Why Gravity Sensors Go to Sea (and Air)

In this uncertain time of pandemic, many of us dream of traveling. If we are forced to stay home to save lives, this is not the case with GIRAFE, the Embedded Cold Atom Research Interferometric Gravimeter, a sensor developed by ONERA researchers that makes it possible to measure gravity with extreme precision. This instrument connects the measurement campaigns at sea and in the air and testifies to the emergence of quantum technologies, moreover supported by the quantum plan announced in January by the French government. Among the targets for this investment of 1.8 billion euros over 5 years are, in addition to the quantum computer and quantum communication, quantum sensors.

Popularization video about quantum physics and the quantum computer, produced by “l’esprit sorcier” and CEA

The advantage of these quantum sensors for measuring gravity is that we can make a measurement that is not only very accurate, but also absolute and stable over time, as their operation is based on the laws of quantum physics. The use of absolute quantum gravimeters ranges from navigation in the absence of a GPS signal to fundamental physics, via subterranean exploration and seabed mapping, which recently triggered an order from the Direction Générale de Equipping quantum gravimeters for the French Navy’s hydrographic and oceanographic service at ONERA.

What is gravity?

Gravity is the force that is responsible for the attraction of all massive bodies to each other: the Sun and the planets, the Earth and the Moon, the apple that falls from the tree … On our planet has a body that is only exposed to gravity, a speed increasing by about 9.8 meters per second, every second.

However, this value is not constant in space and time! It varies on the one hand depending on the place: the earth is e.g. not exactly spherical, and it varies the acceleration of gravity g from 9.83 meters per second squared at the poles to 9.78 meters per second squared at the equator. The precise distribution of masses – mountains, buildings, soil density – also affects the value of g.

On the other hand, g varies over time, for example with the phenomenon of the tide, which affects the sixth decimal of g, or also with the melting of ice and variations in atmospheric pressure.

GIRAFE 2 quantum gravimeter developed by ONERA boardes an aircraft for in-flight measurements to measure the local value of the gravitational field with precision © Malo Cadoret

In fact, the whole environment affects the local value of g to another decimal level. For certain underground detection applications, it is sometimes even necessary to be able to measure variations as small as one billionth of a gthat is, 8 digits behind the decimal point.

How does a quantum gravimeter work?

At the core of quantum physics is wave-particle duality: all bodies (such as atoms) can behave like waves under certain experimental conditions. It is this undulating behavior of atoms that is utilized in the quantum gravimeter to produce atomic interferometers that are extremely sensitive to the value of g.

The general principle of an interferometer is simple: overlay waves that propagate in an environment to extract either information about those waves or about the environment. We are used to observing waves. For example, when you throw a rock into a lake, you create a circular wave on the surface of the water. By throwing two pebbles into the water, two circular waves propagate and end up overlapping each other. These two waves give rise to a wave of greater amplitude if the bump of one wave falls on the bump of the other, or with lower amplitude, or even zero amplitude if the bump of one falls on the hole of the other . This is the phenomenon of interference. Substance is described as a wave phenomenon, and it is therefore possible to cause atoms to interfere!

Two pebbles fall into the water at the same time. The waves they generate interfere. The interference pattern contains drops and bumps, shown in blue and yellow © Wikimedia CC BY-SA 4.0 (via The Conversation)

In an atomic interferometer, atomic waves are monitored by causing them to interact with laser light. It is a special light whose properties make it possible to manipulate the atoms very finely in very short moments. An atomic wave is thus separated into two waves, which take two different paths and which recombine at a point to cause them to interfere. Just as the sum of a bump and a punching of the circular wave on the surface of water can give rise to the absence of deformation, a superposition of two matter waves at the output of the interferometer can also give an “absence of matter”! The resulting interference signal is very sensitive to small changes in the environment.Since the atoms are subjected to gravity along the two orbits, it is possible to relate the interferences obtained to an accurate gravity measurement.

A cloud of free-falling atoms approaching absolute zero

Photo of the cloud of atoms near the absolute zero point in its light trap © Malo Cadoret

In the GIRAFE quantum gravimeter, the source of matter waves is a millimeter-sized cloud of a few million rubidium atoms, captured and cooled to a temperature on the order of one millionth of a degree above the absolute zero point by the laser beams used. The development of these methods for cooling and capturing atoms by laser gave the 1997 Nobel Prize in Physics to Steven Chu, Claude Cohen-Tannoudji and William D. Phillips. Having such a cloud of cold atoms makes it possible not only to exhibit the wave nature of the atoms, but also to control the direction of the matter waves.

The atomic gas is then released from its light trap and falls into free fall for a fraction of a second under the influence of gravity in a tube in which there is a vacuum. The interferometer is made by causing the cloud of atoms to interact with three flashes of light at three different times during their fall to separate, deflect and then recombine the two waves of matter. An interference signal is then observed that reflects a difference in path between the two waves due to gravity. We can then go back to the value of the latter.

Embedded absolute gravity measurements: a world first for quantum sensors

Until recently, this atomic interferometry technique was limited to laboratory instruments, certainly very powerful (capable of measuring small fluctuations of g, of one in a billion), but too voluminous and fragile for field applications. . GIRAFE quantum gravimeter is the first prototype that demonstrated that the gravitational field could be measured absolutely and accurately under operational conditions, at sea on board a ship and in the air on board an aircraft. .

GIRAFE 2 quantum gravimeter developed at ONERA is mounted on an aircraft for measurements during the flight. The gravimeter is mounted on a gyro-stabilized platform (orange) to maintain the measuring axis of gravity © Malo Cadoret

These measurements have made it possible to produce unprecedented gravity maps with quantum precision at the level of one millionth of the value of g. These results were obtained despite difficult operating conditions. For example, during the sea campaign, the gravimeter was exposed to a swell that could sometimes reach 5 to 6 meters.

ONERA’s quantum gravimeter has also proven itself in the air and is demonstrating airborne gravity measurements with an atomic sensor for the first time. It was during an airborne campaign in Iceland in 2017 that GIRAFE mapped the force of gravity over the volcanic zone of the Vatnajökull Glacier. These maps are of particular interest to geologists for whom these measurements are difficult to perform from the ground due to the relief.

Gravity map in the Meriadzec area (North Atlantic). The red lines correspond to BHO’s trajectory. 1mGal = 0.000001g © Malo Cadoret

In 2020, GIRAFE improved its performance during new naval and airborne campaigns by measuring gravity fluctuations of less than one in a million. These results show that a first generation of cold atom quantum gravimeter is ready to be industrialized for geodesy, geophysics or defense applications.

While many quantum technologies are still under development, the precision enabled by quantum gravimeters demonstrates the potential of this area of ​​activity.

This analysis was written by Malo Cadoret, a teacher-researcher in quantum physics, and Jeanne Bernard, a doctoral student in quantum physics (both at the Conservatoire National des Arts et Métiers).
The original article was published on the site of The conversation.

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