Mini Magnetic Sensor May Have Biomedical, Security Applications

In NIST’s new mini-magnetometer, light from a laser (small gray cylinder at left) passes through a small container (green cube) containing atoms in a gas. The cell and any sample being tested are placed inside a magnetic shield (large grey cylinder). When no sample is present (top) the atoms’ “spins” align themselves with the laser beam, and virtually all the light is transmitted through the cell to the detector (blue cube). In the presence of a sample emitting a magnetic field, such as a bomb or a mouse, the atoms become more disoriented as the field gets stronger, and less light arrives at the detector. By monitoring the signal at the detector, scientists can determine the strength of the magnetic field. Copyright Loel Barr View hi-resolution image

A tiny sensor that can detect magnetic field changes as small as 70 femtoteslas—equivalent to the brain waves of a person daydreaming—has been demonstrated at the National Institute of Standards and Technology (NIST). The sensor could be battery-operated and could reduce the costs of noninvasive biomagnetic measurements such as fetal heart monitoring. The device also may have applications in homeland security screening for explosives.

Described in the November issue of Nature Photonics,* the prototype device is almost 1,000 times more sensitive than NIST’s original chip-scale magnetometer demonstrated in 2004 (“Tiny, Atom-based Detector Senses Weak Magnetic Fields”) and is based on a different operating principle. Its performance puts it within reach of matching the current gold standard for magnetic sensors, so-called superconducting quantum interference devices or SQUIDs. These devices can sense changes in the 3- to 40-femtotesla range but must be cooled to very low (cryogenic) temperatures, making them much larger, power hungry, and more expensive.

The NIST prototype consists of a single low-power (milliwatt) infrared laser and a rice-grain-sized container with dimensions of 3 by 2 by 1 millimeters. The container holds about 100 billion rubidium atoms in gas form. As the laser beam passes through the atomic vapor, scientists measure the transmitted optical power while varying the strength of a magnetic field applied perpendicular to the beam. The amount of laser light absorbed by the atoms varies predictably with the magnetic field, providing a reference scale for measuring the field. The stronger the magnetic field, the more light is absorbed.

“The small size and high performance of this sensor will open doors to applications that we could previously only dream about,” project leader John Kitching says.

The new NIST mini-sensor could reduce the equipment size and costs associated with some noninvasive biomedical tests. (The body’s electrical signals that make the heart contract or brain cells fire also simultaneously generate a magnetic field.) The NIST group and collaborators have used a modified version of the original sensor to detect magnetic signals from a mouse heart. The new sensor is already powerful enough for fetal heart monitoring; with further work, the sensitivity can likely be improved to a level in the 10 femtotesla range, sufficient for additional applications such as measuring brain activity, the designers say.

For additional details and a video interview with project leader John Kitching, see “New NIST Mini-Sensor May Have Biomedical and Security Applications”.

* V. Shah, S. Knappe, P.D.D. Schwindt and J. Kitching. Femtotesla atomic magnetometry with a microfabricated vapor cell. Nature Photonics. 1 Nov. 2007.