Noah Lindsell Qualifying Exam

Two photon absorption (TPA) is a widely used technique in physics, chemistry, biology, and medicine for applications such as imaging, microfabrication, and spectroscopy. However, TPA is a highly inefficient process due to its nonlinearity, resulting in extremely small cross sections compared to single photon absorption schemes. A high amount of laser power is required which can damage samples.

Recently, exciting experimental results have appeared within the literature reporting many orders of magnitude enhancement of the TPA efficiency when the sample is excited with time-energy entangled photon pairs rather than typical coherent states, a so-called “quantum advantage”. However, several groups have questioned these results, putting forward tight theoretical bounds to this enhancement several orders of magnitude lower than has been reported, and failing to recreate the experimental results. To date, active debate still exists as to the nature and scale of the possible enhancement.

Here, we perform an investigation of entangled two-photon absorption (ETPA) in an atomic vapor (Rubidium-85 and 87) which possesses a far greater TPA cross section than the materials studied in previous ETPA experiments. We discuss the underlying physics of two-photon absorption, and provide a discussion of the current debate within the literature. We have created an entangled photon pair source via spontaneous parametric downconversion (SPDC) in a periodically-poled lithium niobate (PPLN) waveguide. We have characterized this source using coincidence counting via single photon detectors, and found a high degree of second order coherence, indicating a reliable source of photon pairs. We have also created and characterized an atomic spectroscopy setup with a cell of warm rubidium vapor and performed classical two-photon absorption spectroscopy. Finally, we have created infrastructure to collect and count fluorescence photons from our rubidium cell with high sensitivity. We present current results as well as plans for the future.