Quantum technologies in space: towards a new era of Earth Observation

Observing the Earth from space has become an essential lever for protecting our planet. In particular, its gravity field reveals crucial information about the distribution of water and the mechanisms that govern climate: for example, when a glacier melts or the monsoon hits a continent, the mass distribution and thus the gravity field change.

To refine measurements of the Earth's gravity field, a technological revolution could emerge thanks to quantum technologies.

It is this ambitious challenge that the CARIOQA space mission proposes to take up, by sending the very first quantum accelerometer into orbit, a key step towards next-generation space gravimetry missions. The project is completing its feasibility phase at the end of this year, with a launch scheduled for 2030.

Space gravity missions

The Earth's gravity field varies by region and fluctuates over time. Its study is essential in various fields such as geophysics (monitoring tectonic movements), oceanography (monitoring sea levels) and navigation (guiding boats and submarines).

Before the advent of space gravimetry, terrestrial measurements of gravity were local and limited in coverage, without the possibility of monitoring globally and continuously the variations of the gravitational field.

As early as the 2000s, the CHAMP space mission made it possible to measure gravity thanks to an orbiting satellite equipped with an accelerometer. Indeed, the position of a satellite in orbit depends on the Earth's gravity field and other types of accelerations, linked for example to the friction of the atmosphere.

Thus, to accurately measure the Earth's gravity field and its variations, the position of the CHAMP satellite is precisely measured using GNSS (GPS technology), which is corrected using an on-board accelerometer measuring the non-gravitational effects experienced by the satellite.

In 2002, the GRACE (Gravity Recovery and Climate Experiment) mission provided the first time maps of the Earth's gravitational field, using two low-Earth orbit satellites, each equipped with an accelerometer. By following the variation in distance between the two satellites and rejecting the non-gravitational accelerations, the fluctuations in the gravitational field are deduced. In 2018, the accuracy of this distance measurement between the two satellites was further improved thanks to a laser interferometer on board the GRACE Follow-On mission.

The restitution of the gravity field on a global scale offers new perspectives in the field of Earth sciences, allowing a better understanding and anticipation of climate change.

Quantum accelerometers: a technological breakthrough

Current space gravity missions rely on the measurement of non-gravitational accelerations using precision accelerometers. These instruments measure the movements of a test mass, for example a metal cylinder weighing about a few hundred grams, to accurately detect the forces involved. Today, this principle can be applied by replacing this mass with a cloud of gaseous atoms in a vacuum, manipulated by lasers, to develop quantum accelerometers.

The contribution of quantum physics lies in the exceptional stability of the measurement over time: like atomic clocks, quantum accelerometers use the internal properties of atoms to offer an accuracy that remains constant, unlike classical accelerometers, whose measurements tend to drift.

In a vacuum chamber, a gas of rubidium atoms is trapped, and the movements of the atoms within the cloud are slowed down using precisely controlled lasers. The reduction in the speed of atoms is associated with a decrease in temperature: these are called clouds of cold atoms. In these extreme conditions, close to absolute zero, atoms reveal a behaviour governed by the laws of quantum physics: matter behaves like a wave. Like waves on the surface of the oceans, waves of matter can be added or cancelled out to create a phenomenon of quantum interference.

This is the principle behind the atomic interferometer technology that will be used for acceleration measurement on board CARIOQA. Laser pulses are used to split, manipulate and recombine cold atoms in free fall, thus creating interferences that contain the information of interest for the measurement: the relative acceleration between the cloud of atoms, free-falling in the chamber, and the laser field that interrogates it.

Although the performance of quantum gravimeters is currently better than that of classical gravimeters under certain conditions (better resolution of low spatial frequencies, for example), it is not always easy to estimate.

CARIOQA: a demonstration mission to bridge the technology gap

Atomic accelerometers have been studied since the 1990s in the laboratory, having demonstrated their ability in fundamental physics tests in airplanes developing inertial navigation, or studying gravity on the slopes of Mount Etna.

The next step? Earth orbit!

The CARIOQA project, which began in 2022, aims to demonstrate the viability of this technology on board a satellite, preparing for future space gravity missions. This ambitious project brings together 17 partners, including the French and German space agencies (CNES and DLR), industrial players such as Airbus, Exail, Teletel and Leonardo, as well as a consortium of European laboratories. The first part of CARIOQA allows the development of a prototype for the final instrument, intended for the flight phases.

It is by combining the expertise of space agencies, industry and laboratories that Europe is at the forefront of this technological revolution, paving the way for a new era of exploration and understanding of the Earth's gravity.

Measuring Earth's gravity from space with the CARIOQA mission. Source: CNES CARIOQA mission.

Célia Pelluet is an Optical Engineer, Physics and Quantum Sensors, with the Centre National d'éEtudes Spatiales (CNES), is republished from The Conversation under a Creative Commons license. Read the original article.