Quantum-Assisted Measurement of Atomic Diamagnetism

We report the first measurement of ground-state diamagnetism of isolated neutral atoms in an atomic beam. We realize this measurement using magnetic deflection of fringes in a long-baseline matter-wave interferometer. The observed diamagnetic susceptibilities of −5.8±0.2±0.4×10⁻⁹  m3/kg for barium and −7.0±0.3±0.7×10⁻⁹  m3/kg for strontium are in good agreement with the theoretical values and correspond to a measured force on the order of 10⁻²⁶  N. The high force sensitivity also allows us to observe the isotope dependence of the interference visibility due to the nuclear permanent magnetic moment, thereby demonstrating a new method for neutral isotope selection. The universality of the technique allows the magnetism of a wide range of atoms and molecules to be studied in the gas phase.