China has launched the world’s first satellite dedicated to testing the fundamentals of quantum communication in space. The $100m Quantum Experiments at Space Scale (QUESS) mission was launched today from the Jiuquan Satellite Launch Center in northern China at 01:40 local time. For the next two years, the craft – also named “Micius” after the ancient Chinese philosopher – will demonstrate the feasibility of quantum communication between Earth and space, and test quantum entanglement over unprecedented distances.
Work on QUESS – a collaborative endeavour between the Chinese Academy of Sciences and the Austrian Academy of Sciences – began in 2011 and the assembly was completed earlier this year. The 600 kg satellite will now be put into Earth orbit around 500 km above ground. The craft’s main instrument is a “Sagnac” interferometer that is used to generate two entangled infrared photons by shining an ultraviolet laser on a non-linear optical crystal.
Physicists have previously managed to separate entangled photons by distances up to 300 km on Earth. But because photons scatter when they travel down optical fibres or encounter atmospheric turbulence when sent between telescopes, it is hard to send entangled photons longer distances unless the experiments are performed in space.
Quantum encryption
The main goals of QUESS will be to demonstrate quantum key distribution (QKD) between the satellite and two stations on the ground – the Nanshan 25 m telescope at the Xinjiang Astronomical Observatory in western China and the Xinglong Observatory in Yanshan, around 200 km south of Beijing. A quantum key is a string of ones and zeros, representing the quantum states of particles. These can be used to encode and decode messages, which would be secure from eavesdroppers.
QUESS will perform a test of Bell’s inequality at a separation of over 1200 km – the greatest distance to date – to prove that entanglement can exist between particles separated by such a large distance. QUESS will also quantum teleport a photon state from the Ali observatory on the Tibetan Plateau to the satellite. “These goals will be performed solely by the Chinese team,” says Jianwei Pan from the University of the Science and Technology of China, who is QUESS’s chief scientist.
Pan adds that, once these targets have been met, the Chinese team will then collaborate with Anton Zeilinger and colleagues at the University of Vienna to create an “intercontinental” QKD channel between Beijing and Vienna, with the option of including stations in Italy and Germany. “QUESS will be the first test of quantum communication with a satellite,” says Zeilinger. “It can also be seen as a very significant step towards a future worldwide quantum internet.” Indeed, China is planning to launch a number of similar satellites to create a quantum communications network by 2030.
Challenges ahead
QUESS first faces a number of technical challenges in orbit, especially to make sure that the receiving telescopes on the ground can precisely track the satellite, which will be travelling at 8 km/s. “It’s very challenging to create a perfect quantum channel between the satellite and the ground station,” says Pan. “We have developed a high-frequency and high-accuracy acquiring, pointing and tracking technique to do that.”
Quantum physicist Alexander Ling, from the National University of Singapore, who is not part of the QUESS team, says he is looking forward to the data that will emerge from QUESS adding that there are now about a dozen groups worldwide working on space-based quantum experiments. Last January, his group launched and demonstrated a quantum entanglement source inside a cube-sat – a type of miniaturized satellite for space research weighing no more than 1.5 kg and with each side being around 10 cm long. “Since the access to space is becoming more easily available with the emergence of new satellite technologies such as nanosatellites, we should be able to see more quantum experiments in space,” he adds.
Thomas Jennewein at the University of Waterloo in Canada and colleagues are working on a quantum payload prototype that could fit onto a small satellite. They have already performed several feasibility experiments, such as sending quantum signals to a payload on a moving truck. Later this year, meanwhile, the European Space Agency is expected to carry out a detailed study into the possibility of using the International Space Station for quantum communication experiments.
QUESS is one of four missions belonging to the National Space Science Center’s strategic priority programme in space science. The others are the Dark Matter Particle Explorer, which took off in December 2015, Shijian-10, launched in April, and the Hard X-ray Modulation Telescope, which is set to take off in November.
