Comparison between classical and entangled two-photon saturation absorption spectroscopy in Rubidium-87

Multi-photon absorption spectroscopy utilizes narrow linewidth transitions for applications including optical clocks, plasma diagnostics, and trace species detection, but is fundamentally limited by extremely small absorption cross sections, typically requiring high intensity pulsed lasers. Entangled photon pairs, through their time-energy correlations, have been proposed to enhance two-photon absorption rates at dramatically lower intensities. However, implementing ETPA in dilute media is challenging: the enhancement in absorption cross section must overcome the limited photon pair flux from current entangled sources. Using the Rb-87 (5^2 S 1/2 -> 5^2 D 5/2) two-photon transition at 778.1 nm, we investigate the trade-offs between classical and entangled Doppler-free two-photon absorption (TPA/ETPA). To generate entangled photons, we frequency double to 389 nm photons using second harmonic generation, then produce degenerate entangled pairs at 778.1 nm using spontaneous parametric down-conversion, both using periodically poled lithium-niobate waveguides. We present calculated power-dependent absorption rates in both regimes, validate the expected quadratic intensity, and report on experimental progress towards demonstrating ETPA absorption enhancement.