Researchers demonstrate successful quantum key distribution between the space lab and four ground stations

Researchers report an experimental demonstration of a space-to-ground quantum key distribution (QKD) network using a compact QKD terminal aboard China’s Tiangong-2 space laboratory and four ground stations. The new QKD system is less than half the weight of the system the researchers developed for the Micius satellite, which was used to conduct the world’s first quantum-encrypted virtual teleconference.

The demonstration represents an important step towards a practical QKD based on constellations of small satellites, a configuration considered one of the most promising ways to create a global quantum communication network.

“QKD provides unconditional security by using single photons to encode information between two remote terminals,” said Cheng-Zhi Peng, a research team member from the University of Science and Technology of China. “The compact system we have developed can reduce the cost of implementing QKD by allowing the use of small satellites.”

Peng and researchers from other institutions in China describe their new system and experimental results in Optical, Optica Publishing Group’s journal for high-impact research. They also found that QKD performance can be improved by building an array of satellites orbiting at different angles, or inclinations, relative to the equator.

“Our new work demonstrates the feasibility of a space-to-ground QKD network based on a compact satellite payload combined with satellite constellations with different orbit types,” Peng said. “In the near future, this type of QKD system could be used in applications requiring high security such as government affairs, diplomacy and finance.”

Reduce QKD system

QKD uses the quantum properties of light to generate secure random keys to encrypt and decrypt data. In previous work, the research group demonstrated satellite-to-ground QKD and satellite-based intercontinental quantum networks using the Micius satellite. However, the QKD system used on board this satellite was cumbersome and expensive. The size of a large refrigerator, the system weighed around 130 kg and required 130 W of power.

Under China’s Quantum Constellation Plan, researchers sought to develop and demonstrate a more practical space-to-ground QKD network. To do this, they developed a compact payload that enabled the Tiangong-2 Space Lab to act as a QKD satellite terminal. The QKD payload – consisting of a tracking system, a QKD transmitter and a laser communication transmitter – weighed around 60 kg, required 80 W of power and was about the size of two microwave ovens.

“This payload has been integrated as much as possible to reduce bulk, weight, and cost while achieving the high performance needed to support space-to-ground QKD experiments,” Peng said. “It also had to be very durable to withstand harsh conditions such as the high vibrations experienced during launch and the extreme thermal vacuum environment of space.”

Researchers conducted a total of 19 QKD experiments in which secure keys were successfully distributed between the Space Lab terminal and four ground stations on 15 different days between October 2018 and February 2019. These experiments were conducted at night to avoid the influence of daylight background noise. .

The researchers found that the space lab’s mid-inclination (~42°) orbit allowed multiple passes over a single ground station in one night, increasing the number of keys that could be generated. They also built a model to compare the performance of satellite QKD networks with different orbit types. They found that combining satellites with a medium-inclination orbit like the Space Lab with a sun-synchronous orbit that travels over the polar regions achieved the best performance.

Next steps

The researchers are currently working to improve their QKD system by increasing the speed and performance of the QKD system, reducing costs, and exploring the feasibility of daytime satellite-to-ground QKD transmission. “These enhancements would make it possible to create a practical quantum constellation by launching multiple satellites into low orbit,” Peng said. “The constellation could be combined with a quantum satellite in medium-to-high orbit and fiber-based QKD networks on the ground to create an integrated space-to-ground quantum network.”

Although not part of this work, an even smaller quantum satellite developed by the Hefei National Laboratory and the University of Science and Technology of China and other research institutes in China has been successfully launched into space. July 27. This satellite, known as a micro/nano satellite, weighs about one-sixth the weight of the Micius satellite and contains a QKD system that is about one-third the size of the one demonstrated in the Optical paper. This satellite is designed to perform real-time satellite-to-ground QKD experiments, which represents another important step towards practical and low-cost quantum satellite constellations.

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