Quand les technologies quantiques prennent le large
Now we will embark on ultra-precise quantum sensors — here a measurement campaign of the sector in the open sea, on the hydrographic and oceanographical building Beautemps-Beaupré.
Malo Cadoret, author supplied
At this turbulent moment of pandemic, many of us are dreaming of flying.
If we are compelled to stay with us to save lives, this is not the case of the GIRAFE Gravimeter Interferometric of Cold Atoms Science Embarquable, a sensor built by ONERA researchers that can calculate gravity with extreme precision.
This instrument ties up measuring campaigns at sea and in the air and represents the growth of quantum science, sponsored by the quantum initiative announced in January by the French Government.
Among the goals of this project of 1.8 billion euros over 5 years are, besides quantum computer and quantum communication, the quantum sensors.
The interest of these quantum sensors to measure gravity is that we can take a measurement that is not only very precise, but also absolute and constant over time, since its operation depends on the laws of quantum mechanics.
The uses of absolute quantum engravings range from navigation in the absence of GPS signals to the fundamental mechanics, to underfloor discovery and underwater depth mapping, recently spurred a commission by the Directorate General for Armament of quantum engravings for the Hydrographic and Oceanographic Operation of the National Sea with the ON.
What does gravity mean?
Gravity is the force which causes all the massive bodies to attract each other, the Sun and the planets, the Earth and the Moon, the apple falling from the tree... On our planet a body subject to gravity alone has a rate that rises by approximately 9.8 meters per second, every second.
However this value is not continuous in space and time!
It varies on the one side by location: Earth is not exactly spherical for example, and this varies the acceleration of gravity g from 9.83 meter per second square to poles from 9.78 meters per second square to the equator.
The exact distribution of masses – cliffs, buildings and soil density – also influences the value of g.
Furthermore, g varies over time, for example with the occurrence of tides affecting the sixth decimal plate of g, or even with the loss of ice and changes of air pressure.
The GIRAFE 2 quantum gravimeter developed at ONERA is used for aircraft measurements in flight, to precisely calculate the local gravity field value.
Malo Cadoret, author supplied
In truth, the entire world influences the local value of g at a different decimal level.
For certain subsurface detection applications, it is often also important to be able to calculate differences as low as the billionth of g, i.e. 8 digits behind the comma.
How does a quantum gravimeter work?
At the heart of quantum mechanics is the wave-particle duality: all bodies (such as atoms for example) will behave as waves under certain experimental conditions.
It is the wavelike behavior of the atoms that is manipulated in the quantum gravimeter to realize atomic interferometers highly sensitive to the value of g.
The basic theory of an interferometer is simple: superimpose waves spreading in an atmosphere to extracted either information about these waves or about the environment.
We are accustomed to observing waves.
For example, when a stone is tossed into a lake, a circular wave is produced on the surface of the water.
Throwing two stones into the sea, two circular waves scatter and end up superimposing.
All of these waves give rise to a wave of greater amplitude, if the hump of one wave falls on the hump of the next, or of lower range, or even of zero amplitude, if the hump of the one falls on the hollow of the other.
This is the phenomenon of intrusion.
Matter being represented as an undulatory phenomenon, it is therefore possible to allow atoms to interfere!
Two stones fell into the stream at the same time.
The waves they produce intervene.
The interruption pattern includes hollows and bumps, depicted in blue and yellow.
In an atomic interferometer, atomic waves are controlled by interfering with laser light.
This is a particular light whose properties allow to control atoms in a very fine way for very brief moments.
An atomic wave is therefore separated into two waves which follow two opposite ways, and which are recombined in one stage to allow them interfere.
Thus, just as the sum of a hump and a hollow of the spherical wave on the surface of the water will give rise to no deformation, an overlay of two material waves at interferometric output can also give rise to a « absence of matter »!
The interference signal received is very susceptible to small variations in the environment.
Since atoms are exposed to the force of gravity along the two directions, it is possible to relate the interference obtained to an exact measure of gravity.
A cloud of free floating atoms near to absolute zero.
In the quantum giraffe, the source of waves of material is a vapor of millimetric size of a few million rubidium atoms trapped and cooled to a temperature of the order of one millionth of a degree above absolute zero by means of laser beams.
The development of these methods of cooling and atom trapping by laser has received the 1997 Nobel Prize of Physics in Steven Chu, Claude Cohen-Tannoudji and William D. Phillips.
Having such a cloud of cold atoms not only causes the wavelike form of atoms to be exhaled, it also influences the trajectory of material waves.
Atom gas is freed from its light prison, which goes into a free fall for a fraction of a second under the influence of gravity in a tunnel within of which the vacuum reigns.
The interferometer is conducted by allowing the cloud of atoms to interact with three pulses of light at various intervals during their collapse to detach, reflect and then recombine the two waves of matter.
A pulse of interference is then detected indicating a difference in the direction between the two waves due to gravity.
We should then go back to the value of the above.
Absolute gravity tests embedded: a world first for quantum sensors.
Until recently, the technique of atomic interferometry was limited to laboratory devices, while high performance instruments (capable of calculating minor variations of g of one billion per billion), but too voluminous and brittle for field-scale applications.
The GIRAFE quantum gravimeter is the first prototype to show that the heaviness field can be measured absolute and reliably under operating conditions, at sea on board a ship, and in air on board an aircraft.
The quantum gravimeter GIRAFE 2 built at UNERA is on board an airplane for calculation in flight.
The gravimeter is mounted on a gyrostabilized (orange) base in order to preserve the gravity axis.
Malo Cadoret, author supplied
These steps helped to realize new maps of gravity to the quantum precision, at the level of one millionth of the magnitude of g.
These findings were obtained under challenging operating circumstances.
For eg, during the marine campaign, the gravimeter was exposed to a swell up to 5 to 6 metres.
The quantum gravimeter of ONERA has also proven itself in the air, demonstrating for the first time airborne gravitation measurements using an atomic sensor.
It was during an airborne campaign in Iceland in 2017 that GIRAFE mapped the gravity over the volcanic region of the Vatnajökull glacier.
This maps are of special importance to geologists, for whom these calculations are difficult to do from land because of the landscape.
In 2020, GIRAFE enhanced its efficiency during new maritime and airborne campaigns by calculating variations of less than one per million.
These results show that a first generation of embedded cold-atom quantum gravimeters are ready to be industrialized for geodesy, geophysics, or defense applications.
Although several quantum technologies are still under progress, the precision allowed by quantum gravimeters indicates the promise of this area of activity.
The original version of this article was published on La Dialogue, a non-profit news site devoted to the exchange of ideas between scholarly experts and the general public.
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