Measuring magnetism is important in applications ranging from the detection of landmines to the diagnosis of health conditions, such as epilepsy and cardiac arrhythmias, caused by disruptions in electrical signals. Over the last several years, physics professor Michael Romalis and his team have developed some of the world’s most sensitive magnetometers.
     These magnetometers are so sensitive because they quantify magnetic fields by measuring a quantum mechanical property known as spin, which is present in electrons and other subatomic particles. “Electrons respond to the presence of a small magnetic field by changing the orientation of their spin,” Romalis said. “Our magnetometers measure this orientation.”
     In the detectors developed by the Romalis group, laser light is used to line up the spins of the electrons in a cloud of potassium gas. A second laser probes the alignment of electrons and acts as a detector to measure how the direction of spin changes in response to a magnetic field. The process includes a trick for preventing the spins from relaxing back into random directions, so it is called the spin-exchange relaxation free (SERF) regime.
     Romalis’ group was the first to use these magnetometers to detect brain magnetic fields, which can help diagnose disorders and contribute to basic research about how the brain works. More recently, the researchers have demonstrated the detection of explosives and of the Earth’s magnetic field. This latest application involves a portable magnetometer that can be taken into the field. In addition, the magnetometers are being developed to explore fundamental physics, said Romalis, who has been funded by the National Science Foundation, the National Institutes of Health and other federal agencies.
     Today, these magnetometers are becoming commercially available through a Princeton startup, Twinleaf LLC, which was started by Romalis’ former graduate student Thomas Kornack, and Elizabeth Foley. Both earned their doctoral degrees from Princeton in 2005.