Measurement of the electron electric dipole moment using YbF molecules

The most sensitive measurements of the electron electric dipole moment $d_{e}$ have previously been made using heavy atoms. Heavy polar molecules offer a greater sensitivity to $d_{e}$ because the interaction energy to be measured is typically 10$^{3}$ times larger than in a heavy atom. We report the first measurement of this kind, for which we have used the molecule YbF. Together, the cylindrical symmetry of the molecule and the large interaction energy make our experiment essentially free of the systematic errors that limit $d_{e}$ measurements in atoms. Our first result $d_e = (- 0.2\pm 3.2)\times 10^{- 26} {\rm e} {\rm cm}$ is limited only by counting statistics and demonstrates the power of the method.