A 3D quantum accelerometer that could allow ships to bypass GPS when needed

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An international team of researchers has designed a device based on quantum effects, capable of measuring its position in three dimensions of space with very high precision. This type of device, which proves to be more precise than its non-quantum counterparts, could advantageously replace a GPS system in the event of malfunction or unavailability.

An accelerometer is an instrument which makes it possible to measure the linear non-gravitational acceleration of the object which carries it; it detects changes in movement and thus makes it possible to follow the position of an object. Smartphones (or tablets) all have an accelerometer: it is thanks to it that the screen tilts when you hold the device horizontally, and it is on it that step tracker type applications are based. It consists of a fixed part and a mobile part; in the event of movement, the moving part moves and the variation in electrical capacitance between the two parts makes it possible to determine the direction and extent of the movement.

A quantum accelerometer is based on the measurement of the properties of the quantum waves emitted by the atoms during acceleration – which makes it possible to deduce the displacement, and therefore the position with respect to time. It turns out to be much more sensitive and more precise than conventional accelerometers. The very first commercially viable quantum accelerometer was developed in 2018, by a team at Imperial College London. Their device is based on ultra-cold atoms, cooled using powerful lasers; it does not depend on any external signal.

An acceleration based on ripple patterns

Conventional accelerometers measure the evolution of the speed of an object over time. If we know its starting point, it is therefore possible to deduce its position. But these devices cannot maintain their accuracy over long periods of time without an external reference. Quantum accelerometers are completely autonomous, in addition to being more precise. However, most devices built so far can only measure position in one dimension, along a straight line.

However, it would be much more useful to be able to measure displacements in the different directions (longitudinal, transverse and vertical) – as the accelerometers integrated into smartphones already do. This objective has now been achieved: a team of researchers has managed to build a quantum accelerometer capable of measuring in three dimensions.

(a) Concept and geometry of the “triad of quantum accelerometers” (QuAT). (b) 3D model of the sensor head mounted on a rotation stage that can be tilted around the z and x axes, at angles θz and θx, respectively. © S. Templier et al.

The device, dubbed QuAT (for Quantum Accelerometer Triad) is in the form of a box 40 centimeters long. Inside is a second, smaller glass box containing a cloud of rubidium atoms held at a temperature slightly above absolute zero. At this extreme temperature, atoms behave both like matter and like a wave.

Three lasers point the box of atoms in all directions (length, width and height). By exciting the atoms, the lasers generate collisions causing undulations whose nature depends on the movements of the device. The analysis of the ripple patterns makes it possible to calculate the acceleration in the three directions.

An interest in seismology and mining

The device has proven itself: the researchers fixed it on a “moving” table (which vibrates and turns on itself). They then found that if the acceleration measurements were used to calculate the device’s position in space, after a few hours they would be about 20 meters inaccurate. At the same time, a non-quantum standard version would have been off by one kilometer!

If this device is so precise, it is because the atoms can be controlled with great precision. ” Its ultra-low bias allows the acceleration vector to be tracked over long timescales, resulting in a 50-fold increase in stability (6×10-8 g) compared to our classic accelerometers. We record the acceleration vector at a high data rate (1 kHz), with an absolute magnitude accuracy of less than 10 μg and a pointing accuracy of 4 μrad », Underlines the team in its article.

These results suggest that this device would be very useful on board large vehicles that absorb a lot of vibrations, such as ships. A ship fitted with this quantum accelerometer could stay on course if the GPS signal malfunctions. The QuAT’s pointing accuracy, coupled with its long-term stability, is a promising alternative for high-resolution measurements of tidal inclination, the researchers add. The device could also help improve seismic models and characterize the sources of earthquakes.

Other experts, who were not involved in the design of the device, point out that it could also be very useful for more accurately mapping the Earth’s interior for mining purposes – the gravitational acceleration varying slightly depending on the underground composition, for example above an oil well.

Source: S. Templier et al., ArXiv

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