Trapping radium atoms

Some paths toward unifying nature’s fundamental forces invoke sources of CP violation. Those same sources may shift the charge distribution of nuclei to give them a tiny but potentially measurable electric dipole moment (see Physics Today, June 2003, page 33 ). Physicists are already looking for nuclear EDMs in trapped atoms. Heavy atoms work best. Their relativistic outer electrons incompletely screen the nuclear charge from an external electric field, exposing the putative EDM. If the nucleus has spin, the EDM will respond to parallel and antiparallel electric fields in a detectably different way. Because of its high atomic number and nuclear spin, the radium-225 nucleus is already a promising EDM candidate. Its octopole shape, which boosts any EDM 100-fold, makes it an even better one. Unfortunately, radium is hard to cool and trap because it lacks a strong atomic transition for removing kinetic energy. Even so, Jeffrey Guest of Argonne National Laboratory and his colleagues have recently trapped ²²⁵Ra (and ²²⁶Ra). Their scheme works as follows. Laser light at 714 nm drives the ¹ S ₀ → ³ P ₁ transition. Instead of obligingly dropping back to ¹ S ₀ for another round of cooling, the atoms occasionally de-excite to the ³ D ₁ state. That detour is not disastrous. Light from a second laser, at 1429 nm, drives the ³ D ₁ → ¹ P ₁ transition. From ¹ P ₁, atoms promptly de-excite to the ¹ S ₀ ground state by emitting a 483-nm photon. The scheme worked far better than the Argonne researchers expected. At first they worried that too many atoms would make a ³ D ₁ → ³ P ₀ transition and miss out on excitation to ¹ P ₁. That leak was plugged by a surprising source: Room-temperature blackbody photons from the glass walls of the trap chamber repopulate the ³ D ₁ state. For perhaps the first time, ambient photons have helped, not hindered, a cold-atom experiment. (J. R. Guest et al., Phys. Rev. Lett. 98 , 093001, 2007.)