A team of physicists from Austria has sent pairs of entangled photons, which can be used to encrypt messages with complete security, between telescopes spaced 144km apart in the Canary Islands. The researchers say that preserving entanglement over this distance shows the feasibility of carrying out quantum cryptography using a worldwide network of satellites.

Quantum cryptography exploits the laws of quantum mechanics to create uncrackable keys for encoding and decoding messages. Such keys are made up of the states of quantum particles, such as the polarization of photons, and their values are therefore not independent of observation. So any eavesdropper hoping to read off the value of a secret key will reveal his or her presence in the process.

Anton Zeilinger and colleagues exploit a bizarre feature of quantum mechanics called “entanglement” whereby the act of measuring the state of one particle can instantaneously change the state of another. This effect can be used for cryptography because measuring the polarization of one entangled photon will induce the same state in the other entangled photon. If individual photons from each pair are sent to two different locations, the same security code can be shared across a network of people.

Entangled Canaries

Two year’s ago, Zeilinger’s group showed it was possible to send one photon from an entangled pair between two observatories on separate Canary Islands separated by 144 km. However, it was hard to develop their system because of the distorting effects of the atmosphere, making it hard to distinguish the photons in the beam from stray light.

Now, the researchers have upgraded their source and transmitted both entangled photons using two separate telescopes on La Palma. They produced the entangled photons by firing a laser beam into a crystal to create pairs. A very slight time delay between the transmission of each photon within a pair then allowed the individual photons to be identified when arriving at a receiving telescope on Tenerife.

Vienna team member Rupert Ursin told physicsworld.com that this distance is more than enough to establish the feasibility of satellite-based quantum cryptography. However, the difficult part is preserving entanglement as the beams travel through the Earth’s atmosphere. But Ursin believes this could be overcome by sending beams from satellite to satellite and then vertically down to the receiver — then they would only need to traverse a few kilometres of the atmosphere.

“If you want to do worldwide quantum cryptography then you need to go to space. This is the final proof of principle demonstration,” says Ursin, who adds that fibre-optic cable is not a viable alternative to this free-space communication because entangled-particle beams are completely attenuated after passing through anything more than about 100km of cable.

The Austrian researchers are now working with physicists in Spain to develop a prototype source of entangled photons that Ursin says could be put into space by around 2014. He adds that the rocket used to carry the equipment, to be launched by the European Space Agency, could also accommodate a source of single photons, providing a test of a rival approach to quantum-key distribution.

This research was published in Nature Physics.