Frequency measurements of superradiance from the strontium clock transition

TL;DR

This paper characterizes superradiant light from the ultra-narrow strontium clock transition, demonstrating its potential as a high-precision, portable optical frequency reference with remarkable stability and accuracy.

Contribution

It provides the first spectral characterization of superradiant emission from the strontium clock transition and compares its frequency stability to existing standards.

Findings

Achieved fractional Allan deviation of 6.7e-16 at 1 second

Established absolute frequency accuracy at 2 Hz (4e-15 fractional)

Demonstrated insensitivity to environmental perturbations

Abstract

We present the first characterization of the spectral properties of superradiant light emitted from the ultra-narrow, 1 mHz linewidth optical clock transition in an ensemble of cold $^{87}$Sr atoms. Such a light source has been proposed as a next-generation active atomic frequency reference, with the potential to enable high-precision optical frequency references to be used outside laboratory environments. By comparing the frequency of our superradiant source to that of a state-of-the-art cavity-stabilized laser and optical lattice clock, we observe a fractional Allan deviation of $6.7(1)\times 10^{-16}$ at 1 second of averaging, establish absolute accuracy at the 2 Hz ($4\times 10^{-15}$ fractional frequency) level, and demonstrate insensitivity to key environmental perturbations.