Nonclassical Excitation for Atoms in a Squeezed Vacuum

The two-photon transition ${6S}_{1/2}\to {6D}_{5/2}$ is investigated for trapped atomic cesium excited by squeezed light. The rate $R$ of two-photon excitation versus intensity $I$ is observed to be consistent with the functional form $R={\beta }_{1}I+{\beta }_2{I}^2$, extending into a region with slope 1.3. This departure from the quadratic form for classical light sources is due to the fundamental alteration of atomic radiative processes by the nonclassical field.