Brillouin-Mandelstam scattering in telecommunications optical fiber at millikelvin temperatures

TL;DR

This study measures Brillouin-Mandelstam scattering in standard optical fiber at millikelvin temperatures, revealing interactions with microscopic defects and opening avenues for quantum science and low-temperature sensing applications.

Contribution

First to observe coherent acoustic interactions with two-level systems in optical fiber at millikelvin temperatures, extending understanding of low-temperature Brillouin scattering.

Findings

Observation of TLS interactions at millikelvin temperatures.

Agreement of linewidth behavior with models of ultrasonic attenuation.

Extension of temperature range for Brillouin scattering measurements.

Abstract

Brillouin-Mandelstam scattering is a strong and readily accessible optical nonlinearity enabling a wide array of applications and research directions. For instance, the three-wave mixing process has been employed to great success for narrow-linewidth lasers, sensing applications, microscopy, and signal processing. While most of these avenues focus on room temperature operation, there is now increasing interest in cryogenic operation owing to the scattering mechanism's significant potential for applications and fundamental physics at low temperatures. Here, we measure the Brillouin scattering spectrum in standard single-mode telecommunications optical fiber at millikelvin temperatures using a closed-cycle dilution refrigerator and optical heterodyne detection. Our experiments are performed with a cryostat temperature from 50 mK to 27 K, extending previously reported measurements that utilized liquid helium-4 cryostats with temperatures greater than 1 K. At millikelvin temperatures, our experiment observes coherent acoustic interaction with microscopic defects of the amorphous material - two-level-systems (TLS) - which has not been previously observed in optical fiber. The measured behaviour of the linewidth with temperature is in agreement with well-established models of ultrasonic attenuation in amorphous materials comprising a background intrinsic scattering, thermally-activated scattering, and incoherent and coherent TLS interaction. This work provides a foundation for a wide range of applications and further research including sensing applications, new approaches to investigate TLS physics, and Brillouin-scattering-based quantum science and technology.