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Record breaking quantum encryption

July 10, 2002

The latest steps towards an uncrackable code have been taken by scientists in both Switzerland and the US, who have found novel ways of generating and transmitting "quantum keys" respectively. Both techniques are reported today, 12 July 2002, in New Journal of Physics, published jointly by the Institute of Physics and the German Physical Society.

Sharing a secret key is vital to code-maker, Alice, to enable her to send secret message to her friend Bob without eavesdropper, Eve, being able to intercept and decrypt the message. Until now this has been a serious problem - how can Alice send her key to Bob who lives on the other side of town without Eve being able to pick it up too?

Hugo Zbinden of the University of Geneva, and his colleagues there and at Swiss electronics company "id Quantique", realised that if Alice sends her key to Bob as a series of specially ordered light particles, or photons, then the laws of quantum physics prevent Eve from being able to read the key without both Alice and Bob knowing about it. In the quantum world of very small particles the very act of observing a photon disturbs its path so that the key sent by Alice will be "wrong" by the time it reaches Bob if Eve has tapped into it en route. Alice can keep discarding keys until one gets through that Eve has missed.

Until recently, the idea of quantum key distribution has been tested only in the physics laboratory. Now, Zbinden and his colleagues have demonstrated the first fully integrated quantum cryptography prototype machine across a telecommunications network.

Once the key is checked for eavesdropping and proven secure, the message is encrypted and sent as normal via the phone lines or the Internet. When the message reaches Bob he uses his copy of the quantum key to decrypt Alice`s message.

A world record was set when the researchers tested the system across the Swisscom fibre-optic telephone network and were able to exchange quantum keys between Alice and Bob even when they were 67 kilometres apart.

Richard Hughes and his colleagues at the Los Alamos National Laboratory, in New Mexico, and several other teams have been working on a new way to share a quantum key transmitted by photons. Until now, researchers have only been able to share quantum keys over fibre-optic cable. But, as Hughes points out, it is impossible practically to connect a satellite orbiting the earth to a ground station, for instance, or allow wireless networks to be secured on earth. "One must face the much more challenging problem of transmitting the quantum key distribution (QKD) photons through the atmosphere and reliably selecting them out from the huge background of light that is present even at night."

The Los Alamos team describes a portable, self-contained device that can be set up quickly and easily to make QKD transmissions. The device works day and night over a range of 30 kilometres. The device could be used to make the final encrypted connection from a network base station to a user across town. "No matter what technology an adversary might develop in the future, the QKD transmissions will remain secure forever," explains Hughes, "thereby taking secure communications to new levels of security."

Soon, Alice whether she is a spy, a company executive, or simply Bob`s pen pal will be able to keep her messages top secret thanks to the laws of physics.

Institute of Physics