Two-photon Doppler-free spectroscopy of the 6s² ¹S₀→6s23s ¹S₀ transition for black body thermometry in optical clocks

Rydberg states have been proposed as in-situ probes of black body-radiation (BBR) in optical lattice clocks (OLCs). Although Rydberg states posses a high sensitivity to BBR of $\approx$18 Hz/K, which surpasses clock transitions' sensitivity by approximately three orders of magnitude, BBR sensing with Rydberg states is hampered by their sensitivity to Doppler shifts, external fields, and density shifts. Yb possesses a unique level structure, which facilitates degenerate two photon spectroscopy of the 6s$^2$ $^1$S$_0$ $\rightarrow$ 6s23s $^1$S$_0$ ground-to-Rydberg transition. The 6s23s $^1$S$_0$ state is BBR sensitive and immune to first order Zeeman shifts, electric quadrupole shifts, and predicted to have a small density shift. Here, we perform Doppler-free spectroscopy of the 6s$^2$ $^1$S$_0$ $\rightarrow$ 6s23s $^1$S$_0$ transition via MOT depletion in a Planckian BBR environment. We observe lifetime-limited linewidths below 200 kHz and resolve the Rydberg line to a statistical uncertainty of $ \Delta \nu / \nu \approx 5\times 10^{-13}$ corresponding to $\approx$ 27 K temperature resolution. These results demonstrate the potential use for the 6s23s $^1$S$_0$ as an in-situ BBR probe for thermometry in OLCs.