Another Step Towards Unbreakable Quantum Encryption

Long-distance relay proves concept of foolproof security using entangled photons

A group of European scientists succeeded in beaming an encryption code based on photons across about 90 miles of ocean. The experiment took place recently in the Canary Islands, from a light source on the island of La Palma to a receiving station on nearby Tenerife. The researchers published their exploits in this month's issue of the academic journal Nature Physics.

According to the European Space Agency, which funded the study, the experiment allows ESA to move closer to harnessing entanglement as a means of "communicating with satellites with total security."

Entanglement is an unusual property of quantum mechanics used to describe the manner in which photons naturally interact with each other. Using a non-linear process called Spontaneous Parametric Down Conversion (SPDC), it is even possible to directly create pairs of entangled photons. These entangled photons lend themselves to encryption applications because of their unique linkage. If one photon is exposed to a third particle, both of the paired photons will change their state instantaneously -- even though one of the photons is physically separated from its mate.

The degree of change in the state of the paired photons is completely random and unpredictable, according to ESA, theoretically foiling any attempt by a third party to decode the message without actually intercepting the quantum encryption key. However, the very act of detecting the photons would change the state of the photon pair, betraying the presence of an eavesdropper. "These changes would be obvious to the legitimate receiving station and the presence of the eavesdropper would be instantly detected," ESA wrote in its official announcement of the experiment's success. "Such behavior has the potential to allow messages to be swapped with complete confidence."

Prior to the inter-island experiment, it was not known whether entanglement would be adversely affected when the beam of entangled photons was transmitted through the atmosphere over a considerable distance. The favorable results of the experiment have given the space agency confidence that entanglement does not decay over long distances, indicating that quantum encryption can be harnessed for securing satellite communications.

Possible applications for the unbreakable code might include transmissions of financial data between banks, military communications and even the distribution of feature films, according to ESA officials.