Magnetometer Has Subfemtotesla Sensitivity

Paula M. Powell

Researchers at Princeton University in New Jersey, working with colleagues at the University of Washington in Seattle, have developed an all-optical atomic magnetometer more sensitive than superconducting quantum interference devices where sensitivity typically ranges from 0.9 to 1.4 fT Hz^-1/2.

Michael V. Romalis at Princeton said the optical system demonstrates magnetic field sensitivity of 0.54 fT Hz^-1/2 with a measurement volume of 0.3 cm³. It can reportedly detect a magnetic field as small as 0.5 fT with 2-mm resolution. He believes that this sensitivity and spatial resolution, combined with the fact that cryogenic cooling is not needed, as in the superconducting devices, could facilitate its use in biomagnetic imaging of the brain and other organs, as well as applications to test fundamental symmetries of nature.

An all-optical atomic magnetometer developed at Princeton offers sensitivity and resolution competitive with superconducting quantum interference devices, but without requiring cryogenic cooling.

Atomic magnetometers measure the Larmor precession of spin-polarized atoms in a magnetic field. All-optical devices do this in three basic steps, which Romalis said could occur simultaneously. In simplest terms, atoms within a test cell are polarized by the pumping laser beam. Under an applied magnetic field (the one to be measured), they change their spin direction. This is One common problem with all-optical atomic magnetometers has been that of spin-exchange collisions, whereby collisions between atoms cause their depolarization. In the spin-exchange relaxation-free magnetometer developed at Princeton, researchers resolved this issue by using a very high density of atoms so that the atoms do not have enough time to depolarize in the short time between collisions. The researchers also add a dense helium buffer gas to the vapor cell. This slows any diffusion of the alkali atoms and, combined with spatially resolved optical detection, allows measurement of localized magnetic fields.

Romalis said the optical atomic magnetometer is still in an early development stage, so there are a variety of possible ways to tweak its efficiency. The scientists are working on setting up a system to image magnetic fields generated by the brain that will use a two-dimensional photodiode array with 256 elements. Because the thermal magnetic noise produced by the brain is on the order of 0.1 fT Hz^-1/2, further noise reduction of the optical magnetometer is still necessary.

If the researchers succeed in doing this, they believe the device could enable noninvasive studies of individual cortical modules in the brain.