Scientists have used satellite technology for the first time to generate and transmit entangled photons — particles of light — across a record distance of 1,200 kilometres on Earth.
The feat, published today in the journal Science, is more than 10 times the distance previously achieved using land-based fibre optic technologies.
The experiment by a group of Chinese researchers, led by Professor Jian-Wei Pan of the Chinese Academy of Sciences (CAS), takes us a step closer to achieving instant, unhackable communication.
A cornerstone of quantum physics is a process called entanglement, where the properties of two particles — such as spin, position and momentum — intimately affect each other, even when those particles are separated by large distances.
“This [experiment] has extended those boundaries by an order of magnitude.
“Quantum science has also become a new resource as real as energy and is being applied to cryptography, teleportation and quantum computing. This new knowledge can be instantly applied to these areas.”
World’s first quantum satellite
Until now, quantum entanglement had been demonstrated over distances of up to 100 kilometres using optical fibres.
However, the “noise” or interference rapidly makes the signal too weak to detect.
This limitation is removed by going into space, said Dr Ben Buchler, a quantum physicist from the Australian National University’s (ANU) Centre for Quantum Computation and Communication Technology.
“Previously, if you wanted to distribute entanglement over a very large distance you’re limited by the loss of photons in optical fibres, but with satellites you can go a lot further because you’ve only got to get through the atmosphere,” he said.
After five years of development, the Chinese Academy of Sciences launched the world’s first quantum satellite, called Micius, in August 2016.
Late last year, the satellite, which orbits at a height between 500 and 2,000 kilometres above Earth, directed beams of entangled photon pairs at telescopes up to 1,203 kilometres apart between Delingha in Qinghai and Gaomeigu Observatory in Lijiang.
When the photons were received, Professor Lu and his colleagues were able to demonstrate that, despite the distance between them, the individual photons at each location were still “entangled” with their counterparts at the other ground station.
“[The satellite] sends one half of the pair to one base station on Earth and the other half of the pair to a different base station on Earth,” explained Dr Buchler.
“Those photons are entangled so then you have an entangled pair separated by a very long way.
“And that’s been hard to do.”
Dr Chunle Xiong of the Australian National University, who is in the process of developing Australia’s own quantum satellite, said the result was a major milestone.
Paving the way for secure communication
Because any change in one particle will be apparent in the other instantly, quantum entanglement has been considered a potentially powerful tool for sending information securely.
In particular, this could enable the secure distribution of keys to unlock codes encrypting secret information.
Because of the nature of entanglement, if any third party tried to use the key, it would be detected by the sender and receiver and they would know they had been hacked.
“What’s extra cool about this work, is that the particular way they can distribute the key is based on a source of quantum entanglement that’s [generated] in space — in the satellite,” said Dr Buchler.
Here in Australia, the work on a quantum satellite led by Professor Xiong in collaboration with the European Space Agency, could see Australia have similar or even better quantum science capacity as soon as 2021.
By Annabel McGilvray