Quantum Satellite Links Extend More Than 1,000 Kilometers

New system one step closer to practical quantum cryptography

2 min read
Delayed exposure photograph of the Satellite-to-Earth link in China’s quantum entanglement experiment.
This delayed exposure photograph shows the satellite’s passing through the Nanshan optical ground station (OGS). The green line is the beacon laser from the satellite to the ground. The red light is the beacon laser from the ground station to the satellite.
Photo: Zhu Jin

A space-based, virtually unhackable quantum Internet may be one step closer to reality due to satellite experiments that linked ground stations more than 1,000 kilometers apart, a new study finds.

Quantum physics makes a strange effect known as entanglement possible. Essentially, two or more particles such as photons that get linked or "entangled" can influence each other simultaneously no matter how far apart they are.

Entanglement is an essential factor in the operations of quantum computers, the networks that would connect them, and the most sophisticated kinds of quantum cryptography, a theoretically unhackable means of securing information exchange.

The maximum distance over which researchers have thus far generated quantum cryptography links between stations on Earth is roughly 144kilometers. In principle, satellites could serve as relays between ground stations to greatly boost the lengths to which quantum links can extend.

In 2017, scientists in China used the satellite nicknamed Micius, which is dedicated to quantum science experiments, to connect sites on Earth separated by up to roughly 1,200 kilometers via entanglement . Although those experiments generated about 5.9 million entangled pairs of photons every second, the researchers were able to detect only one pair per second, an efficiency rate far too low for useful entanglement-based quantum cryptography.

Now, the same researchers have achieved their goal of entanglement-based quantum cryptography using the Micius satellite. The scientists, who detailed their findings online in the 15 June edition of the journal Nature, say they again connected two observatories separated by 1,120 kilometers. But this time, the collection efficiency of the links was improved by up to four-fold, which resulted in data rates of about 0.12 bits per second.

Illustration of the Micius satellite and the two ground stations. The satellite flies in a Sun-synchronous orbit at an altitude of 500 km. The physical distance between Nanshan and Delingha ground station is 1120 km.The Micius satellite flies in a sun-synchronous orbit at an altitude of 500 km. The physical distance between the Nanshan and Delingha ground stations is 1120 km.Illustration: Micius Team

The scientists employed two ground stations, in Delingha and Nanshan, in China. Each site had a newly built telescope 1.2 meters wide that was specifically designed for the quantum experiments.

To boost the efficiency of the quantum cryptography links, the researchers focused on improving the systems used to acquire, orient toward and track targets at both the satellite and ground stations. They also made sure to improve the receiving and collection efficiencies of the lenses and other optical equipment on the ground.

“A remarkable feature of the entanglement-based quantum cryptography that we demonstrated here is that the security is ensured even if the satellite is controlled by an adversary,” says study senior author Jian-Wei Pan, a quantum physicist at the University of Science and Technology of China at Hefei. In other words, even if an unwelcome third party controls the satellite, they cannot eavesdrop on communications through the satellite without the other participants knowing.

The scientists note they could increase the brightness of their spaceborne entangled-photon source by roughly 100-fold. This could boost the data rate of the system to tens of bits per second.

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Two men fix metal rods to a gold-foiled satellite component in a warehouse/clean room environment

Technicians at Northrop Grumman Aerospace Systems facilities in Redondo Beach, Calif., work on a mockup of the JWST spacecraft bus—home of the observatory’s power, flight, data, and communications systems.


For a deep dive into the engineering behind the James Webb Space Telescope, see our collection of posts here.

When the James Webb Space Telescope (JWST) reveals its first images on 12 July, they will be the by-product of carefully crafted mirrors and scientific instruments. But all of its data-collecting prowess would be moot without the spacecraft’s communications subsystem.

The Webb’s comms aren’t flashy. Rather, the data and communication systems are designed to be incredibly, unquestionably dependable and reliable. And while some aspects of them are relatively new—it’s the first mission to use Ka-band frequencies for such high data rates so far from Earth, for example—above all else, JWST’s comms provide the foundation upon which JWST’s scientific endeavors sit.

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