Compact High-Spectral Purity Frequency Combs for Strontium Quantum Clock

Integrated quantum clocks exemplify ultracold-atom-based quantum sensors that rely on lasers as a crucial component. Precise control over the quantum states of ions and atoms used in such devices necessitates lasers with narrow linewidths, high spectral stability, and minimal phase noise. To transfer the absolute spectral characteristics to the cooling and trapping lasers, frequency combs come into play. A reduction of the intrinsic linewidths of frequency combs below a few kHz without need of locking to an optical stabilization cavity would simplify quantum clock experiments significantly. Frequency combs based on mode-locked Er:fiber oscillators are state-of-the-art systems exhibiting several advantages over solid-state lasers like compactness, alignment-free operation and robustness against environmental influences. By employing supercontinuum generation, amplification stages and nonlinear conversion processes, the wavelength range of fiber frequency combs can be extended from 420 to more than 2000 nm. Fiber frequency combs typically have comb lines with an intrinsic optical linewidth in the range of several tens of kilohertz. The broadening of the linewidth is attributed to factors such as pump-induced noise, sensitivity on environmental effects as well as on quantum noise effects. In our recent work we have demonstrated frequency combs exhibiting exceptionally low phase noise resulting in comb linewidths as low as 700 Hz. In this work we employ this technique aiming intrinsically narrow linewidths at wavelengths used in an integrated quantum clock experiment based on Strontium atoms (813 nm, 689 nm).