Setting Experimental Bounds on Entangled Two-Photon Absorption Cross Sections*

Two-photon absorption (2PA) is widely used in microscopy for deep, sub-cellular imaging. However, the efficiency of 2PA is limited by the properties of both the absorber and the excitation light. Entangled photon pairs produced via spontaneous parametric downconversion (SPDC) exhibit correlations in time and space that may improve the excitation efficiency relative to a classical laser. The most significant improvement is expected at low photon flux where isolated pairs interact with the absorber. In this regime, the rate of the entangled two-photon absorption (E2PA) process scales linearly with photon flux and the E2PA cross section. Despite over a decade of publications claiming to measure huge cross sections that suggest a quantum advantage exists of up to 10 orders of magnitude, recent work from multiple groups has shown strong counterevidence.

In our work to investigate these claims, we have developed experimental apparatuses that enable sensitive measurements of E2PA via two techniques. In one technique we collect fluorescence from samples excited with SPDC. In the other technique we measure SPDC transmittance through samples. In both studies, our samples are two-photon absorbing chromophores in room-temperature liquids. Despite the high sensitivity of the techniques, we could not resolve a signal in any of the measurements. We set upper bounds on the cross section of the samples that are up to five orders of magnitude lower than previously published cross sections. We propose that the discrepancy between results in this field originates from misinterpretation of the origin of measured signals.