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

TL;DR

This study directly measures the Casimir-Polder force on ultracold strontium atoms near a dielectric surface in the intermediate regime by observing energy level shifts through spectroscopy, confirming QED predictions.

Contribution

First direct spectroscopic measurement of the Casimir-Polder force in the intermediate regime, bridging experimental gaps and validating theoretical models.

Findings

Measured CP-induced frequency shifts agree with QED calculations.

Results differ from short-range approximation and exclude long-distance one.

Demonstrated the potential for studying atom-surface interactions in various geometries.

Abstract

The Casimir-Polder (CP) effect -- the force between a neutral atom and an uncharged conducting plate in empty space -- is an intriguing consequence of quantum vacuum fluctuations. The typically attractive CP potential crosses over from a scaling of $z^{-3}$ at short separations to $z^{-4}$ at long distances, where retardation effects due to the finite speed of light become important. At intermediate distances, where the atom--surface separation is of the order of the wavelength of the dominant atomic transition, experiments have so far relied on indirect methods, such as diffraction or quantum reflection, to observe the CP effect. Here, we directly reveal the CP force between strontium atoms and a dielectric surface via the induced shifts in the atomic energy levels in the intermediate regime. We spectroscopically probe the CP-induced kHz-frequency shift of ultracold atoms confined by a magic-wavelength optical lattice at 189(2)~nm from the surface -- on the scale of the dominant 461-nm transition. Our measurements agree well with QED calculations and differ from the short-range approximation, while excluding the long-distance one. This paves the way for studying the CP effect across various surface properties and geometries, as well as exploring the tensor nature of the atom-surface potential -- all important for the development of hybrid atomic optical-magnetic quantum devices.