Observation of the electric Breit–Rabi effect

The response of an atom to external electric and magnetic fields can reveal fundamental atomic properties. It has long been verified that, in a static magnetic field, those atomic energy levels with hyperfine interactions shift according to the Breit-Rabi formula, which introduces nonlinear dependence on the magnetic field. On the other hand, the corresponding Breit-Rabi dependence on a static electric field has not been observed before due to a combination of experimental challenges. Here, we precisely measure the Stark shift of the [Formula: see text] transition of [Formula: see text]Yb ([Formula: see text] = 1/2) with cold atoms held by an optical dipole trap in a static electric field up to 120 kV/cm. We observe the electric Breit-Rabi effect displaying high-order ([Formula: see text] and [Formula: see text]) DC Stark shifts. These effects arise from the influence of the strong electric field on hyperfine interactions.