First Measurement of the Atomic Electric Dipole Moment of ${}^{225}\mathrm{Ra}$

The radioactive radium-225 (${}^{225}\mathrm{Ra}$) atom is a favorable case to search for a permanent electric dipole moment. Because of its strong nuclear octupole deformation and large atomic mass, ${}^{225}\mathrm{Ra}$ is particularly sensitive to interactions in the nuclear medium that violate both time-reversal symmetry and parity. We have developed a cold-atom technique to study the spin precession of ${}^{225}\mathrm{Ra}$ atoms held in an optical dipole trap, and demonstrated the principle of this method by completing the first measurement of its atomic electric dipole moment, reaching an upper limit of $|d({}^{225}\mathrm{Ra})|<5.0\times {10}^{-22}e\mathrm{cm}$ (95% confidence).

Figure 1

Diagram of the center of the science chamber. A standing-wave ODT is aligned between a pair of copper electrodes. A collimated beam at 483 nm both optically pumps and probes the atoms, providing absorption images of the atom cloud. A traveling-wave ODT propagating in the direction normal to the page delivers atoms from the 3D MOT to the standing-wave ODT. Inset: CCD image of atoms loaded in the standing-wave ODT; the image is $450\mu \mathrm{m}\times 450\mu \mathrm{m}$.

Figure 2

Sequence of polarization (Pol.) and image pulses used in the EDM measurement. Half-period denotes half of the spin precession period.

Figure 3

Precession curves from the two experimental runs. Panels (a) and (b) are based on the first run, which used a 3 mCi ${}^{225}\mathrm{Ra}$ source, and panels (c) and (d) are based on the second run with 6 mCi. The panels show data with the $E$ field parallel to the $B$ field (red), $E$ field antiparallel to the $B$ field (blue), and $E$ field off (black). Between the two runs the bias $B$ field was deliberately altered, resulting in the two different precession frequencies. An EDM would cause a phase difference between the $E$ field parallel and $E$ field antiparallel fit curves. The global fits for the 3 and 6 mCi runs yield ${\chi }^2/24=1.11$ and ${\chi }^2/28=1.35$, respectively.