transition for quantum sensing

Yb is unique among all Alkaline Earth-like elements (AEL) as the strong dipole allowed transition from the ground 6s$^2$ $^1$S$_ 0 \rightarrow$ 6s6p $^1$P$_1$ state is $\approx$50\% of the ionization energy. This property enables Doppler free excitation to several low-lying Rydberg states that are useful for quantum sensing at low probe intensities. Here we present degenerate two photon Doppler free spectroscopy of the 6s$^2$ $^1$S$_0$ $\rightarrow$ 6s23s $^1$S$_0$ ground-to-Rydberg transition. The 6s23s $^1$S$_0$ state is black-body radiation (BBR) sensitive and immune to first order Zeeman shifts, electric quadrupole shifts, and predicted to have a small density shift. We observe lifetime-limited linewidths and resolve the line center to a statistical uncertainty of $\Delta \nu / \nu \approx 5 \times 10^{-13}$, corresponding to $\approx$27 K temperature resolution, via MOT depletion in a Planckian BBR environment. These systematics may be favorable for next-generation AEL vaporcell quantum sensors including: two-photon optical clocks with PHz carrier frequency, microwave electrometers, and high-field DC voltage standards.