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First Use of Atom Interferometer to Measure Aircraft Acceleration

Atom interferometers have always been too delicate to measure ordinary accelerations. Until now

Matter has the mind-boggling ability to behave like waves as well as particles but it has taken physicists some time to exploit this effect. In recent years, however, various groups around the world have perfected the art of making laser-like beams of atoms and allowing them to interfere to generate interference patterns.

So-called atom interferometers have huge potential. Since the wavelength of atoms and molecules can be smaller than light’s, the interferometers can be much more accurate.

What’s more, unlike light, atoms are influenced by Earth’s gravity, which allows measurements of this force with unprecedented accuracy. This is done either in special underground laboratories where the devices can be isolated from outside influence or in free falling experiments, where the devices can experience a short period of 0g.

But there’s another thing atom interferometers ought to be able to do: measure acceleration. In theory, these devices have the potential to act as accelerometers which are at least as sensitive as modern inertial navigation systems. And they ought to be more robust too, not least because they operate with no (conventional) moving parts.

But there’s a problem. Atom interferometers are so sensitive that the slightest vibration overwhelms the results. And that as ruled them out as useful inertial sensors.

Until now. Today, Remi Geiger at the Laboratoire Charles Fabry in Paris and group of amis, have built the first atom interferometer that can measure the movements of an aircraft. They’ve even tested their device in an Airbus A300, saying that its capable of measuring accelerations 300 times smaller than the aircraft’s motions.

The trick these guys have perfected is a way to strip out the effects of big vibrations that would otherwise overwhelm their measurements. This they do with mechanical accelerometers attached to their kit, which record the large scale movement of the aircraft.

They then simply take these measurements away from the acceleration measured by the atom interferometer. This reveals the much small variations measured by the atom interferometer.

“Our instrument consists of a hybrid sensor which is able to measure large accelerations thanks to the mechanical devices, and able to reach a high resolution thanks to the atom accelerometer,” say Geiger and co.

That could have a significant impact on navigation systems since this kind of accuracy could help correct the errors that creep in to conventional inertial navigations systems.

But the technique could also help in other areas too, such as geodesy and gravimetry, which measure small changes in the Earth’s gravitational field.

It could also make fundamental physics experiments in microgravity easier. One important experiment is to place limits on a principle known as the Universality of Free Fall or the weak equivalence principle. This is the idea that all bodies fall at the same rate, regardless of their internal structure.

Physicists have measured this to one part in 10^13 but some theories predict that more precise measurements should reveal a deviation. In other words, a body’s internal structure should influence the way it falls under gravity, but only by a tiny amount.

Until now, all the experiments to measure this in microgravity have used two different atom interferometers to cancel out the noise. In effect, they measure the difference in the way two different atoms falls. But this kind of relative measurement is not ideal.

The new technique will give physicists a way of measuring the acceleration of a single type of atom directly, a technique that could lead to measurements of one part in 10^15 in space-based experiments.

And since the European Space Agency has selected just such a test for its next generation of experiments known as Cosmic Vision 2020-22, we may just see a version of this in orbit by then.

Ref: Detecting Inertial Effects With Airborne Matter-Wave Interferometry

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