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 AE 0.2 AE 0.4 10 -9 m 3 =kg for barium and -7.0 AE 0.3 AE 0.7 10 -9 m 3 =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.