Quand les technologies quantiques prennent le large
Les technologies quantiques sont déjà une réalité. Les gravimètres quantiques permettent de faire des mesures avec une précision inégalée – malgré les embruns et la houle.
theconversation.com
The interest of these quantum sensors to measure gravity is that we can make a measurement not only very accurate, but also absolute and stable in time because their operation is based on the laws of quantum physics. The applications of absolute quantum gravimeters range from navigation in the absence of a GPS signal to fundamental physics, including subsoil prospecting and seabed mapping, which recently led to an order from the French Navy's Direction Générale de l'Armement for quantum gravimeters for the French Navy's Hydrographic and Oceanographic Service.
What is gravity?
Gravity is the force responsible for the attraction of all massive bodies between them: the Sun and the planets, the Earth and the Moon, the apple falling from the tree... On our planet, a body subjected only to the force of gravity has a speed that increases by about 9.8 meters per second, every second.
However this value is not constant in space and time! It varies according to location: the Earth is not exactly spherical, for example, and this causes the acceleration of gravity g to vary from 9.83 meters per square second at the poles to 9.78 meters per square second at the equator. The precise distribution of mass - mountains, buildings, soil density - also affects the value of g.
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On the other hand, g varies in time, for example with the phenomenon of tides that influence the sixth decimal place of g, or also with the melting of ice and the variations of atmospheric pressure.
The quantum gravimeter GIRAFE 2 developed at ONERA is mounted on board an aircraft for in-flight measurements, in order to measure the local value of the gravity field with precision. Malo Cadoret, Author provided
In fact, the whole environment affects the local value of g at a different decimal level. For some subsurface sensing applications, it is sometimes even necessary to be able to measure variations as small as a billionth of g, i.e. 8 digits behind the decimal point.
How does a quantum gravimeter work?
At the heart of quantum physics is the wave-particle duality: all bodies (like atoms for example) can behave like waves under certain experimental conditions. It is this wave-like behavior of atoms that is exploited in the quantum gravimeter to realize atomic interferometers extremely sensitive to the value of g.
The general principle of an interferometer is simple: superimpose waves propagating in an environment to extract either information on these waves or on the environment. We are used to observe waves. For example, when we throw a pebble into a lake, we create a circular wave on the surface of the water. By throwing two pebbles into the water, two circular waves propagate and end up superimposed. These two waves give rise to a wave of greater amplitude if the hump of one wave falls on the hump of the other, or of smaller amplitude, or even zero amplitude, if the hump of one falls on the hollow of the other. This is the phenomenon of interference. Matter is described as a wave phenomenon, so it is possible to make atoms interfere!
Two pebbles fall into the water at the same time. The waves they generate interfere. The interference pattern contains hollows and bumps, represented in blue and yellow. Source: Wikipedia
In an atomic interferometer, atomic waves are controlled by making them interact with laser light. This is a particular light whose properties allow to manipulate in a very fine way the atoms during very brief instants. An atomic wave is thus separated into two waves that take two different paths, and that are recombined at a point to make them interfere. Thus, just as the sum of a bump and a trough of the circular wave on the surface of water can give rise to the absence of deformation, a superposition of two waves of matter at the output of an interferometer can also give an "absence of matter"! The interference signal obtained is very sensitive to minute changes in the environment. As the atoms are subjected to the force of gravity along both paths, it is possible to relate the interference obtained to an accurate measurement of gravity.
A cloud of atoms in free fall near absolute zero
Photo of the cloud of atoms near absolute zero in its light trap. Malo Cadoret, Author provided
In the GIRAFE quantum gravimeter, the source of matter waves is a millimeter-sized cloud of a few million rubidium atoms, trapped and cooled to a temperature of the order of a millionth of a degree above absolute zero using laser beams. The development of these laser cooling and trapping methods earned Steven Chu, Claude Cohen-Tannoudji and William D. Phillips the 1997 Nobel Prize in Physics. Having such a cloud of cold atoms allows not only to exhibit the wave nature of atoms, but also to control the direction of the matter waves.
The gas of atoms is then released from its light trap, and falls in free fall for a fraction of a second under the effect of gravity in a tube inside which reigns the vacuum. The interferometer is realized by making the cloud of atoms interact with three flashes of light at three different times during their fall, so as to separate, deflect, and then recombine the two matter waves. We then observe an interference signal reflecting a difference in path between the two waves due to gravity. We can then go back to the value of the latter.
Absolute gravity measurements on board: a world first for quantum sensors
Until recently, this technique of atomic interferometry was limited to laboratory instruments, which were certainly very efficient (capable of measuring minute fluctuations in g, of one in a billion), but too bulky and fragile for field applications. The GIRAFE quantum gravimeter is the first prototype that has demonstrated that the gravity field can be measured absolutely and accurately in operational conditions, at sea on a ship, and in the air on an airplane.
The GIRAFE 2 quantum gravimeter developed at ONERA is mounted on board an aircraft for in-flight measurements. The gravimeter is mounted on a gyrostabilized platform (orange) in order to maintain the measurement axis on gravity. Malo Cadoret, Author provided
These measurements allowed the realization of new maps of gravity with quantum precision, at the level of the millionth of the value of g. These results were obtained despite difficult operational conditions. For example, during the marine campaign, the gravimeter was subjected to swell that could sometimes reach 5 to 6 meters.
ONERA's quantum gravimeter has also proved itself in the air, demonstrating for the first time airborne gravity measurements with an atomic sensor. It was during an airborne campaign in Iceland in 2017 that GIRAFE mapped gravity over the volcanic area of the Vatnajökull glacier. These mappings are of particular interest to geologists, for whom such measurements are difficult to make from the ground because of the terrain.
Gravity map in the Meriadzec area (North Atlantic). The red lines correspond to the trajectory of the BHO. 1mGal=0.000 001 g. Malo Cadoret, Author provided
In 2020, GIRAFE improved its performance during new sea and airborne campaigns by measuring gravity fluctuations lower than one per million. These results demonstrate that a first generation of onboard cold atom quantum gravimeter is ready to be industrialized for geodesy, geophysics or defense applications.
While many quantum technologies are still under development, the accuracy of quantum gravimeters demonstrates the potential of this field.
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