On January 23, Thales Alenia Space announced that it had been selected by the European Space Agency as prime contractor for the TeQuantS (Technological development for space-based Quantum reSource Distribution) project, which aims to develop quantum communications technologies between space and Earth.

This contract, which is part of the 4.0 Core Competitiveness component of the Arets (Advanced Research in Telecommunications Systems) program, is supported by Cnes and the Austrian Space Agency.

As Mathias Van Den Bossche, Director of Research, Technology and Products at Thales Alenia Space, explains, the challenge is to " anticipate the development of quantum technologies needed for the future quantum internet network, to make future quantum computers and sensors communicate with each other, and achieve exponential performance gains "

At the heart of strategic issues

Next computer revolution, " of the same order of magnitude as the advent of our current computers ", quantum computing is at the heart of the strategic stakes of the States. The power of this new type of computer will make it possible to solve problems of currently unattainable complexity such as optimizing the management of a fleet of vehicles, predicting the properties of a drug or breaking into encryption systems. That said, if "one of the first uses of these quantum computers will be to break through the most common cryptographic system in use today, it is also important to know that quantum technologies also offer solutions to guarantee the security of communications. The whole issue is which capability will mature first.

The performance of quantum sensors and quantum computers will be enhanced tenfold by their networking, which will allow them to perform their tasks in a distributed fashion. But to connect them, it will be necessary to deploy networks of a new type, which can transfer not classical information but quantum information. Quantum information can be transmitted by teleportation of quantum particle states, an operation that has been known since 1996 and for which Anton Zeilinger was awarded the 2022 Nobel Prize in Physics. As we will see, satellites will be needed to enable long-distance communications, which explains the interest of one of the world leaders in space telecommunications, Thales Alenia Space.

In the field of quantum information networks, China is " more or less ten years ahead of the rest of the world ". In terms of cryptanalysis, it has already succeeded in " breaking through an encryption key of a few tens of bits " although still far from breaking through a current RSA encryption key, the cryptography algorithm used to exchange confidential data on the Internet, which counts between 1024 and 2048 bits. China has also achieved from space a quantum link between two ground stations more than 1 200 km apart. " An undeniable technological achievement ", and we will explain why.

On the cusp of a revolution

In Europe, the delay is certainly important but, it should be known that " most of the quantum technologies were invented in our laboratories ". The challenge for TeQuantS is to make them " mature so that they become operational and compatible with their use environments, in space and on Earth ". This interest in quantum computing is explained by its potential. It is so great that " we can be sure that we have not yet imagined all the possibilities that it opens ". We expect a " revolution', as the Internet was in its time when it was a question of networking our computers ". When the Internet was invented in the 1960s, " no one imagined the applications and services that are part of our daily lives today ".

Successfully commissioning a quantum network and " thus making several quantum computers or sensors communicate with each other ", will multiply " their capabilities for applications in many fields, such as cybersecurity, chemistry, medicine, energy or logistics for example. " When in use, these quantum systems (computers and sensors) " will pave the way for systems capable of solving complex real-world problems that today's best computers will never be capable of " We're talking about computations " that we can't do now because they would require centuries, even millennia, but that networked quantum computers will be able to do in very short "

Better yet, before going through the experiment, some current scientific problems " that remain difficult to address because of insufficient computing power will be more easily solved " Today, in many fields, we know how to write equations but we do not have enough computing power to solve them.  Problems that are considered today as extremely complex, requiring very long computing times, " will be very simplified by quantum computing ".

An exceptional field of research

Concretely, quantum computers - and even more so after they are networked - will be able to be used to " explore with far fewer limitations the properties of new molecules or materials," design drugs more "quickly by accurately simulating their chemical properties, a calculation too difficult for today's supercomputers ", provide more " accurate systems for predicting weather and forecasting the impact of climate change  or solve very complex problems such as optimizing electricity distribution by energy providers. " Artificial intelligence and blockchains will also take advantage of quantum computers with a multiplicity of computing power. Another fascinating example is imagining using quantum sensors to monitor " the electrical activity of neurons, one by one. This opens up a whole new field of application for understanding the brain and detecting the diseases associated with it. But that's not all. Tomorrow, associated with quantum sensors, such as cold atom accelerometers, they will be "able to detect the deformation of the earth's gravity field generated by the presence of submarines, and thus potentially render obsolete the nuclear deterrent which relies on the use of submarines that are impossible to locate". In another domain, we can also imagine the appearance of networked quantum gaming. The very popular Minecraft, which is a virtual construction game that allows you to create your own universe using elementary blocks made of various materials (wood, stones, glass... and quantum blocks whose nature is only determined when you inspect them) could have real quantum blocks - for now these quantum blocks are simply simulated.

A challenge for cybersecurity

In the field of cybersecurity, it is important to know that a quantum computer can theoretically " break through the most common encryption key-sharing systems, which rely on solving a very complex problem, in a very short time, by performing all the steps of a computation in parallel, even if there are very many of them," where a classical computer must " process the problem sequentially, one step after another " Conversely, quantum cryptography consists in " using the quantum properties of light to generate encryption keys by two remote users located anywhere in the world ". These quantum encryption keys will address " threats of quantum computing because they do not rely on a complex problem as is the case with current encryption keys, and provide keys that are true randomness with controlled privacy : the Holy Grail of encryption keys, because quantum computers do not have a complex problem to solve in order to crack them. "

Why a constellation ?

To understand the need for a constellation of satellites, let's recall that one of the problems encountered is the " need to transfer qubits reliably, i.e., without loss and without errors " To get rid of these two constraints, the constellation is the solution and perhaps even the only one, to " realize long distance quantum communications. " The terrestrial optical fibers allow to transmit a quantum information only on a few tens of kilometers. That's good, but " very insufficient to envision the creation of a quantum internet " so the " best option for pushing the limits of transmission is through the use of satellites "

To communicate, quantum devices must " send and receive qubits using a communication channel established by light particles in an entangled state ". However, these qubits turn out to be sensitive to the environment they cross. The environment of the optical fibers " is dense, and for this reason knows high rates of absorption of light what loses qubits ". However, we cannot amplify a quantum signal at the risk of destroying its properties. It is therefore necessary to live with the transmission losses, by seeking a medium where they are as low as possible. This is the case of free space, " which is an empty medium where nothing can absorb the light ". That's why satellites are pretty much a must for long-distance quantum communications.

Set to go live by 2035

Still, it needs to be demonstrated. That's the whole point of the TeQuantS project, which plans to " build a satellite and two optical ground stations " with by 2026 the demonstration that " these two stations can be linked from this satellite in orbit by creating a long-distance quantum communications link ". By 2030, the goal is to demonstrate " that we can make an experimental network, that is to say link several links, and from there begin to propose systems on the market of telecom operators for a production around 2035 ". The idea is to use a laser on board the satellite to illuminate a crystal whose particular property is to generate "pairs of photons linked in an entangled state". It is from this state, which has the property of quantum entanglement, "that a quantum communication channel can be created and that information teleportation protocols can be implemented. Concretely, in the satellite, the "laser excites the crystal so that photons from the laser generate two correlated photons. Then we separate these pairs by sending a photon of each pair to each of the ground stations. Once these photons have been received by the ground stations, a state teleportation protocol can be implemented to transfer information from one photon to the other. Strictly speaking, the information does not flow through the satellite, which is only used to produce the photon pairs to create the channel.

The development of this satellite will require the removal of several technological locks. First, it will be the " first time that an experiment to make entangled photons will be flown " There are a number of uncertainties on the " behavior of a nonlinear crystal in space " and for the optical part it will be " necessary to realize telescopes with a precise pointing to direct the photons towards the ground stations, which will require a satellite of a very great stability for a cost as low as possible ". Concerning the ground stations, " we need to develop equipment and receivers capable of recovering the entangled photons, emitted by a satellite that moves on an orbit, generating a Doppler effect ". Atmospheric turbulence is also a concern that astronomers are familiar with (the scintillation of stars seen from the ground).

Finally, clouds that stop the signal " are a very big hard point ". To get rid of this strong constraint that " we will obviously never be able to lift, the idea is to keep photons distributed by clear weather ". This leads to other problems, such as the one related to " quantum memories to store entanglement (information will come later when we want to communicate), which raises the question of the storage time of photons ". Today, we manage to keep them thinking only a few seconds  " The goal is to keep them for several days ", and the teams at Thales Alenia Space are working today on this objective.