Sr around a nanotapered optical fiber*

Interfacing ultracold atoms with the large evanescent fields around a nanotapered optical fiber offers a rich toolbox for quantum science, such as probing subtle quantum effects. In particular, strontium is an excellent candidate for high-resolution spectroscopy on such platforms due to its 7.6 kHz-linewidth intercombination transition. However, the high-intensity optical fields required for trapping at the nanotapered region can introduce unwanted position- and state-dependent energy shifts on the atoms. Employing magic wavelengths with proper polarization of the evanescent fields ensures the atomic transition is insensitive to the trap. We demonstrate optical trapping of ⁸⁸Sr atoms in a two-color, state-insensitive trap around a 215-nm diameter tapered fiber at well-defined distances of hundreds of nanometers from the fiber surface. In this two-color trap, adjusting the attractive and repulsive intensities can precisely tune the atom-surface separation. This platform opens the door to performing high-resolution spectroscopy measurements of the Casimir-Polder interaction between the trapped atoms and the fiber dielectric surface.