Quantum entangled two-photon absorption for selective, localized, and low intensity pumping of excited state populations in plasma

Entangled two-photon absorption (ETPA) has the potential to pump a fluorescing excited state population with low incident intensity and high state selectivity for innovative quantum-enhanced plasma spectroscopy measurements. The time-frequency entanglement of entangled photon pairs allows for the simultaneous arrival of entangled pairs at the target location and a narrow bandwidth for the sum frequency. The simultaneous arrival gives a linear scaling of the ETPA cross section with the incident photon flux compared to a quadratic scaling for two-photon absorption (TPA) with classical light. ETPA reduces incident flux requirements for chemical sensing and biological microscopy by up to seven orders of magnitude compared to classical TPA [1], and we are hopeful to realize a similar quantum enhancement for ETPA in plasma. The lower incident flux for ETPA can be compatible with a low-intensity continuous laser, in contrast to classical TPA which requires a high-intensity, pulsed laser. Finally, the narrow bandwidth of the sum frequency can precisely target an excited state with minimal contamination into non-target states, and the two-photon process is inherently compatible with cross-beam spatial localization. Potential diagnostic schemes include pumping low-n states for low-Z impurities, high-n Rydberg states for high-Z impurities, and charge-exchange populations.

[1] M. Raymer et al., J. Chem. Phys. 155, 081501 (2021); A. Eshun et al., Acc. Chem. Res. 55, 991 (2022)