Observation of mHz-level cooperative Lamb shifts in an optical atomic clock

TL;DR

This paper reports the direct observation of cooperative Lamb shifts caused by electric dipole-dipole interactions in a strontium-87 optical clock, revealing spatially-dependent shifts that can be suppressed or enhanced through geometric control, opening new avenues for quantum many-body physics studies.

Contribution

It provides the first direct measurement of cooperative Lamb shifts in an optical atomic clock and demonstrates control over these shifts via geometric configuration.

Findings

Cooperative Lamb shifts observed at mHz precision.

Shifts can be suppressed below systematic uncertainties.

Excitation near a Bragg angle enhances the effects nearly tenfold.

Abstract

We report on the direct observation of resonant electric dipole-dipole interactions in a cubic array of atoms in the many-excitation limit. The interactions, mediated by single-atom couplings to the shared electromagnetic vacuum, are shown to produce spatially-dependent cooperative Lamb shifts when spectroscopically interrogating the mHz-wide optical clock transition in strontium-87. We show that the ensemble-averaged shifts can be suppressed below the level of evaluated systematic uncertainties for state-of-the-art optical atomic clocks. Additionally, we demonstrate that excitation of the atomic dipoles near a Bragg angle can enhance these effects by nearly an order of magnitude compared to non-resonant geometries. Given the remarkable precision of frequency measurements and the high accuracy of the modeled response, our work demonstrates that such a clock is a novel platform for studies of the quantum many-body physics of spins with long-range interactions mediated by propagating photons.