Quantum Experiments at Space Scale (QUESS; ), is a Chinese research project in the field of quantum physics.

Tiangong-2 Tiangong-2 () was a Chinese space laboratory and part of the Project 921-2 space station program. Tiangong-2 was launched on 15 September 2016. It was deorbited as planned on 19 July 2019. Tiangong-2 was neither designed nor planned to be a ...

is China's second Space Laboratory module which was launched on 15 September 2016.

carries a total of 14 mission and experiment packages, including Space-Earth

quantum key distribution Quantum key distribution (QKD) is a secure communication method which implements a cryptographic protocol involving components of quantum mechanics. It enables two parties to produce a shared random secret key known only to them, which can then ...

() and laser communications experiment to facilitate space-to-ground quantum communication. A satellite, nicknamed Micius or

Mozi Mozi (; ; Latinized as Micius ; – ), original name Mo Di (), was a Chinese philosopher who founded the school of Mohism during the Hundred Schools of Thought period (the early portion of the Warring States period, –221 BCE). The an ...

() after the ancient Chinese philosopher, is operated by the

Chinese Academy of Sciences The Chinese Academy of Sciences (CAS); ), known by Academia Sinica in English until the 1980s, is the national academy of the People's Republic of China for natural sciences. It has historical origins in the Academia Sinica during the Repub ...

, as well as ground stations in China. The

University of Vienna The University of Vienna (german: Universität Wien) is a public research university located in Vienna, Austria. It was founded by Duke Rudolph IV in 1365 and is the oldest university in the German-speaking world. With its long and rich h ...

and the Austrian Academy of Sciences are running the satellite's European receiving stations. QUESS is a proof-of-concept mission designed to facilitate

quantum optics Quantum optics is a branch of atomic, molecular, and optical physics dealing with how individual quanta of light, known as photons, interact with atoms and molecules. It includes the study of the particle-like properties of photons. Photons have ...

experiments over long distances to allow the development of

quantum encryption Quantum key distribution (QKD) is a secure communication method which implements a cryptographic protocol involving components of quantum mechanics. It enables two parties to produce a shared random secret key known only to them, which can then ...

and quantum teleportation technology. Quantum encryption uses the principle of entanglement to facilitate communication that can absolutely detect whether a third party has intercepted a message in transit thus denying undetected

decryption In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can dec ...

. By producing pairs of entangled photons, QUESS will allow ground stations separated by many thousands of kilometres to establish secure

quantum channel In quantum information theory, a quantum channel is a communication channel which can transmit quantum information, as well as classical information. An example of quantum information is the state of a qubit. An example of classical information i ...

s. QUESS itself has limited communication capabilities: it needs line-of-sight, and can only operate when not in sunlight. Further Micius satellites were planned, including a global network by 2030. The mission cost was around US$100 million in total.

Mission

The initial experiment demonstrated

(QKD) between Xinjiang Astronomical Observatory near

Ürümqi Ürümqi ( ; also spelled Ürümchi or without Umlaut (diacritic), umlauts), formerly known as Dihua (also spelled Tihwa), is the capital of the Xinjiang, Xinjiang Uyghur Autonomous Region in the far Northwest China, northwest of the Peopl ...

and Xinglong Observatory near

Beijing } Beijing ( ; ; ), alternatively romanized as Peking ( ), is the capital of the People's Republic of China. It is the center of power and development of the country. Beijing is the world's most populous national capital city, with over 21 ...

– a

great-circle distance The great-circle distance, orthodromic distance, or spherical distance is the distance along a great circle. It is the shortest distance between two points on the surface of a sphere, measured along the surface of the sphere (as opposed to a st ...

of approximately . In addition, QUESS tested Bell's inequality at a distance of – further than any experiment to date – and teleported a photon state between Shiquanhe Observatory in

Ali ʿAlī ibn Abī Ṭālib ( ar, عَلِيّ بْن أَبِي طَالِب; 600 – 661 CE) was the last of four Rightly Guided Caliphs to rule Islam (r. 656 – 661) immediately after the death of Muhammad, and he was the first Shia Imam ...

Tibet Autonomous Region The Tibet Autonomous Region or Xizang Autonomous Region, often shortened to Tibet or Xizang, is a province-level autonomous region of the People's Republic of China in Southwest China. It was overlayed on the traditional Tibetan regions of ...

, and the satellite. This requires very accurate

orbital maneuver In spaceflight, an orbital maneuver (otherwise known as a burn) is the use of propulsion systems to change the orbit of a spacecraft. For spacecraft far from Earth (for example those in orbits around the Sun) an orbital maneuver is called ...

ing and satellite tracking so the base stations can keep line-of-sight with the craft. In 2021 full quantum state teleportation was demonstrated over at ground, based on entanglement distributed by the satellite. Once experiments within China concluded, QUESS created an international QKD channel between China and the

Institute for Quantum Optics and Quantum Information The Institute for Quantum Optics and Quantum Information (IQOQI) (german: Institut für Quantenoptik und Quanteninformation) is a member institute of the Austrian Academy of Sciences and was founded in November 2003, to create an Austrian resear ...

, Vienna, Austria − a ground distance of , enabling the first intercontinental secure quantum video call in 2016.

Launch

The launch was initially scheduled for July 2016, but was rescheduled to August, with notification of the launch being sent just a few days in advance. The spacecraft was launched by a

Long March 2D The Long March 2D (), also known as the Chang Zheng 2D, CZ-2D and LM-2D, is a Chinese two-stage orbital carrier rocket mainly used for launching LEO and SSO satellites. It is manufactured by the Shanghai Academy of Spaceflight Technology (SA ...

rocket from

Jiuquan Jiuquan, formerly known as Suzhou, is a prefecture-level city in the northwesternmost part of Gansu Province in the People's Republic of China. It is more than wide from east to west, occupying , although its built-up area is mostly located in ...

Launch Pad 603, Launch Area 4 on 17 August 2016, at 17:40 UTC (01:40 local time).

Multi-payload mission

The launch was a multi-payload mission shared with QUESS, LiXing-1 research satellite, and ³Cat-2 Spanish science satellite. * LiXing-1: LiXing-1 is a Chinese satellite designed to measure upper atmospheric density by lowering its orbit to 100–150 km. Its mass is 110 kg. On 19 August 2016, the satellite reentered into the atmosphere, so the mission is closed. * ³Cat-2: The 3Cat-2 (spelled "cube-cat-two") is the second satellite in the 3Cat series and the second satellite developed in Catalonia at

Polytechnic University of Catalonia The Technical University of Catalonia ( ca, Universitat Politècnica de Catalunya, , es, link=no, Universidad Politécnica de Cataluña; UPC), currently referred to as BarcelonaTech, is the largest engineering university in Catalonia, Spai ...

’s NanoSat Lab. It is a 6-Unit

CubeSat A CubeSat is a class of miniaturized satellite based around a form factor consisting of cubes. CubeSats have a mass of no more than per unit, and often use commercial off-the-shelf (COTS) components for their electronics and structure. CubeSat ...

flying a novel

GNSS A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location (longitude, latitude, and altitude/elevation) to high p ...

Reflectometer (GNSS-R) payload for Earth observation. Its mass is 7.1 kg.

Secure key distribution

The main instrument on board QUESS is a "

Sagnac effect The Sagnac effect, also called Sagnac interference, named after French physicist Georges Sagnac, is a phenomenon encountered in interferometry that is elicited by rotation. The Sagnac effect manifests itself in a setup called a ring interferomet ...

interferometer Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber o ...

. 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 Quantum key distribution (QKD) is a secure communication method which implements a cryptographic protocol involving components of quantum mechanics. It enables two parties to produce a shared random secret key known only to them, which can then ...

(QKD) – the transmission of a secure

cryptographic key A key in cryptography is a piece of information, usually a string of numbers or letters that are stored in a file, which, when processed through a cryptographic algorithm, can encode or decode cryptographic data. Based on the used method, the 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 Polarization or polarisation may refer to: Mathematics *Polarization of an Abelian variety, in the mathematics of complex manifolds *Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...

, with each party receiving one half of the pair. This secret key can then be used as a

one-time pad In cryptography, the one-time pad (OTP) is an encryption technique that cannot be cracked, but requires the use of a single-use pre-shared key that is not smaller than the message being sent. In this technique, a plaintext is paired with a r ...

, 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. QKD has been attempted on Earth, both with direct line-of-sight between two observatories, and using

fibre optic An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means ...

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. In addition, QUESS tests some of the basic foundations of

quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, q ...