NIST researchers demonstrated the first microfabricated atomic vapor-cell magnetometer a decade ago, and have improved the devices continuously since. The chip-scale units have many applications, including the measurement of interplanetary magnetic fields and sensing of extremely faint* magnetic fields, such as those produced by nerve signals in the human heart or brain.
The measurement of magnetic fields around planets is an important method for determining the internal physical structure of the planet. Magnetometers have therefore been sent aboard both deep space and earth-orbiting satellites to monitor the magnetic fields from the Earth or other planets in our solar system and learn more about their structure. Most such space magnetometers have readings that drift over time, even when the magnetic field they measure is not because of instrumental imperfections. Chip-scale atomic magnetometers, which are low-power, lightweight and yet which can measure fields with high accuracy, may therefore provide new opportunities for space science and are well-matched to the new generation of low-cost CubeSats.
Biomagnetic signals are conventionally detected by devices called superconducting quantum interference devices (SQUIDs), which are highly sensitive, but must be cooled to the temperature of liquid helium (4 K). That requires a substantial amount of insulation between the SQUID and a human head. But magnetic fields from brain waves, which are feeble to begin with, fall off exponentially with distance.
Current models measure the effect of magnetic fields on a population of rubidium atoms enclosed in a glass vapor cell about a cubic millimeter in size. A laser raises the temperature to about 150 °C, resulting in a vapor of a hundred trillion atoms. A circularly polarized, “pump” laser beam is directed through the vapor, aligning the spins of all the atoms in the same direction. The same laser is used as a “probe” beam: It shines through the vapor, out of a window in the cell, and into a special detector that measures the polarization of the arriving light.
In the absence of a magnetic field, the atoms’ spins would retain their original orientation, and the polarization of the probe beam would remain unchanged. But when a field is present, it tilts the atoms’ spin orientation slightly. That, in turn, changes the polarity, and hence the amount, of the light entering the detector. The magnitude of that change is a measure of magnetic field strength.
The latest generation of chip-scale magnetometers is now being tested for potential medical use in applications such as studying brain patterns in epilepsy. But the proven sensitivity of the devices also enables them to serve other measurement functions -- for example, as part of a novel NIST design for a chip-scale detector of electric current.
* The Earth’s magnetic field strength is a few tens of microtesla (10-6 T). NIST’s chip-scale magnetometers can measure fields of a few femtotesla (10-15 T), more than a billion times fainter.