High sensitivity detection of magnetic fields is a fundamental capability that enables a large number of diverse applications, from locating unexploded ordnance and underground structures, to the detection of biomagnetic fields associated with heart and brain activity. Superconducting quantum interference device (SQUID) magnetometers have long been the only practical option for high sensitivity magnetic field detection. However, they require bulky and expensive cryogenic cooling. Recent advances in atomic magnetometry have allowed world-record sensitivity (less than one femtotesla) to be achieved without the use of cryogenic cooling, thus drastically reducing the size and operating expense of a magnetometer. Our research group is exploring novel designs for atomic magnetometers while developing devices for specific applications. The main effort of our current research is to develop magnetometers for magnetoencephalography (MEG), the measurement of the magnetic fields produced by the human brain.
Atomic Magnetometer Background: The figures show the fundamental elements of a highly sensitive atomic magnetometer. The magnetic field is sensed by measuring the interaction between a magnetic field and the electronic spins of an atomic vapor contained in a glass cell. In order to achieve the high sensitivities, the atom density and magnetic properties of the gas must be maximized. High atom density is achieved by heating a glass cell containing a droplet of alkali metal such as potassium, rubidium, or cesium. The low melting temperature of the alkalis results in a high vapor pressure at temperatures of a hundred degrees Celsius and above. The magnetic response of the cloud of atoms is maximized through a process called optical pumping. In this process a circularly polarized “pump” laser beam passes through the cloud of atoms and aligns nearly all of the electron spins. This results in a greatly enhanced response because the signals from all the atoms add coherently. The collective response of the atoms to the magnetic field results in a change in the index of refraction of the atomic gas which is measured by detecting the optical rotation of a linearly polarized “probe” laser beam.
The highest possible sensitivity is achieved when the magnetometer is operated at a low magnetic field. In this so called spin exchange relaxation-free (SERF) regime, atom-atom spin exchange collisions in the atomic vapor are no longer a source of decoherence, and thus the atom density can be dramatically increased to improve the sensitivity of the magnetometer.