibersdorf Research. So far, the system works well enough for a single test, but it is not quite ready for sale. “We hope that all problems of implementation will be solved within three years,” says Zeilinger.