Magnetic-field detectors, or magnetometers, are currently used for a variety of applications including medical imaging, contraband detection, materials imaging and even geological exploration. Current devices however are somewhat limited, expensive and inefficient.

Researchers at MIT think that they have devised a more sensitive device that could be used for a broader range of applications that is so energy efficient a hand-held, battery powered version is possible.

Magnetometers rely on synthetic diamonds with nitrogen vacancies. Pure diamonds are made of a latticework of carbon atoms which do not interact with magnetic fields. Nitrogen vacancies (NVs) occur when there is a missing atom in the latticework that is adjacent to a nitrogen atom.

When light particles, photons, strike electrons in the NV it boots it to a higher energy state. The presence of a magnetic field can alter the spin of an electron and the stronger the magnetic field, the greater the variation in the spin. Scientists are able to measure the change in these electrons which allows the magnetometer to perform its various functions.

Taking accurate measurements, however, requires capturing as many photons as possible when they leave the diamond. To accomplish this some devices rely on gas-filled chambers and others work only in narrow frequency bands which limits their accuracy and usefulness.

“In the past, only a small fraction of the pump light was used to excite a small fraction of the NVs. We make use of almost all the pump light to measure almost all of the NVs,” says Dirk Englund, the Jamieson Career Development Assistant Professor in Electrical Engineering and Computer Science and one of the designers of the new device in a statement.

By making a few changes to the device, the angle at which the light enters the diamond and the cut of the stone, there researchers were able to dramatically increase the efficiency of the device and improve the accuracy and possible applications.

Now, when the light enters it bounces around “like a tireless cue ball ricocheting around a pool table”, reaching every part of the crystal before it is spent.

“You can get close to a meter in path length. It’s as if you had a meter-long diamond sensor wrapped into a few millimeters,” says Englund.

According to the researchers, the geometry of the NVs allows a crystal at one end to capture 20 percent of the escaping photons and re-focus them onto a light detector. The result is a 1,000 fold improvement in efficiency.

“We gain an enormous advantage by adding this prism facet to the corner of the diamond and coupling the laser into the side. All of the light that we put into the diamond can be absorbed and is useful,” said Hannah Clevenson, a graduate student in electrical engineering and first author of a paper on the advance in Nature Physics.