Optical Magnetometer Array for Fetal Magnetocardiography
  We describe an array of spin-exchange relaxation free optical magnetometers designed for detection of fetal magnetocardiography (fMCG) signals. The individual magnetometers are configured with a small volume with intense optical pumping, surrounded by a large pump-free region. Spin-polarized atoms that diffuse out of the optical pumping region precess in the ambient magnetic field and are detected by a probe laser. Four such magnetometers, at the corners of a 7 cm square, are configured for gradiometry by feeding back the output of one magnetometer to a field coil to null uniform magnetic field noise at frequencies up to 200 Hz. Using this array, we present the first measurements of fMCG signals using an atomic magnetometer. 
Robert Wyllie, Matthew Kauer, Ronald T. Wakai, and Thad G. Walker
physics.atom-ph