Espace Presse – Quantum sensors, measuring instruments with unmatched precision

What is a quantum sensor?

A sensor is by definition a tool that measures one or more physical quantitiesexplains Preden Roulleau, research director at CEA-Iramis.
They are said to be “quantum” when they use quantum properties to function, such as

the principle of superposition, quantum interference, or entanglement of their quantum states These sensors may contain different types of particles (photons or electrons) or atoms that physicists are able to place in a given quantum state. The latter have the special feature of being extremely sensitive: the slightest disturbance in the surrounding environment can change it. . From where
the unmatched accuracy and sensitivity of these sensors.

Since when has this idea of ​​a quantum sensor been developed?

The origins of quantum sensors are quite old: yes,
from the 1950sthat
atomure has demonstrated the interest in using a quantified and perfectly reproducible system for the realization of frequency standards – a property at the origin of the redefinition of the other from a transition of the cesium atom. Today, atomic clocks make it possible to
realize and keep in sync all the time scales realized in the world.


NIST-F1 Cesium Atom Fountain Atomic Clock. This watch is the primary time and frequency standard in the United States, with an uncertainty of 5.10-16 (from 2005). © Public domain

What are the uses of these ultra-sensitive sensors?

The development of atomic clocks has resulted in many applications in the fields
transportcertification of
transactions and of course
satellite positioning systems.

In addition, new types of sensors are also derived from these first clocks:
cold atomic sensors or interferometers. Their applications are several: gravimeters (measuring gravity) can be used for
predict earthquakes or explore underground. Some are already marketed by the French company Muquans (the first and only company in the world to do so), which manufactures gravimeters that are used to probe the earth and detect, for example, the formation of a pocket of magma in a volcano. That
accelerometer (which measures acceleration) can be used for tuning
autonomous navigation systems.

Other sensors that work thanks to a defect contained in
a diamond crystal (NV Center* diamond)are they capable of measure magnetic fields with exceptional spatial resolution, as one can approach extremely close to a source of magnetic field, to a few nanometers “, Emphasizes Patrice Bertet, researcher at CEA-Iramis. They are especially used for
characterize the magnetic properties of certain materials. These sensors also have the huge advantage that they can be used under ambient pressure and temperature conditions, which facilitates and expands their use.

Cold atom gyrometer

Cold atom gyroscope © A. Landdragin / SYRTE / CNRS

If they are currently the most developed, interferometers and NV center diamond sensors are not the only types of existing quantum sensors. ”
The quantum properties of light can also be used to improve the performance of an interferometerexplains Patrice Bertet.
“Compressed” quantum states have been implemented for two years in Lirgo and Virgo, the two gravitational wave interferometers A true achievement because these sensors have made it possible

discovered for the first time, in 2015, waves that have so far been undetectable
as a result of the violent fusion of two black holes located 1.3 billion light-years from Earth.

There is also
circuit-based sensors

superconductors, developed especially by Patrice Bertet and his team. ”
With these sensors, we perform ultra-sensitive detection of magnetic resonance, with a sensitivity that extends from a few tens of spins, and which we hope to be able to push to single spin in the near future. Finally, some of the sensors developed at CEA, as many bricks as one day, by responding to certain challenges related to quantum information processing, could contribute to the design of a quantum computer.

Mag4Health: quantum sensors in the service of cutting-edge medical imaging

Created quite recently in 2021, the Mag4Health start-up from CEA-Leti has developed, using quantum sensors developed at CEA-Leti, a new generation of magnetoencephalographs (MEG) that operate at room temperature. Magnetoencephalography or MEG is a neuroimaging method that consists of recording, in real time, the electromagnetic activity in the brain, thus improving the diagnosis of certain neuronal diseases. Today, there are around one hundred MEG units in hospitals due to their cost, up to 2.5 million euros per year. unit, but also because of their weight, more than 5 tons. Mag4Health intends to revolutionize access to the MEG with the introduction of a non-invasive device, five times cheaper and weighing approximately 300 kg. This democratization should make it possible to bring forward new clinical applications, such as rehabilitation after a stroke or early diagnosis of Alzheimer’s disease. Since 2021, the startup has implemented its prototypes based on quantum sensors in a CHU environment.

How do these quantum sensors work?

In the case of NV-centered diamonds,
a defect contained in synthetic diamonds is used A nitrogen atom and a free space that replaces a carbon atom. This defect, which is exposed to a laser, emits a light whose intensity depends on the spin state of the electrons surrounding it – a spin state is a state of rotation of the pure quantum electron and sensitive to the magnetic field. Thus, in the presence of an external magnetic field, these spin states will change and modify the intensity of the light emitted by the defect. This is how we can track the value of the magnetic field very accurately.

Interferometers work a little differently:
laser cooled atoms are used at very low temperatures (some micro-wines from the absolute zero point, ie -273.15 ° C), which has the effect of slowing them down (their speed approaches zero). This low velocity of the cooled atoms makes it possible to measure extremely accurately the forces to which they are subjected. ”
The resulting interferometers are very sensitive to rotation, gravity and acceleration Says Patrice Bertet.

Diagram of an atomic interferometer using free-falling atoms manipulated by laser beam splitters.

Diagram of an atomic interferometer using free-falling atoms manipulated by laser beam splitters. © A. Landdragin / SYRTE / CNRS

What are the R&D challenges around quantum sensors?

The very
high sensitivity of quantum sensors goes hand in hand with a very high fragility compared to sources of decoherence, which explains why the use of most of these sensors still exists today.
limited to very specific markets. The R & D issues confirmed in the national strategy for quantum technologies will consist of
consolidate France’s position on cold nuclear sensors and to develop the next generation of inertial sensors, magnetic sensors and atomic clocks.


Graphene sheet, element of the quantum sensor that made it possible for the first time to observe the electric dipole associated with electronic spin waves (2021) © P. Roulleau – P. Jacques / CEA

Especially for the quantum sensors developed at CEA, the research lines consist on the one hand in exploring systems that would make it possible to overcome the limitation of very low temperatures and on the other hand in developing the production of circuits on a larger scale than laboratory scale . Steps are being taken in this direction, with, for example, the realization of a silicon Qubit at CEA-Leti, in Grenoble in 2016 – the very well-mastered silicon technology can enable easier realization of circuits and easy upscaling. The research continues in connection with the ERC Synergy QuCube project, led by CEA and CNRS researchers, and the H2020 QLSI project led by CEA-Leti. The national quantum plan offers
QuantAlps great opportunities in the field with very well researched projects
as part of PEPR which starts in a little while.

* NV for nitrogen free position

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