Optical pumping technique for measuring small nuclear quadrupole shifts inS01atoms and testing spatial isotropy

Using ${}_{}{}^{201}{}_{}{}^{}\mathrm{Hg}$ (I=(3/2) atoms, we have demonstrated a new technique for studying the static quadrupole interactions of nuclei in ${}_{}{}^1{}_{}{}^{}\mathrm{S}_0^{}{}_{}{}^{}$ atoms. The nuclear-spin precession frequency is made sensitive to quadrupole energy shifts by inducing, through optical pumping, both dipole and quadrupole spin polarization in a vapor of ${}_{}{}^{201}{}_{}{}^{}\mathrm{Hg}$ atoms. The sensitivity predicted from measurements of the polarization agrees with the calibrated value found by inducing a known quadrupole light shift. By rotating the mercury-vapor cell, quadrupole interactions with the cell walls were observed with the expected cos2φ azimuthal variation. In an application of this technique, a search for possible dependence of the ${}_{}{}^{201}{}_{}{}^{}\mathrm{Hg}$ spin precession frequency on the orientation of the precession axis in space has yielded the null result Δf<5×${10}^{\mathrm{-}7}$ Hz, which reduces the previous limits on spatial anisotropy by over three orders of magnitude and places stringent new bounds on violations of Lorentz invariance.