For decades, physicists have looked to use the behavior of particles of light to securely send information. The basic science underlying quantum cryptography has been determined over the past 40 years, but a slew of papers published this summer by physicist Jian-Wei Pan establishes China as the early leader in deploying the technology on a global scale.
Why it matters: Networks using quantum keys theoretically allow for very private communications and safe transactions — because if attacked, the key would be altered and the parties would know it wasn't secure. That would be valuable for financial transactions or voting that involves transmitting information between two points. But beyond a handful of field tests, there hasn't been a commitment to develop the technology at this scale until now.
How it works: Two people who want to communicate would share a number key encoded in a string of single photons (particles of light) that can be used to encrypt and decrypt a message. It's secure because if someone tries to intercept the message, the photons would be physically altered and the key would no longer work, but the data would be secure.
The vision: Optical fibers carry photons short distances on the ground (anything more than about 200 kilometers and the fiber absorbs the photon signal). So researchers want to pair them with satellites that can relay the signal and then drop it back down to a receiver on Earth. That goes on and on, ultimately carrying the information around the globe to the intended receiver.
What they did: China built a 2,000-km fiber optic network between Beijing and Shanghai and launched a satellite last year — both dedicated to basic research on quantum satellite communications. So far, they've used it to:
- Send photons from the satellite to telescopes 1,200 km apart on the ground that acted as receivers.
- Transmit quantum-encoded information from the ground to the satellite.
- Distribute an actual quantum key string of photons from the satellite to the ground.
- Shared the key between two ground receivers — during the day. (That's key because light from the sun, moon and cities on Earth can drown out the photon signal. The current satellite only operates at night.)
"They all together prove that a number of different concepts relevant for the quantum internet really do work in a space setting," says Anton Zeilinger, a quantum physicist at the University of Vienna who was Pan's advisor.
The bottom line: China's achievements are more technological than scientific, but they represent a true advance in the development and deployment of these technologies, says Ray Newell of Los Alamos National Laboratory.
He points out that many of the fundamental science and technologies for quantum key distribution were invented in the United States. (Satellite-based quantum key distribution was invented at Los Alamos, which holds the original patent for the technology.) Other countries possess the knowledge to build these systems, but China is the first to make a major investment.
"In China, the decision to build it was done at the beginning, and then they went through with a lot of manpower and money," says Norbert Lutkenhaus from the University of Waterloo.
Keep in mind: A so-called quantum internet isn't likely to replace the internet as we know it, but it could operate in parallel. "In the short term, it's going to be a niche system for people who need high security," says Alexander Ling at the National University of Singapore's Centre for Quantum Technologies.
What's next: Pan, from the University of Science and Technology of China, has said he wants to launch another quantum communications satellite, but to 20,000 km above Earth in order to cover a larger swath of the planet. There are also plans to equip China's forthcoming space station with quantum communications capabilities and see if a quantum key can be distributed between Beijing and Vienna.
Three ground stations have been built in Europe — and three more are on the way. "The aim is to have intercontinental quantum communication, which is crucial for the future quantum internet," says Zeilinger.
Other key players:
Japan: A team of researchers there has sent entangled photons from a satellite in low Earth orbit to the ground. Key innovation: they are using a lightweight, less expensive satellite — one avenue for setting up a large number of nodes in space.
Germany: While most researchers are using low Earth orbit satellites, a team in Germany is trying to send quantum information using geostationary satellites more than 36,000 km above the Earth. That would allow for continuous coverage. The challenge: even more photons will likely be lost traveling that distance.
Canada: It plans to launch a small (60 kg) satellite that will be less expensive and more relevant to commercial applications.
Singapore: They've had a quantum light source in orbit since 2015 and, in collaboration with the University of New South Wales - Canberra, have proposed a mission to test key distribution between two small satellites.
United States: There are proposals to explore quantum communications from the International Space Station, but there currently aren't any plans (at least public ones) to launch quantum-enabled satellites.
"I think that's a shame. The ability to generate enduring value to the country is something that could be done with straightforward and modest investment," says Newell of Los Alamos, who is working on land-based quantum communication systems to secure the electric grid.
Editor's note: This story has been updated to include Singapore's satellite-based quantum communications projects.