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 operate on a single AA battery and reduce the costs of non-invasive biomagnetic measurements such as fetal heart monitoring.

The NIST prototype consists of a low-power infrared laser and a rice-grain-sized container that 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 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 sensor may have applications in MRI or in airport screening for explosives based on detection of nuclear quadrupole resonance in nitrogen compounds. The sensitivity of the sensor can likely be improved to the 10 femtotesla range, sufficient for measuring brain activity. With further improvements, the NIST sensor also might be used to pinpoint tumors or determine function of various parts of the brain.

Read more here .