China's quantum satellite makes breakthrough in secure communications

Using the satellite, named Micius after an ancient Chinese philosopher, the researchers beamed intertwined photon pairs down to the cities of Delingha in northern China and Lijiang in southern China. Each of them were separated by more than 1,200-km. This distance has remained a technical barrier because entangled photons are eventually absorbed by the medium they are moving through.

The system will open more avenues for further quantum communications practical and fundamental experiments on quantum optics at distances that were previously not accessible on the ground.

Launched in 2016, the one-of-a-kind satellite is laying the groundwork for a space-based network of quantum communication.

"Chinese researchers deserve a greatest praise and acknowledgement of their skills, persistence, and devotion to science", said Sergienko.

This experiment was made through two satellite-to-ground downlinks with a total length varying from 1,600 to 2,400 kilometers.

The obtained link-efficiency is many times higher than that of the direct bi-directional transmission of the two photons through telecommunication fibres, said Pan, who is also the lead scientist of QUESS. That is good enough for quantum communications between neighbouring towns, but it can not work for much greater distances, because the signal is gradually lost the more optic fibre it travels down. Optical fibers, however, are vulnerable to interference, making it hard for scientists to establish a quantum communication network capable of transmitting over very long distances.

Another approach is making use of satellite-based and space-based technologies, as a satellite can conveniently cover two distant locations on Earth.

Photons are extremely fragile: they travel more smoothly in the near vacuum of space than in the earth's atmosphere. Over the past decade, physicists have been able to transmit pairs of entangled photons over increasing distances, both in the air and along optical fibres. Aiming is also a challenge because of the high speeds of the satellite and its distance to the ground. The pairs are split and individual photons are directed at two different receiving stations - covering distances of 500-2000 km. Regardless of the distance, the photons sustained their entanglement and were successfully received by the ground stations. To optimize the link efficiency, Chinese scientists combined a narrow beam divergence with a high-bandwidth and a high-precision acquiring, pointing, and tracking (APT) technique. Yin, along with his team, exhibited an improved method in order to achieve global quantum networks through satellite technology and laser beams, reports Science Alert.

An immediate application of distributed entangled photons, said Pan, is for entanglement-based quantum key distribution to establish secure keys for quantum communication.

"For the first time, we're testing the physical law of the micro world on a space scale, and laying the foundation for exploring more basic laws in physics in the future". "They are now clearly the world leader in quantum satellites", he says.

In another study, researchers in Germany found they could measure the quantum features of laser signals transmitted by a satellite 38,600 km away. When combined with the correct encryption algorithm, this system is uncrackable even if encrypted messages are sent over normal communication channels, experts have said. In an e-mail to The Globe and Mail, Dr. Pan revealed that he and his team were able to get the experiment working within a couple of months of the satellite's launch last August.

Currently, QUESS can only send photons at night.

One test would see whether changing gravitational fields affect entanglement.

  • Carolyn Briggs