Quantum Experiments at Space Scale

Xinglong

Ürümqi

Ali

Vienna

Ground stations

The initial experiment demonstrated quantum key distribution (QKD) between Xinjiang Astronomical Observatory near Ürümqi and Xinglong Observatory near Beijing – a great-circle distance of approximately 2,500 kilometres (1,600 mi).[3] In addition, QUESS tested Bell's inequality at a distance of 1,200 km (750 mi) – further than any experiment to date – and teleported a photon state between Shiquanhe Observatory in Ali, Tibet Autonomous Region, and the satellite.[3] This requires very accurate orbital maneuvering and satellite tracking so the base stations can keep line-of-sight with the craft.[3][14] In 2021 full quantum state teleportation was demonstrated over 1,200 km (750 mi) at ground, based on entanglement distributed by the satellite.[15]

Once experiments within China concluded, QUESS created an international QKD channel between China and the Institute for Quantum Optics and Quantum Information, Vienna, Austria − a ground distance of 7,500 km (4,700 mi), enabling the first intercontinental secure quantum video call in 2016.[3][6]

The main instrument on board QUESS is a "Sagnac effect" interferometer.[3] This is a device which generates pairs of entangled photons, allowing one of each to be transmitted to the ground. This will allow QUESS to perform Quantum key distribution (QKD) – the transmission of a secure cryptographic key that can be used to encrypt and decrypt messages – to two ground stations. QKD theoretically offers truly secure communication. In QKD, two parties who want to communicate share a random secret key transmitted using pairs of entangled photons sent with random polarization, with each party receiving one half of the pair. This secret key can then be used as a one-time pad, allowing the two parties to communicate securely through normal channels. Any attempt to eavesdrop on the key will disturb the entangled state in a detectable way.[13] QKD has been attempted on Earth, both with direct line-of-sight between two observatories, and using fibre optic cables to transmit the photons. However, fibre optics and the atmosphere both cause scattering which destroys the entangled state, and this limits the distance over which QKD can be carried out. Sending the keys from an orbiting satellite results in less scattering, which allows QKD to be performed over much greater distances.[3]

In addition, QUESS tests some of the basic foundations of quantum mechanics. Bell's theorem says that no local hidden-variable theory can ever reproduce the predictions of quantum physics, and QUESS will be able to test the principle of locality over 1,200 km (750 mi).[3]

QUESS lead scientist Pan Jianwei told Reuters that the project has "enormous prospects" in the defence sphere.[17] The satellite will provide secure communications between Beijing and Ürümqi, capital of Xinjiang, the remote western region of China.[17] The US Department of Defense believes China is aiming to achieve the capability to counter the use of enemy space technology.[17] Chinese Communist Party general secretary Xi Jinping has prioritised China's space program, which has included anti-satellite missile tests, and the New York Times noted that quantum technology was a focus of the thirteenth five-year plan, which the China government set out earlier that year.[18] The Wall Street Journal said that the launch put China ahead of rivals, and brought them closer to "hack-proof communications".[19] Several outlets identified Edward Snowden's leak of US surveillance documents as an impetus for the development of QUESS, with Popular Science calling it "a satellite for the post-Snowden age".[14][20][21]

QUESS is the first spacecraft launched capable of generating entangled photons in space,[7] although transmission of single photons via satellites has previously been demonstrated by reflecting photons generated at ground-based stations off orbiting satellites.[22] While not generating fully entangled photons, correlated pairs of photons have been produced in space using a cubesat by the National University of Singapore and the University of Strathclyde.[22] A German consortium has performed quantum measurements of optical signals from the geostationary Alphasat Laser Communication Terminal.[23] The US Defense Advanced Research Projects Agency (DARPA) launched the Quiness macroscopic quantum communications project to catalyze the development of an end-to-end global quantum internet in 2012.

In 2024, ESA intends to launch the Eagle-1 quantum key distribution satellite, with a goal similar to that of the Chinese QUESS. It will be part of the development and deployment of the European Quantum Communication Infrastructure (EuroQCI).[24]

• Chinese space program
• Quantum information
  • Quantum cryptography
  • Quantum computing
  • Quantum key distribution
  • Quantum teleportation

• QUESS Launching Archived 2017-11-06 at the Wayback Machine(Chinese)
• 3Cat-2 Satellite web site at Polytechnic University of Catalonia