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arXiv:2403.09814 · Physical Review A

Doppler-free three-photon spectroscopy on narrow-line optical transitions

Guglielmo Panelli, S. C. Burd, Erik J. Porter, Mark A. Kasevich

Published Mar 17, 2025• 3 citations
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Abstract

We demonstrate coherent Doppler-free three-photon excitation of the $^{1}S_{0}\phantom{\rule{0.16em}{0ex}}\ensuremath{\leftrightarrow}\phantom{\rule{0.16em}{0ex}}^{3}P_{0}$ optical clock transition and the $^{1}S_{0}\phantom{\rule{0.16em}{0ex}}\ensuremath{\leftrightarrow}\phantom{\rule{0.16em}{0ex}}^{3}P_{1}$ intercombination transition in free-space thermal clouds of $^{88}\mathrm{Sr}$ atoms. By appropriate orientation of the wave vectors of three lasers incident on the atoms, the first-order Doppler shift can be eliminated for all velocity classes. Three-photon excitation of the $^{1}S_{0}\phantom{\rule{0.16em}{0ex}}\ensuremath{\leftrightarrow}\phantom{\rule{0.16em}{0ex}}^{3}P_{1}$ transition enables high-contrast Ramsey spectroscopy with interrogation times comparable to the $21\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{s}$ natural lifetime using a single near-resonant laser source. Three-photon spectroscopy on the $^{1}S_{0}\phantom{\rule{0.16em}{0ex}}\ensuremath{\leftrightarrow}\phantom{\rule{0.16em}{0ex}}^{3}P_{0}$ clock transition, using only laser frequencies nearly resonant with the $^{1}S_{0}\phantom{\rule{0.16em}{0ex}}\ensuremath{\leftrightarrow}\phantom{\rule{0.16em}{0ex}}^{3}P_{0}$ and $^{1}S_{0}\phantom{\rule{0.16em}{0ex}}\ensuremath{\leftrightarrow}\phantom{\rule{0.16em}{0ex}}^{3}P_{1}$ transitions, enables a reduction in Doppler broadening by two orders of magnitude and a corresponding $\ensuremath{\sim}470\phantom{\rule{0.16em}{0ex}}\mathrm{Hz}$ linewidth without a confining potential.

DOI
10.1103/physreva.111.033112

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Three-photon laser excitation of Rydberg states by three different laser beams can be arranged in a starlike geometry that simultaneously eliminates the recoil effect and Doppler broadening. Our analytical and numerical calculations for a particular laser excitation scheme $5{S}_{1/2}\ensuremath{\rightarrow}5{P}_{3/2}\ensuremath{\rightarrow}6{S}_{1/2}\ensuremath{\rightarrow}nP$ in Rb atoms have shown that, compared to the one- and two-photon laser excitation, this approach provides much narrower linewidth and longer coherence time for both cold atom samples and hot vapors, if the intermediate one-photon resonances of the three-photon transition are detuned by more than respective single-photon Doppler widths. This method can be used to improve fidelity of Rydberg quantum gates and precision of spectroscopic measurements in Rydberg atoms.
Two-photon Doppler-free spectroscopy of the 6s2  1S0→6s23s  1S06s^2\;^1S_0 \rightarrow 6s23s\;^1S_06s21S0​→6s23s1S0​ Rydberg transition for quantum sensing in ytterbium
Here we report atomic structure calculations and experimental observations of the two-photon 6s2  1S0→6s23s  1S06s^2\;^1S_0 \rightarrow 6s23s\;^1S_06s21S0​→6s23s1S0​ transition in Yb. The unique level structure of Yb places the 6s6p  1P16s6p\;^1P_16s6p1P1​ state's energy at ≈ ⁣49.7%\approx\!49.7\%≈49.7% of the ionization energy, which facilitates two-photon Doppler-free excitation of the 6s23s  1S06s23s\;^1S_06s23s1S0​ Rydberg state from the 6s2  1S06s^2\;^1S_06s21S0​ ground state at a relatively low optical intensity. This J=0→J=0J= 0 \rightarrow J=0J=0→J=0 transition is immune to first-order Zeeman and electric quadrupole shifts, has reduced sensitivity to higher-order Zeeman shifts, and is further predicted to have relatively low sensitivity to density shifts. Using configuration interaction theory, we compute relevant properties of the 6s23s  1S06s23s\;^1S_06s23s1S0​ Rydberg state for sensing, including the C6C_6C6​ coefficient, dc polarizability, blackbody radiation (BBR) shift, and BBR depopulation rate. As a first experimental step, we demonstrate two-photon Doppler-free spectroscopy of the 6s2  1S0→6s23s  1S06s^2\;^1S_0 \rightarrow 6s23s\;^1S_06s21S0​→6s23s1S0​ transition. From our experimental spectra, we estimate the natural linewidth (lifetime) of the 6s23s  1S06s23s\;^1S_06s23s1S0​ state to be 90 kHz (1.8 μμμs). We believe this transition, or other 6s2  1S0→6sns  1S06s^2\;^1S_0 \rightarrow 6sns\;^1S_06s21S0​→6sns1S0​ transitions in Yb with n≈23n\approx23n≈23, may find use in a variety of sensing applications, potentially including a two-photon clock, electrometer, or BBR thermometer.