Brillouin-Mandelstam Scattering-based Cooling of Traveling Acoustic Waves from Cryogenic Temperatures

Abstract

Thermal phonons are a major source of decoherence in quantum mechanical systems. Operating in the quantum ground state is therefore often an experimental prerequisite. Additionally to passive cooling in a cryogenic environment, active laser cooling enables the reduction of phonons at specific acoustic frequencies. Brillouin cooling has been used to show efficient reduction of the thermal phonon population in waveguides at GHz frequencies down to 74 K. In this letter, we demonstrate cooling of a 7.608 GHz acoustic mode by combining Brillouin active cooling with precooling from 77 K using liquid nitrogen. We show a 69 % reduction in the phonon population, resulting in a final temperature of 24.3 K, 50 K lower than previously reported.

Figures (4)

• Figure 1: Sketch of Stokes (top) and anti-Stokes (bottom) Brillouin-Mandelstam scattering.
• Figure 2: A) Diagram of the experimental Setup. CW: Continuous wave, EOM: electro-optical modulator, EDFA: erbium doped fiber amplifier, BPF: band pass filter, AOM: acousto-optical modulator, PC: polarization controller, ESA: electrical spectrum analyzer, var. att: variable attenuator B) Back-scattered spectrum of the anti-Stokes process with 9.34 dBm pump power. The resonance at 7.355$\pm 0.002$ GHz occurs in the untapered room temperature part of the fiber, whereas the 7.608 $\pm 0.002$ GHz resonance origins in the tapered part of the fiber at 77 K. C) Back-scattered spectrum of the anti-Stokes process at 17.37 dBm pump power.
• Figure 3: Logarithmic peak height (top) and FWHM (bottom) of the Stokes (red triangles) and the anti-Stokes (blue diamonds) resonances as a function of pump power. A linear fit to the linear part of the logarithmic peak height was used to determine the Brillouin gain of the fiber $G_B = 196.1 \pm 1.9 \,\mathrm{W^{-1} m^{-1}}$.
• Figure 4: Theoretical (solid black line) and measured (blue triangles) thermal phonon population and effective temperature of the $\frac{\Omega_B}{2\pi} = 7.608\,$GHz mode as a function of pump power. At maximum power, the thermal phonon population is reduced by $69\pm3$ %, resulting in a final phonon population of $66 \pm 6$ phonons.