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United States – 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 prototype device, which is suitable for use in screening for explosives, is almost 1,000 times more sensitive than NIST’s original chip-scale magnetometer introduced in 2004, and is based on a different operating principle. Its performance almost matches the current gold standard for magnetic sensors—termed 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 (mW) IR laser and a rice-grain-sized or 3x2x1mm container, which 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.

To make a complete portable magnetometer, the laser and vapor cell would need to be packaged with miniature optics and a light detector. The vapor cell can be fabricated and assembled on semiconductor wafers using existing techniques for making microelectronics and microelectromechanical systems (MEMS). This design, adapted from a previously developed NIST chip-scale atomic clock, offers the potential for low-cost mass production.

NIST scientists demonstrated that the prototype mini-sensor produces a strong signal that changes rapidly with the strength of a magnetic field from the outside world. The device exhibits a consistent minimum level of electromagnetic static, or “white noise,” which indicates a stable limit on its overall sensitivity. They estimated that a well-designed compact magnetometer with present sensitivity can operate continuously for weeks on a single AA battery. Magnetometers need to be designed with applications in mind; smaller vapor cells require less power but are also less sensitive. Thus, an application for which low power is critical will benefit from a very small magnetometer, whereas a larger magnetometer will be more suitable for a different application requiring high sensitivity. The NIST work evaluates the tradeoffs among size, power and performance in a quantifiable way.