Europium forms a Bose–Einstein condensate

Ultracold atomic gases are common systems for quantum simulations. (See, for example, Physics Today, August 2017, page 17 , and “The early universe in a quantum gas,” Physics Today online, 17 November 2022 .) The use of magnetic atoms offers the opportunity to observe dipole–dipole interactions in addition to the usual contact interactions, which arise from atoms bumping into one another. The long-range interactions allow magnetic Bose–Einstein condensates (BECs) to access novel quantum phases, such as supersolids (see the article by Tim Langen, Physics Today, March 2022, page 36 ).

In the past decade or so, researchers have become interested in what new physics might emerge if a spin degree of freedom is added to a magnetic BEC. The relative strengths of the spin-dependent contact interactions and the dipole interactions depend on the type of atoms and can’t be tuned. So researchers must prepare BECs of different species to explore the full range of potential behaviors. In 2011 chromium atoms were cooled into a BEC with both magnetic and spin interactions, and until recently they were the only such species.

Now Mikio Kozuma of the Tokyo Institute of Technology and his colleagues have created a BEC of europium atoms. Whereas Cr atoms have much larger spin-dependent contact interactions than dipole–dipole interactions, Eu has unusually small spin-dependent interactions and a dipolar length four times as large as that of Cr. Eu thus offers a markedly different regime to explore the interplay of spin and dipole effects.

The process for cooling most lanthanide atoms involves slowing them down in a magneto-optical trap then evaporatively cooling them in a pair of optical traps. For those lanthanides, the first cooling step relies on an optical transition, but Eu’s optical transition doesn’t reliably relax to the ground state. In 2021 Kozuma and his colleagues identified and pumped a metastable state that did.

In the new study, the researchers cooled 5 × 10⁴ Eu atoms to about 349 nK. They confirmed the influence of spin and magnetic dipoles in their BEC by measuring the relative speeds and extents of the gas’s expansion along different axes relative to various applied external magnetic field orientations.

Now that Eu BECs have been created and characterized, there should be plenty of new physics to explore. Magnetic BECs with spin are expected to demonstrate unusual phenomena. For example, theorists have predicted that such BECs will develop swirling vortices because of the Einstein–de Haas effect, in which a change in the magnetic moment causes rotation. (Y. Miyazawa et al., Phys. Rev. Lett. 129, 223401, 2022 .)