China making the quantum future in space

China plans to launch a medium-to-high Earth orbit quantum satellite soon, extending its claimed lead to a decade

When China announced the successful docking of its Shenzhou-16 spacecraft and Tiangong-3 space station on Tuesday, state media said three Chinese astronauts will have a chance to study “novel quantum phenomena.”

Beyond that teaser, there were no more details. Since quantum is the pay dirt that some real connoisseurs of China’s space program are most seriously interested in, the announcement created frustration. 

Fortunately, there is enough open-source information out there to enable assembly of a progress report – one that starts with the fact that, in a change from four years ago, China now relies on special satellites instead of a space station to conduct its main quantum experiments. 

The country’s existing Micius satellite, a quantum laboratory, has already been credited with a series of scientific achievements since it was launched into low orbit (500 kilometers above sea level) in August 2016.

Next, Micius will conduct intercontinental experiments with other countries such as Russia, Italy, Sweden and South Africa.

China successfully launched Jinan-1, a low-orbit quantum satellite, last July. It plans to launch a medium-to-high Earth-orbit satellite and several smaller low orbit ones in the coming few years. 

If all these experiments are successful, China can achieve unhackable data transmission with quantum key distribution (QKD), an encryption technology, and provide relevant services to banks and government customers.

“We are now developing the first medium-to-high orbit quantum satellite, which is planned to be launched around 2026,” Pan Jianwei, professor of physics at the University of Science and Technology of China, said in an opening speech during the BEYOND Expo 2023 in Macau on May 10.

“Apart from testing QKD, the quantum satellite will also provide a new platform for quantum precision measurement (or quantum clock),” Pan said. “With this, quantum entanglement distribution over a distance of more than 10,000 kilometers can be realized.”

Quantum entanglement is a phenomenon that explains how two photons can be linked to each other and have the same polarisation state even if they are very far apart. When the state of one of them is changed, the other will also change. Such a phenomenon can be applied in encryption for data transmission.

This type of encryption is unbreakable as it relies on the foundations of quantum mechanics. Traditional public key cryptography, used by banks and governments to protect their data transmissions, relies on mathematical functions, which can be decrypted with supercomputers or quantum computers.

In layman’s terms:

• Quantum entanglement distribution involves sending two photons to two places while maintaining their state.

• QKD is sending a photon along a data transmission channel but keeping another to ensure that no hacking is occurring. 

• Quantum teleportation is sending information about the state of a photon.

From theory to reality

The theory of quantum entanglement was first proposed by Austrian physicist Erwin Schrödinger, who won the Nobel Prize in Physics in 1933. In 1984, engineers Charles Bennett and Gilles Brassard invented the first QKD protocol called BB84, which emits polarized optical pulses using a 1,550 nanometer laser source.

In 1998, Austrian physicist Anton Zeilinger achieved a breakthrough in quantum teleportation, which is an essential concept in many quantum information protocols and an important possible mechanism for building gates within quantum computers. 

Zeilinger, together with American physicist John Clauser and French physicist Alain Aspect, won the Nobel Prize for Physics last year. He was the academic advisor of Pan, who gained his doctoral degree at the University of Vienna in Austria in 1999.

In 2001, Pan returned to China. In 2009, his team conducted quantum teleportation at a distance of over 16 kilometers, a world record at that time. In 2016, Pan led China’s Quantum Science Experimental Satellite project that launched the Micius satellite. In 2017, the satellite used the BB84 laser to send signals to the ground and completed a series of quantum experiments. 

Top Chinese quantum scientist Pan Jianwei. Photo: CGTN

“In order to realize global quantum communication, it is necessary to overcome the current difficulties faced by quantum satellites,” Pan said. “A single low-orbit satellite cannot cover the entire world. Also, current satellites can only send signals at night in fine weather.”

He said the problems can be solved by launching more satellites into low orbit to cover a broader area on the ground and a bigger satellite into medium-to-high orbit to connect them.

In comparison, Elon Musk’s Starlink satellites are now operating in low orbit, 550 kilometers above sea level. Medium orbit refers to a height of 20,000 kilometers above sea level, where GPS satellites and China’s Beidou satellites are operating. High orbit or geostationary orbit, about 36,000 kilometers above sea level, is suitable for traditional satellites that send telecommunication and television signals.

The fall of Tiangong-2

Pan’s idea of setting up a QKD satellite network was implemented when China’s Tiangong-2 space station was launched into low-orbit in September 2016. The space station sent QKD signals to the ground between 2018 and 2019 and worked with the Micius satellite.

But the experiments ended as Tiangong-2 made a controlled reentry to Earth and burned up over the South Pacific Ocean in July 2019. China had originally planned to combine Tiangong-2 with Tiangong-3 in 2022.

Details about what quantum experiments had been done by Tiangong-2 weren’t made public until last August.

“The Micius satellite is only a starting point,” CAS Academician Wang Jianyu said in a published interview last August. “From a practical perspective, we should build a network of satellites in low, medium and high orbits in order to cover all the world’s quantum communication networks.”

“At present, we are at least five years more advanced than other global players in this area. If we can successfully launch a medium-to-high quantum satellite, we will lead the world by at least 10 years,” Wang said.

Liao Sheng-Kai, a professor at the University of Science and Technology of China (USTC), said Jinan-1 weighs 98 kg in total with a 23-kg QKD emitter while the Micius satellite is 640 kg with a 80kg QKD emitter. He said the size reduction can help lower research costs significantly.

While China is spending more on quantum in space, western firms prefer to stay on the ground –figuring that, if QKD can be transmitted via fibers some day, transmission through satellites will prove comparatively uneconomical and China will lose its bet in business terms.

In June 2021, Arqit Quantum Inc, an encryption startup in the United Kingdom, said it would launch two QKD satellites in 2023. But a Wall Street Journal report said in April last year that the company might have overstated its prospects.

Last December, Arqit said it was abandoning its plan to launch quantum satellites as it would rely on its QuantumCloud to provide encryption services to customers.

Alice and Bob

On the ground, QKD is usually done through optical fibers from Alice to Bob (two fictional characters representing the sender and the receiver in quantum communication.) But the transmission distance is largely limited by signal loss. 

On May 25, Pan and a group of Chinese scientists said in a paper published by the Physical Review Letters, a weekly academic journal, that they achieved a 1,002-km point-to-point long-distance QKD in optical fibers.

In January last year, a team led by Chinese scientist Guo Guangcan achieved a QKD transmission distance of 833 km, breaking the record of 605 km achieved by researchers from Toshiba’s Cambridge Research Laboratory in October 2021.

Back in September 2017, China had already constructed a 2,000 km-long quantum fiber network connecting Beijing, Jinan, Hefei and Shanghai.

Researchers said European countries, Japan and the United States have built and are expanding their own QKD networks as they see a rising demand for encryption services from banks and government customers.

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