European researchers saidTuesday they have proved an effect in quantum physics works over long distances, which means itcould one day be used to make electronic communications infallibly secure.

Scientists at the European Space Agency said they found that the properties of quantum entanglement — a state in which two light particles or photons' states become linked after they interact with each other — is maintained even when the photons are as far as 144 kilometres apart and must travel through the atmosphere, into space and back.

The finding is particularly notable for its potential applications, the scientists say.

If a third particle interacts with one of the pair of entangled photons, the other will instantaneously change its quantum state without itself having any contact with the third particle.

The effect could allow communications to be securely transmitted because the act of eavesdropping would instantly change the entangled particle, alerting the receiving station that the communication had been compromised.

The first uses of such technology once it is further developed are likely to be military, but it could also be used to keep electronic banking secure or to maintain other private communications.

The ESA's experiment tested whether the quantum entanglement effect could be maintained over long distances through the atmosphere or whether it woulddecay or be destroyed.

In September 2005, the European team pointed the ESA's one-metre telescope on Tenerife, Canary Islands, at the Roque de los Muchachos Observatory on La Palma Island, 144 kilometres away.

There, a quantum optical terminal produced entangled photon pairs, sending one photon to Tenerife and keeping the other for comparison. The results showed that the photons stayed entangled.

Further tests were done in 2006 using a quantum device that produced faint laser pulses instead of entangled photon pairs. Faint laser pulse sources emulate single photon sources but are simpler.

The next step for the ESA team is to conduct the experiment in space.

"Being in space will mean that we can test entanglement over lines of sight longer than 1,000 kilometres," the ESA's Josep Perdigues said in a written statement.