Revealing the Casimir-Polder effect through narrow-linewidth spectroscopy of strontium atoms near a dielectric.

Understanding atom-surface interactions is crucial for developing technologies integrating atoms with nanodevices for quantum sensing and computation. When a neutral atom is near a dielectric substrate, it experiences the Casimir-Polder (CP) force, which depends on the geometry and properties of the substrate, as well as the symmetry of the atom. Alkaline-earth atoms, such as strontium, have narrow linewidth transitions that experience energy-level shifts due to CP forces larger than the excited state's linewidth. In our experiments, strontium atoms are laser-cooled to 1 μK and trapped in an optical lattice formed by a standing wave created with a dielectric mirror inside an ultrahigh-vacuum chamber. As the atoms are moved hundreds of nanometers away from the dielectric, we observe spectroscopically the energy level shifts on the 689-nm kHz linewidth transition. We avoid the Stark shift of this transition with a lattice at the 914-nm magic wavelength. This research opens the door to better optical-atomic integrated technologies by engineering surface properties and geometries, and providing a better fundamental understanding of the interactions.