Using a satellite orbiting 300 miles above the planet, Chinese researchers beamed entangled particles of photons to three ground stations across China, each separated by more than 1,200km. The achievement marks a 10-fold increase in the distance over which entanglement has been maintained. Previously, scattering and coherence decay limited the link separations to only 100km. As such, this successful run is cementing China’s positioning as the leader of the burgeoning field of quantum communication and quantum encryption. The latter might eventually lead to the formation of an allegedly unhackable ‘quantum internet’ with important consequences to society and of immense geopolitical value for those government controlling it.
The beginning of a global quantum network
Quantum entanglement — or what Einstein used to call “spooky action at a distance” — is a physical phenomenon that occurs when pairs or groups of particles are generated or interact in ways such that the quantum state of each particle cannot be described independently of the others. If one of two entangled particles changes its quantum state, it influences the other and directs an instant change no matter if the two particles are spaced apart at the other ends of the universe. Giving one particle an ”up” spin, for instance, might always mean its entangled partner has a ”down” spin. This change is instantaneous.
This phenomenon isn’t some weird, purely theoretical physics caveat that exists solely in the minds of awkward theoretical physicists. It’s quite practical, even. Using the power of entanglement scientists at the National Institute of Standards and Technology (NIST) instantly transferred information from on proton to another one 100 kilometers away in 2015. Physicists call this “quantum teleportation”.
To use entangled photons to communicate, ground-based systems use fiber optic cables or relay the particles through the open air. However, this introduces the risk of collision with ordinary particles disrupting the delicate quantum states of our entangled photons. The longer the particles have to travel, the greater the risk of decoherence, which is why transmission distances were limited to a few hundred kilometers.
An alternative would be to use so-called ‘quantum memory’ modules, linked and spread out in a daisy-chain. But in order to do all that, you have to know you’ve stored the photons without actually measuring them.This is theoretically possible but the technology is too out of reach to be of any practical use in the near future.
It remains that the only current viable option for a global communications network is to beam quantum keys through the vacuum of space then distribute them across tens to hundreds of kilometers using ground-based nodes. This is where China’s 600-kilogram “Micius” satellite comes in.
Micius is the world’s first quantum satellite. Launched in 2016, the satellite named after a famous Chinese philosopher employs a crystal that produces entangled pairs of photons. To communicate, the satellite aligned itself with ground stations in the cities of Delingha and Ürümqi—both on the Tibetan Plateau—as well as in the city of Lijiang in China’s far southwest, and fired entangled particles in pairs to generate a secret key.
“If someone tries to intercept [a particle] when it’s being transmitted, by touching it, they make it burst,” Gregoir Ribordy of Geneva-based quantum cryptography firm ID Quantique said last year.
The distance between Delingha and Lijiang is 1,203 kilometers, which marks a record-setting stretch over which the entangled photon pairs were transmitted. With this success, China is taking establishing itself as the absolute leader in quantum communications.
It should be noted, however, that while Micius’ achievements mark a breakthrough, we’re still pretty far from seeing a practical quantum network. Of the six million or so entangled pairs generated by Micius during each second of transmission, only one pair per second actually reached the ground-based detectors. The rest were altered by Earth’s atmosphere. Scientific American quotes Jian-Wei Pan—a physicist at the University of Science and Technology of China in Hefei who has been planning the experiment ever since 2003 — as saying “[this is like] detecting a single photon from a lone match struck by someone standing on the moon.”
Nevertheless, this is very powerful proof of concept. Already, China has gained enough confidence to green-light more quantum communication satellites which will be tested over the next five years.
The experiment’s findings were described in the journal Science.