Global Courant
When China announced the successful docking of its Shenzhou-16 spacecraft and Tiangong-3 space station on Tuesday, state media reported said three Chinese astronauts will have the opportunity to study ‘new quantum phenomena’.
There were no more details beyond that teaser. Since quantum is the currency that some true connoisseurs of China’s space program are most interested in, the announcement caused frustration.
Fortunately, there is enough open-source information available to compile a progress report — one that begins with the fact that, unlike four years ago, China now relies on dedicated satellites rather than a space station to carry out its main missions. to feed. quantum experiments.
The country’s existing Micius satellite, a quantum lab, has already had a string of scientific achievements since launching into low orbit (500 kilometers above sea level) in August 2016.
Micius will then experiment intercontinentally 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 satellites in the coming years.
If all these experiments succeed, China can achieve unhackable data transmission distribution of quantum keys (QKD), an encryption technology, and provide relevant services to banks and government clients.
“We are now developing the first medium to high orbit quantum satellite, which is expected to be launched around 2026,” said Pan Jianwei, a professor of physics at the University of Science and Technology of China, said in an opening speech at BEYOND Expo 2023 in Macau on May 10.
“Besides testing QKD, the quantum satellite will also provide a new platform for quantum precision measurement (or quantum clock),” Pan said. “With this, quantum entanglement over a distance of more than 10,000 kilometers can be achieved.”
Quantum entanglement is a phenomenon that explains how two photons can be coupled together and have the same polarization state, even if they are very far apart. When the status of one of them changes, the other changes as well. Such a phenomenon can be applied in encryption for data transfer.
This type of encryption is unbreakable because it rests on the foundations of quantum mechanics. Traditional public key cryptography, used by banks and governments to protect their data transfers, is based on mathematical functions, which can be decrypted using supercomputers or quantum computers.
In layman’s terms:
Quantum entanglement distribution involves sending two photons to two places while maintaining their state. QKD sends a photon down a data transmission channel, but traps another photon to ensure no hacking occurs.
Quantum teleportation transmits information about the state of a photon.
From theory to reality
The theory of quantum entanglement was first proposed by an Austrian physicist Erwin Schrodingerwho won the Nobel Prize in Physics in 1933. In 1984, engineers Charles Bennett and Gilles Brassard invented the first QKD protocol called BB84which emits polarized optical pulses using a 1550 nanometer laser source.
In 1998, Austrian physicist Anton Zeilinger achieved a breakthrough in quantum teleportation, an essential concept in many quantum information protocols and an important potential mechanism for building gates within quantum computers.
Zeilinger won the Nobel Prize in Physics last year together with the American physicist John Clauser and the French physicist Alain Aspect. He was academic advisor to Pan, who received his PhD in 1999 from the University of Vienna in Austria.
In 2001, Pan returned to China. In 2009, his team performed quantum teleportation over a distance of more than 10 miles, a world record at the time. In 2016, Pan led the Chinese 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
“To realize global quantum communication, it is necessary to overcome the current difficulties that quantum satellites face,” said Pan. “A single satellite in low orbit cannot cover the whole world. Also, the current satellites can only transmit signals at night in good weather.”
He said the problems could be solved by launching more satellites in low orbit to cover a larger area on the ground and a larger satellite in medium to high orbit to connect them.
For comparison: that of Elon Musk Starlink satellites now operate in low Earth orbit, 550 kilometers above sea level. Medium orbit refers to an altitude of 20,000 kilometers above sea level, where GPS satellites and China’s Beidou satellites are active. High orbit or geostationary orbit, about 36,000 kilometers above sea level, is suitable for traditional satellites that broadcast telecommunications and television signals.
The fall of Tiangong-2
Pan’s idea of establishing a QKD satellite network was implemented when China’s Tiangong-2 space station 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 when Tiangong-2 made a control return to Earth and burned up over the South Pacific in July 2019. Originally China had planned to combine Tiangong-2 with Tiangong-3 in 2022.
Details about what quantum experiments had been finished of Tiangong-2 were not made public until August last year.
“The Micius satellite is just a starting point,” said CAS academician Wang Jianyu said in a published interview last August. “From a practical point of view, we should build a network of satellites in low, medium and high orbits to cover all the world’s quantum communication networks.”
“At the moment we are at least five years ahead of other world players in this field. If we can successfully launch a medium to high quantum satellite, we will lead the world for 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 a total of 98 kg with a 23 kg QKD transmitter, while the Micius satellite weighs 640 kg with a QKD transmitter. transmitter of 80 kg. He said the size reduction could help cut research costs significantly.
While China is spending more on space quantum, Western companies prefer to stay grounded – assuming that, if QKD can one day be transmitted over fiber, transmission via satellites will prove relatively uneconomical and China will lose its bet in business terms. will lose.
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 may have overestimated its outlook.
Last December, Arqit said it abandoned its plan to launch quantum satellites because it would rely on its QuantumCloud to provide encryption services to customers.
Alice and Bob
On the ground, QKD is usually done over optical fibers from Alice to Bob (two fictional characters representing the sender and 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 she is a 1,002km long distance point-to-point QKD in optical fibers.
In January last year, a team led by Chinese scientist Guo Guangcan achieved a QKD transmission distance of 833kmbreaking the record 605km reached by researchers at Toshiba’s Cambridge Research Laboratory in October 2021.
In September 2017, China already had one 2,000 kilometers-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 increasing demand for encryption services from banks and government customers.
Read: China rushes into quantum computing race
Follow Jeff Pao on Twitter at @jeffpao3
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