WHY THIS MATTERS IN BRIEF
- Quantum networks promise almost total secrecy but significant hurdles remain to creating international and internet scale networks, now though China looks like it will be the first to create the first international, cross border, quantum network
Late last year, a green dot appeared in the night sky near the Chinese-Myanmar border, and in itself that would be conspicuous but this was special.
“It was like a very bright green star,” says physicist Chao-Yang Lu, a professor at the University of Science and Technology of China, who saw it from an observing station on the outskirts of the Chinese city of Lijiang.
Without knowing what it was his team had to act fast if their observatory was to figure out what it was, and as it turned out the green light was actually a laser beamed from a satellite orbiting over 300 miles overhead. Within minutes observatories around China locked their telescopes onto the strange phenomenon in search of the real prize within – delicate, single infrared photons produced by a special crystal on the satellite. Filtering out the green light, they all latched on to their target – a quantum signal the likes of which has never been sent before.
This Chinese space experiment was a crucial test for a budding field of emerging security technology called Quantum Cryptography that uses quantum particles, like photons, to send information securely. That said though quantum particles, as you might imagine, are notoriously difficult to transmit and capture. If, for example, you try to send them through optical fibre then the signal gets corrupted after just 150 miles of travel, and that means that without robust and reliable quantum repeater, a technology that, like it says, repeats the signal and that’s still in its very early stages of development, even after a team in from theUS National Institute of Standard and Technology (NIST) recently set a new world record, you won’t be able to create large scale quantum networks.
As a result researchers have long suggested using satellites and other high orbiting platforms to beam quantum particles over long distances but they’d never done it before – until now that is. In this experiment, the satellite distributed single photons between two ground stations 750 miles apart, a new record.
“For extending the distance of quantum communication networks, this is a milestone,” says Eleni Diamanti, the vice director of the Paris Center for Quantum Computing, who was not involved in the work, “there’s no question about it.”
China launched the $100 million satellite, known as “Quantum Experiments at Space Scale”, last August from the Jiuquan Satellite Launch Center in the Gobi Desert. Before the launch, researchers placed a complicated system of lasers, mirrors, and a special crystal on board. When a specific laser shone on the crystal, it would create pairs of light particles known as entangled photons. The crystal makes 6 million pairs of photons at a time, but on the ground, the two ground stations could only detect about one pair per second.
“It’s a challenging task,” says Lu, “it’s the equivalent of trying to see a human hair from 300 meters away.”
Researchers like Lu and his colleagues think quantum cryptography could be the encryption tool of the future. Properly executed, the protocol works like this – first you measure characteristics of photons to generate a key of 1’s and 0’s that you send to your intended recipient. Then, you encrypt your message with the key and send it. If a hacker tried to steal the key in transit, quantum mechanics theory says they’d instantly change it to a different set of numbers.
Think of Schrodinger’s cat, which is both dead and alive when you’re not looking, but becomes purely one or the other when you pay attention. In the same way, the hacker would instantly change the state of the photons that make up the key, which means in theory, it’s physically impossible to hack – although, again, that said recently another team in Canada managed to hack an “unhackable” quantum network, ironically in their case to see if they could create tools to prevent it from being hacked in the first place, so as you can see there’s a lot more development left to do on the technology, and still lots of questions.
Meanwhile China’s launch and the resulting experiment were a long time coming. Jian-Wei Pan of the University of Science and Technology of China, the physicist who led the project, proposed the satellite experiment back in 2003. His team of some 100 people painstakingly designed, built, and tweaked the laser and satellite system over many years. They performed trial experiments on the ground first, delivering quantum keys only a few miles at first, and gradually ramping up the distance to over 2,000km last August.
Compared to the rest of the field , they’ve moved fast though, says physicist Thomas Jennewein of the University of Waterloo in Canada, who recently sent a quantum key from the ground to an airborne airplane, and years ago, Jennewein worked on several proposals to do similar experiments on the International Space Station (ISS).
“None of those projects really went much beyond the study phase because of the complexity and the cost and everything,” he says, “but the Chinese just went ahead and did it. It’s great.”
The reason they could do it so quickly is that people at the highest level of the Chinese government prioritised the project, says Denis Simon of Duke Kunshan University, an expert on Chinese science policy. Because the high-ups wanted it, the group didn’t have to go through the usual bureaucratic funding steps, he says, and the Chinese government is particularly interested in this technology because it wants quantum-secured communications in the national interest.
“The Chinese government wants to communicate with their naval ships, with their South China Sea activities,” he says, “they want to do a lot of things with it.”
Meanwhile, researchers in other countries are attempting similar experiments but with much, much more red tape. Diamanti’s group, for example, is waiting to hear back from the European Space Agency (ESA) on a proposal to distribute quantum particles from the ISS to several potential ground stations in Europe, and Paul Kwiat, a physicist at the University of Illinois at Urbana-Champaign, is leading US efforts to do a similar experiment on the ISS with NASA – something that will now probably get an instant green light from the US government who it has to be said, now seems to be a laggard in the space.
That said though is still doesn’t look like anyone else, even the UK’s government who’s pushing over £270 million via their UK National Quantum Technologies Programme (NQTP), is as ambitious, or as resource rich, as the Chinese and Lu says his group is planning to perform the same experiment from a new satellite with a higher orbit which would be able to send quantum keys between cities that are further apart – they want to exchange quantum keys between China and Austria, where some of their collaborators work, and by 2030, Pan has said that China plans to launch a fleet of these satellites to create a global network.
“We are very lucky and benefit from a fast decision-making system,” says Lu, “there’s nothing like when political and scientific interests align.”
Fast tracked science and fast tracked budgets? I can see the scientists in the western world going green with envy right now…