ESA takes steps toward quantum communications

A team of European scientists has proved within an ESA study that the weird quantum effect called 'entanglement' remains intact over a distance of 144 kilometres. The experiment allows ESA to take a step closer to exploiting entanglement as a way of communicating with satellites with total security.

Quantum entanglement is one of the many non-intuitive features of quantum mechanics. If two photons of light are allowed to properly interact with one another, they can become entangled. One can even directly create pairs of entangled photons using a non-linear process called Spontaneous Parametric Down Conversion (SPDC).

Those two entangled photons can then be separated but as soon as one of them interacts with a third particle, the other photon of the pair will change its quantum state instantaneously. This happens according to the random outcome of the interaction, even though this photon never did interact with a third particle.

Such behaviour has the potential to allow messages to be swapped with complete confidence. This is because, if an eavesdropper listens into the message, the act of detecting the photons will change the entangled partner. These changes would be obvious to the legitimate receiving station and the presence of the eavesdropper would be instantly detected.

A quantum communications system would be a valuable way to transmit banking information, or military communications, or even to distribute feature films without the fear of piracy.

Even though entanglement has been known about for decades, no one has known whether the entanglement decays over long distance. For example, would a beam of entangled photons remain entangled if it passed through the atmosphere of the Earth? On their journey, the photons could interact with atoms and molecules in the air. Would this destroy the entanglement?

If so, entanglement would be useless as a means of communicating with satellites in orbit, because all signals would have to pass through the Earth's atmosphere. Now, an Austrian-German led team have proved conclusively that photons remain entangled over a distance of 144 kilometres through the atmosphere. That means that entangled signal will survive the journey from the surface of the Earth into space, and vice versa.

In September 2005, the European team aimed ESA's one-metre telescope on the Canary Island of Tenerife toward the Roque de los Muchachos Observatory on the neighbouring island of La Palma, 144 kilometres away. On La Palma, a specially built quantum optical terminal generated entangled photon pairs, using the SPDC process, and then sent one photon towards Tenerife, whilst keeping the other for comparison.

Upon comparing the results from Tenerife with those from La Palma, it was obvious that the photons had remained entangled. "We were sending the single-photon beam on a 144 kilometres path through the atmosphere, so this horizontal quantum link can be considered a 'worst case scenario' for a space to ground link," says Josep Perdigues, ESA's Study Manager.

Additional tests with a quantum communication source that generated faint laser pulses instead of entangled photon pairs were performed in 2006. Faint laser pulse sources emulate single photon sources by attenuating the optical power of a standard laser down to single photon regime. Attenuated lasers are technologically much simpler than entangled photon sources or 'true' single photon sources.

The price you have to pay is the unwanted opportunity for information leakage, due to the non-zero probability of having more than one photon per pulse. In practice, this limits the maximum link distance for exchanging securely a key. By implementing a decoy-state protocol in the experiments using a faint laser pulse source, the maximum link distance (yet secure against an eavesdropper’s action) was extended to values representative of a space to ground experiment.

The team are now studying ways to take the experiment into space. "Being in space will mean that we can test entanglement over lines of sight longer than 1 000 kilometres, unfeasible on Earth, thereby extending the validity of Quantum Physics theory to macroscopic scales," says Perdigues. One option is to use the external pallet on the Columbus module of the International Space Station. Another would be to put the quantum optical terminal on a dedicated satellite of its own. The quantum optical terminal is about 100 kg in mass and fits into a one-cubic-metre box.

Credits: ESA

Source: ESA

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New view of Titan

Today, two and a half years after the historic landing of ESA’s Huygens probe on Titan, a new set of results on Saturn’s largest moon is ready to be presented. Titan, as seen through the eyes of Huygens still holds exciting surprises, scientists say.

On 14 January 2005, after a seven-year voyage on board the NASA/ESA/ASI Cassini spacecraft, ESA’s Huygens probe spent 2 hours and 28 minutes descending by parachute to land on Titan. It then sent transmissions from the surface for another seventy minutes before Cassini moved out of range.

On 8 December that year, a combined force of scientists published their preliminary findings in Nature. Now, after another year and a half of patient work, they are ready to add fresh details to their picture of Titan. This time, the papers are published in a special issue of the Planetary and Space Science magazine.

“The added value comes from computer modelling,” says Jonathan Lunine, Huygens Interdisciplinary Scientist from the Lunar and Planetary Laboratory, University of Arizona.

By driving their computer models of Titan to match the data returned from the probe, planetary scientists can now visualise Titan as a working world. “Even though we have only four hours of data, it is so rich that after two years of work we have yet to retrieve all the information it contains,” says François Raulin, Huygens Interdisciplinary Scientist, at the Laboratoire de Physique et Chimie de l'Environnement, Paris.

The new details add greatly to the picture of Saturn’s largest moon. “Titan is a world very similar to the Earth in many respects,” says Jean-Pierre Lebreton, ESA Huygens Project Scientist.Huygens found that the atmosphere was hazier than expected because of the presence of dust particles – called ‘aerosols’. Now, scientists are learning how to interpret their analysis of these aerosols, thanks to a special chamber that simulates Titan’s atmosphere.

When the probe dropped below 40 kilometres in altitude, the haze cleared and the cameras were able to take their first distinct images of the surface. They revealed an extraordinary landscape showing strong evidence that a liquid, possibly methane, has flowed on the surface, causing erosion. Now, images from Cassini are being coupled with the ‘ground truth’ from Huygens to investigate how conditions on Titan carved out this landscape.

Credits: ESA/NASA/JPL/University of Arizona

Source: ESA

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