Subpicotesla Atomic Magnetometry

John Kitching [Photo courtesy: NIST, Boulder]

A team of physicists led by John Kitching of National Institute of Standards and Technology (NIST) has reported the development of a tiny sensor that can detect magnetic field changes as small as 70 femtoteslas—equivalent to the brain waves of a person daydreaming. [A femtotesla is one quadrillionth (or a millionth of a billionth) of a tesla, the unit that defines the strength of a magnetic field. For comparison, the Earth’s magnetic field is measured in microteslas, and a magnetic resonance imaging (MRI) system operates at several teslas].

This compact magnetometer is based on the so-called SERF (spin-exchange relaxation free) principle, which was used by a group at Princeton University in 2003 to enhance the sensitivity of larger, tabletop-sized magnetometers to outperform SQUIDs. The NIST group developed novel approaches and technologies to adapt the SERF concept for tiny and practical devices. The sensor could be battery-operated and could reduce the costs of non-invasive biomagnetic measurements such as fetal heart monitoring.

At zero magnetic field, the atoms’ electron “spins” (which can be roughly visualized as tiny magnetic arrows pointing through the electrons) all point in the same direction as the laser beam, and the atoms absorb virtually no light. As the magnetic field is increased, the electrons jump to higher-energy levels and their spins go out of sync, causing the atoms to absorb some of the light.

Ordinarily, the atoms would collide randomly and the electron spins would change direction in between collisions, degrading the sensor signal. The SERF approach maintains consistent spins for a relatively long time (10 milliseconds) by combining a low magnetic field with high temperatures of 150 degrees C (302 degrees F). The spins have little time to adjust in between the collisions. Like cars on a highway, the atoms behave more consistently when conditions are crowded.

Image credit and copyright: Loel Barr

In NIST’s new mini-magnetometer, light from a single low-power (milliwatt) infrared laser (small gray cylinder at left) passes through a small container (green cube; dimensions: 3 by 2 by 1 millimeters) containing about 100 billion rubidium atoms in gas form. The cell and any sample being tested are placed inside a magnetic shield (large grey cylinder). When no sample is present, as in the top image, the atoms’ “spins” (depicted inside red circle) align themselves with the laser beam, and the 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 (middle and bottom images), the atoms become more disoriented as the field gets stronger, and less light arrives at the detector. A mouse heart produces a stronger signal than many explosive compounds found, for example, in bombs, if both are located the same distance from the sensor; at greater distances, the detected field is reduced. By monitoring the signal at the detector, scientists can determine the strength of the magnetic field.

“This result suggests that millimeter-scale, low-power, inexpensive, femtotesla magnetometers are feasible … Such an instrument would greatly expand the range of applications in which atomic magnetometers could be used,” the paper states. The new NIST mini-sensor could reduce the equipment size and costs associated with some non-invasive biomedical tests. The device also may have applications such as homeland security screening for explosives.

The device could be used in a heart monitoring technique known as magnetocardiography (MCG), which is sensitive enough to measure fields of few picoteslas emitted by the fetal heart from small currents in heart muscle cells, providing complementary and perhaps better information than an electrocardiogram. With further improvements, the NIST sensor also might be used in magnetoencephalography (MEG), which measures the magnetic fields produced by electrical activity in the brain, helping to pinpoint tumors or determine function of various parts of the brain.

Reference:
"Femtotesla Atomic Magnetometry with a Microfabricated Vapor Cell"
Vishal Shah, Svenja Knappe, Peter D.D. Schwindt, and John Kitching,
Nature Photonics, v1, p649 - 652 (1 November 2007) Abstract

[We thank Media Relations, NIST for materials used in this posting]