The ability to send perfectly secure messages from one location on the planet to another has obvious and immediate appeal to governments, the military and various commercial organisations such as banks. This capability is already possible over short distances thanks to the magic of quantum cryptography, which guarantees the security of messages, at least in theory.
For the moment, however, quantum cryptography works only over distances of 100 km or so. That’s how far it is possible to send the single photons that carry quantum messages through an optical fibre or through the atmosphere.
Last year, we watched as European and Chinese physicists battled to claim the distance record for this technology with the Europeans finally triumphing by setting up a quantum channel over 143 kilometres through the atmosphere.
That distance is a good fraction of the way into space. And the reason that’s important is that it’s a stepping stone to sending quantum messages to orbiting satellites which can then route the messages to almost anywhere else on the planet.
Today, the Chinese claim another small victory in this quantum space race. Jian-Wei Pan at the University of Science and Technology of China in Shanghai and a few pals say they’ve bounced single photons off an orbiting satellite and detected them back on Earth. That’s significant because it simulates a satellite sending single photons from orbit to the surface, crossing off another proof-of-principle milestone in their quantum checklist.
The experiment is simple in principle. These guys have two telescopes in a binocular formation which they pointed at a satellite orbiting at an altitude of 400 kilometres. This satellite is covered with reflectors capable of bouncing a laser beam from Earth back to its original location.
They used one of the telescopes to send pulses of light towards the satellite and the other, with a diameter of 60 cm, to look for the reflection.
Of course, the Earth’s atmosphere absorbs a very high percentage of the photons transmitted from the ground. So Jian-Wei and produced each pulse with just enough photons so that, on average, just one would reach the satellite and be reflected back to Earth. The idea was to simulate the satellite itself sending single photons to the surface.
Each pulse began its journey from Earth with about 1 billion photons and, on average, just one started the return journey.
Obviously, many of the returning photons would also be absorbed by the Earth’s atmosphere. So the pulse was repeated many millions of times a second.
The result, say Jian-Wei and co, is that they were able to detect the returning photons at a rate of about 600 per second. “These results are sufﬁcient to set up an unconditionally secure QKD link between satellite and earth, technically,” they add.
That’s a significant stepping stone. “Our results represent a crucial step towards the ﬁnal implementation of high-speed QKD between the satellite and the ground stations, which will also serve as a test bed for secure intercontinental quantum communication,” say the Chinese team.
However, this experiment raises something of a puzzle. The Chinese team say they used a German satellite called CHAMP for their experiment. This was launched in 2000 and its mission was to make a precise gravity map of the Earth by bouncing lasers off it.
What’s curious about the Chinese announcement is that CHAMP deorbited in 2010. So a curious question is when the team did this work. Clearly, the team has been sitting on this result for some time.
Why publish it now? The answer may be a small but significant detail revealed in the final paragraph of the paper. Here Jian-Wei and co announce that they plan to launch the first quantum science experiment into space. The spacecraft is called the Chinese Quantum Science Satellite and it is scheduled for launch in 2016.
A quick Google search shows that the official Chinese news agency, Xinhau, revealed in March that its scientists were planning a quantum information and technology space experiment. But the announcement did not give the name of the satellite and appears to have had little if any coverage in the west.
“We hope to establish a quantum communication network from Beijing to Vienna,” according to Jian-Wei, a plan that will presumably require significant co-operation from their arch-competitors in Europe.
Last year, European scientists themselves proposed sending a quantum communications experiment to the International Space Station, an idea that could be beat the Chinese at their own game and would be relatively cheap and quick. But whether this plan has gained traction isn’t clear.
What is abundantly clear is that the quantum space race is rapidly hotting up. But the embarrassing truth for American science is that the US isn’t yet a player in the quantum space race (at least not publicly). Perhaps that’s something that should change.
Ref: arxiv.org/abs/1306.0672 : Experimental Single-Photon Transmission from Satellite to Earth