Spectroscopic measurement of the Casimir-Polder force in the intermediate regime

We use spectroscopy to measure the Casimir-Polder (CP) force between strontium atoms and a dielectric surface in the intermediate regime. In our approach, we probe the kHz-frequency shift induced by CP on a narrow strontium transition, thereby detecting the effect of the dielectric mirror and the quantized vacuum electromagnetic field. Using a standing-wave magic-wavelength optical lattice, we confine the atoms to the first lattice site at 188(2) nm from the mirror—a position that lies within the strongest strontium transition at 461 nm. Our results align with our theoretical predictions for this intermediate regime. This work opens the door to future CP studies involving surfaces of varied properties and geometries, while also supporting improved control of optical and magnetic atomic traps near surfaces for quantum science and technology.