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 $6s^2\ ^1S_0\ \leftrightarrow\ 6s6p\ ^1P_1$ transition of $^{171}$Yb ($I$ = 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 ($E^4$ and $E^6$) DC Stark shifts. These effects arise from the influence of the strong electric field on hyperfine interactions.