Quantum "key" breaks distance record

11 Jul 2002

A Swiss team distributes an "uncrackable" quantum key over a 67 km standard fiber-optic link between Geneva and Lausanne.

Researchers in Switzerland have built a prototype quantum cryptographic system and used it so send a quantum key down a standard optical fiber.

The team, which includes researchers from the University of Geneva and its spin-out company id Quantique, exchanged the key over a Swisscom telephone network between Geneva and Lausanne, 67 km away.

According to the researchers, who report their work today in the New Journal of Physics, this cryptographic system is the first to offer a "plug and play" capability and to work on a commercial fiber link. They say that similar experiments in the past have been performed in laboratories, demanding nitrogen-cooled detectors or frequent adjustment of polarization and phase.

In quantum cryptography, the key is actually a sequence of linearly-polarized single photons. Because single photons are used, quantum physics theory dictates that the key cannot be intercepted when sent between the two key holders, who are known as Alice and Bob (see the paper for more details).

In id Quantique's system, Bob makes use of a pulsed 1550 nm source as a transmitter. Each pulse is first separated by a 50/50 beamsplitter, before being sent down the fiber to Alice. At Alice, the pulses are attenuated and reflected back to Bob with a phase shift.

Alice and Bob's quantum key modules are both contained within 19-inch boxes, and each is controlled with a standard PC.

Grégoire Ribordy, of id Quantique, told Optics.org that the system should work over any fiber network. However, as the attenuation of optical fiber effectively limits the key distribution to 70 km, he expects the first applications to be in metropolitan links. Banks and financial institutions looking to archive information securely are expected to be the first users of the technology.

"We are in advanced discussions with potential customers, but there is still some work to be done on the product," said Ribordy. While the optics used in the prototype is thought to be sufficiently mature, Ribordy says that in the final product the PCs will be integrated with the Alice and Bob modules.

This upgrade of the prototype system to a commercial product is expected within a year, Ribordy said.

Author
Michael Hatcher is technology editor of Opto and Laser Europe magazine.