19 Mar 2015

Measuring Gravity With Atoms

From Our Changing World, 9:06 pm on 19 March 2015

by Ruth Beran

Cheap lasers such as those found in CD players and a thorough understanding of how atoms and light work could be a game changer when it comes to measuring gravity.

A photo of Mikkel Andersen with the gravimeter on the table

Mikkel Andersen with the gravimeter on the table Photo: RNZ / Ruth Beran

Precise measurements of gravity allow geoscientists to predict volcanic activity and measure the earth’s composition without drilling – but while current gravimeters use springs or light, University of Otago atomic physicist Mikkel Andersen says his revolutionary machine will use atoms.

“There is no atomic gravimeter on the market yet,” says Mikkel, although there are other research groups working on similar projects.

He has been working with his team on the project for more than four years now and it may be a few more before a commercial device is ready. Mikkel and his team have worked on other groundbreaking research which involved individually trapping atoms, you can listen to that story here.

Mikkel's gravimeter uses atom interferometry. Interferometry may be familiar from experiments using light.

Diagram of the double-slit experiment showing interferometry

Diagram of the double-slit experiment showing interferometry Photo: by Lacatosias and Stannered, CC-BY-SA-3.0

When two waves overlap, and light is a wave, the waves interfere and cause dark and bright parallel lines.

“What works for light also works for atoms,” says Mikkel who is a researcher at the Dodd-Walls Centre for Photonic and Quantum Technologies. “According to quantum mechanics all things are waves at the fundamental limit. And so are atoms, that we usually consider as small balls.”

This means that atoms can be used, like light, to interfere with each other.

To make atoms interfere, they first have to be laser cooled. Then, using carefully designed laser pulses, the atoms are sent along different paths. Combining them again, the atoms interfere with each other causing an interference pattern similar to that seen when light waves interfere.

What results is a series of very, very narrow lines, similar to what you’d see on a ruler. If the cloud of atoms is allowed to fall under the force of gravity, the parallel lines in that cloud fall too. Working out how far the lines have fallen indicates how far the cloud of atoms has fallen, similar to watching a ruler being dropped vertically. A very accurate measurement of gravity can then be made by using a laser to measure how far the cloud drops in a given amount of time.

Moving the gravimeter to different locations and comparing the small differences in the drop of atoms allows measurements to be made of the variances in local gravity at these different locations.

Two important things enable this instrument to work. Firstly, the time it takes for the atoms to drop can be measured extremely accurately using atomic clocks. Secondly, it’s relatively easy now to make lasers which have an extremely accurate wavelength.

Mikkel and his team specifically use rubidium atoms because the lasers in CD players are close to the main resonance of rubidium.

“They’re very very cheap,” he says. “So we buy 100 for $3.”

The gravimeter at the University of Otago is currently spread over a large two metre long table, but the idea is to create a portable device measuring about 1 metre by 0.5 metre. To do so, the system will need a smaller vacuum chamber, and optical fibre to replace the field of optics which currently directs the laser.

The device will have a price tag of over $500,000 once commercialised and in terms of accuracy, Mikkel says the team is “hoping to at least parallel the accuracy of the best devices presently on the market.”

You can listen to a previous story on using a gravimeter in Antarctica here.