Brillouin-Mandelstam scattering in telecommunications optical fiber at millikelvin temperatures.

Brillouin-Mandelstam scattering is a strong and readily accessible optical nonlinearity with diverse applications and research directions. For instance, the three-wave mixing process has been employed for narrow-linewidth lasers, distributed sensing, microscopy, and signal processing. While most studies focus on room-temperature operation, there is growing interest in cryogenic operation due to its significant potential for applications and fundamental physics at low temperatures. We measure the Brillouin scattering spectrum in standard single-mode telecommunications fiber at millikelvin (mK) temperatures using a closed-cycle dilution refrigerator and optical heterodyne detection. Our experiments, ranging from 50 mK to 27 K, extend beyond previous studies that used liquid helium-4 cryostats at temperatures above 1 K. At millikelvin temperatures, we observe coherent acoustic interactions with two-level systems (TLS)—microscopic defects in the amorphous material—previously unobserved in optical fiber. The temperature-dependent linewidth aligns with established models of ultrasonic attenuation in amorphous materials, involving intrinsic and thermally-activated scattering as well as incoherent and coherent TLS interactions.