# Kerr Microresonator Soliton Frequency Combs at Cryogenic Temperatures

## TL;DR

This study demonstrates that cryogenic cooling of silicon nitride microresonators significantly reduces thermo-refractive noise, enabling easier generation of single Kerr solitons for applications in optical clocks and precision metrology.

## Contribution

It provides experimental and theoretical evidence that cryogenic temperatures facilitate access to single solitons without degrading key device parameters.

## Key findings

- Thermo-refractive coefficient decreases by two orders of magnitude at cryogenic temperatures.
- Single bright Kerr solitons are accessible below 60 K.
- Cryogenic operation does not affect quality factor, dispersion, or nonlinearity.

## Abstract

We present measurements of silicon nitride nonlinear microresonators and frequency comb generation at cryogenic temperatures as low as 7~K. The resulting two orders of magnitude reduction in the thermo-refractive coefficient relative to room-temperature enables direct access to single bright Kerr soliton states through adiabatic frequency tuning of the pump laser while remaining in thermal equilibrium. Our experimental results, supported by theoretical modeling, show that single solitons are easily accessible at temperatures below 60~K for the microresonator device under study. We further demonstrate that the cryogenic temperature primarily impacts the thermo-refractive coefficient. Other parameters critical to the generation of solitons, such as quality factor, dispersion, and effective nonlinearity, are unaltered. Finally, we discuss the potential improvement in thermo-refractive noise resulting from cryogenic operation. The results of this study open up new directions in advancing chip-scale frequency comb optical clocks and metrology at cryogenic temperatures.