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 have used YbF to make the first measurement of this kind. Together, the large interaction energy and the strong tensor polarizability of the molecule make our experiment essentially free of the systematic errors that currently limit $d_e$ measurements in atoms. Our first result $d_e=(-0.2\mathrm{}\pm 3.2)\times {10}^{-26\mathrm{}}e\mathrm{cm}$ is less sensitive than the best atom measurement but is limited only by counting statistics and demonstrates the power of the method.