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Many photonic quantum technologies require some form of light matter interface in order to be viable. Such an interface may be used to store information encoded within the photons, for example when used in a photonic quantum computer, or to convert photons to telecom wavelengths and to extend entanglement across large distances, which is important for global quantum communication [1] .

Raw and processed experimental data from P. Burdekin et al., "Single-photon-level sub-Doppler pump-probe spectroscopy of rubidium," Phys. Rev. Applied 14, 044046 (2020), with a text file describing the data structure.&nbsp

We propose and demonstrate pump-probe spectroscopy of rubidium absorption which reveals the sub-Doppler hyperfine structure of the $^{5}$S$_{1/2} \leftrightarrow$ $^{5}$P$_{3/2}$ (D2) transitions. The counter propagating pump and probe lasers are independently tunable in frequency, with the probe operating at the single-photon-level. The two-dimensional spectrum measured as the laser frequencies are scanned shows fluorescence, Doppler-broadened absorption dips and sub-Doppler features. The detuning between the pump and probe lasers allows compensation of the Doppler shift for all atomic velocities in the room temperature vapor, meaning we observe sub-Doppler features for all atoms in the beam. We detail a theoretical model of the system which incorporates fluorescence, saturation effects and optical pumping and compare this with the measured spectrum, finding a mean absolute percentage error of 4.17\%. In the future this technique could assist in frequency stabilization of lasers, and the single-photon-level probe could be replaced by a single photon source., 5 page paper, 4 page supplemental material. Comments welcome