Entangled two-photon absorption to drive an atomic excited state population

Entangled two-photon absorption (ETPA) may allow for pumping an atomic excited state population with a continuous, low-intensity laser and with resulting fluorescence suitable for high-bandwidth plasma fluctuation measurements. Here, we investigate excited state populations for pumping and fluorescing transitions in an Argon plasma species to assess ETPA diagnostic capabilities. We focus on pump transitions accessible with a frequency-doubled Ti:Sapphire laser and visible fluorescence. The time-frequency entanglement of entangled photon pairs allows for the simultaneous arrival of entangled pairs at the target location and produces a narrowband sum-frequency that corresponds to the CW laser linewidth. The ETPA cross section scales linearly with the incident photon flux due to photon entanglement, in contrast to the quadratic scaling for two-photon absorption with classical light. Potential diagnostic schemes include pumping low-n transitions for low-Z impurities, high-n Rydberg transitions for high-Z impurities, and transitions in charge-exchange populations.