A compass that lights the way

Instrument senses magnetic field direction optically

A compass made of light promises to be more sensitive than anything in a boy scout’s wildest dreams. A light beam shot through a blob of rubidium atoms can directly and reliably measure the size and orientation of a magnetic field, a team of physicists reports in the Sept. 13 Physical Review A.

Highly sensitive compasses are needed for oil discovery, earthquake detection and navigation (in the catastrophic event of a GPS failure, that is). Recently, highly sensitive compasses guided engineers as they drilled relief wells in the Gulf of Mexico after the Deepwater Horizon blowout. 

These compasses are very good at finding the size of a magnetic field, but typically have to be tweaked to include a built-in local reference magnetic field so that they can also find the field’s direction. This comparison of the external field to an internal reference allows the compass to reconstruct the magnetic field, but the quality of the data can vary greatly, says study coauthor Alexander Zibrov of Harvard University.

Zibrov and his colleagues wanted to create a compass that could directly pinpoint the direction and the size of a magnetic field. To do that, they relied on a technique that used a magnetically sensitive cloud of atoms and a laser. In their experiment, the team trapped rubidium-87 atoms at 113 degrees Fahrenheit in a domino-sized chip and shined linearly polarized light into the atoms. The light was filtered so that it had the same direction, like the light that makes it through polarized sunglasses.

In the presence of a magnetic field, the atoms’ orientation changed in a particular way, and this change was detectable in the light that came through the atom cloud, the team found. This change in transmission allowed the researchers to find the size and direction of the magnetic field at the same time.

Other compasses based on lasers and atoms exist, Zibrov says, but those rely on circularly polarized light and other ways to excite the atoms, and require fancy mathematical models to reconstruct the magnetic field after the measurement has been taken.

In the experiment, the compass detected magnetic fields with a strength between 0.1 gauss, which is less than the Earth’s magnetic field, and 200 gauss, which is stronger than a small iron magnet. The authors write that the performance can be adjusted with design tweaks such as changes to the temperature and size of the chip.

The new study is “a nice piece of work,” says physicist Szymon Pustelny of Jagiellonian University in Kraków, Poland. Although the physics behind the new compass is largely known, “they succeeded in showing its nice application,” he says.

Compared to earlier models, the new compass is more robust against interfering noise coming from random collisions between the atoms and other sources, says study coauthor Valera Yudin of the Institute of Laser Physics in Novosibirsk, Russia. What’s more, these compasses would be small and would consume very little energy.

But before the optical compass appears strapped to Scouts’ belts, the lab prototype must be tested in the field. “To talk about real applications,” Pustelny cautions, “a lot of work needs to be done.”

Laura Sanders

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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