China's quantum satellite clears major hurdle on way to ultrasecure communications

Just months into its mission, the world’s firstquantum-communications satellite has achieved one of its most ambitious goals.

Researchers report in Science1 that, by beaming photons between the satellite and twodistant ground stations, they have shown that particles can remain in a linkedquantum state at a record-breaking distance of more than 1,200 kilometres.That phenomenon, known as quantum entanglement, could be used as the basis of a future securequantum-communications network.

The feat is the first result reported fromChina’s Quantum Experiments at Space Scale (QUESS) mission, also known as Miciusafter an ancient Chinese philosopher. Launched last August, the craft is designed to demonstrateprinciples underlying quantum communication. The team is likely to launch morequantum-enabled satellites to start building a network.

Quantum communication is secure because anyinterference is detectable. Two parties can exchange secret messages by sharingan encryption key encoded in the properties of entangled particles; anyeavesdropper would affect the entanglement and so be detected.

The Micius team has already done experiments exploring whetherit is possible to create such encryption keys using entangled photons, andeven 'teleport' information securely between Earth and space,says Pan Jian-Wei, a physicist at the University ofScience and Technology of China in Hefei and the main architect of the probe.But he says that his team is not yet ready to announce the results.

Bell test

In theory, entangled particles should remain linked at anyseparation. That can be checked using a classic experiment called a Bell test.

Central to QUESS's experiments is a laser beam mounted on the satellite. Forthe Bell test, the beam was split to generatepairs of photons that share a common quantum state, in this case related topolarization. The entangled photons were funnelled into two onboard telescopesthat fired them at separate stations on the ground: one in Delingha, on thenorthern Tibetan Plateau, and the other 1,203 kilometres south, atGaomeigu Observatory in Lijiang. Once the particles arrived, the team used theBell test to confirm that they were still entangled.

The researchers had a window of less than 5 minutes each night when thesatellite, which orbits at an altitude of about 500 kilometres, was in view ofboth observatories. Within weeks of launch, they were able to transmit a pairof entangled photons per second — a rate ten times faster than they had hoped.The crucial experiment was completed before the end of the year, says Pan: “Weare very happy that the whole system worked properly.” The previous recordfor such an experiment was 144 kilometres2.

“This proves that one can perform quantum communications atcontinental distances,” says Frédéric Grosshans, a quantum-communicationsphysicist at the University of Paris South in Orsay. Entangled particles arethe “workhorse” of quantum communications, he adds.


“I am really impressed by the result of the Chinese group,” saysWolfgang Tittel, a physicist at the University of Calgary in Canada. “To me, itwas not clear after the satellite launch if they would succeed,” he says, or whetherthey would use it to learn for the next improved mission.

Pan says that in addition to the quantum-key and teleportationexperiments, the team also plans to use Micius to test how gravity affects thequantum state of photons. And they want to launch a second, improved, quantumsatellite in two years. A major challenge, he says, will be to upgrade thetechnology so that it can send and receive signals during the day, when thereare many more photons around and it is harder to pick out the ones coming fromthe satellite.

For now, Pan feels vindicated about the first spacecraft’s design. Colleaguesthought that it was too ambitious, he says, because it produced the entangledphotons in space and required two photon-firing systems.

Similar missions in the planning stages — such as Canada’s Quantum Encryptionand Science Satellite (QEYSSat) — use a simpler approach, creating theentangled photons on Earth and beaming them to a satellite. In a study3 published last week, theQEYSSat team reported a successful test of its technology, transmitting photonsfrom the ground to an aircraft as much as 10 kilometres in the air.

Thomas Jennewein, who is at the University of Waterloo in Canadaand part of the Canadian mission, says that his group and others around theworld are now racing to catch up with the Chinese effort. “They are now clearlythe world leader in quantum satellites,” he says.

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