Progress in the assessment and certification of entangled two-photon absorption in organic compounds.

Recent investigations suggest that the use of non-classical states of light, such as entangled photon pairs, may open new and exciting avenues in experimental two-photon absorption spectroscopy. Despite several experimental studies of entangled two-photon absorption (eTPA), there is still a heated debate on whether eTPA has truly been observed. This interesting debate has arisen, mainly because it has been recently argued that single-photon-loss mechanisms, such as scattering or hot-band absorption may mimic the expected entangled-photon linear absorption behavior.



In our work we first provide a thorough experimental study of eTPA in the organic molecules Rhodamine B (RhB) and zinc tetraphenyl-porphirin (ZnTPP). We determine the effects of a controllable temporal delay between the signal and idler photons on the strength of the eTPA signal, and on the other hand we use two concurrent and equivalent detection systems with and without the sample in place as a useful experimental check. Through this experimental setup we find that, surprisingly, the purported ETPA signal is not suppressed for a temporal delay much greater than the characteristic photon-pair temporal correlation time. While our results reproduce the previous findings from other authors, our full analysis indicates that the signal observed is not actually due to ETPA but is likely due to linear losses.



Second, focusing on transmission measurements of eTPA, we theoretically explore three different two-photon quantum interferometers in the context of certifying eTPA. We demonstrate that the so-called N00N-state configuration is the only one amongst those considered which is insensitive to linear (single-photon) losses. Remarkably, our results show that N00N states may become a potentially powerful tool for quantum spectroscopy, and place them as a strong candidate for the certification of eTPA in an arbitrary sample.