  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 this work, we focus on
transmission measurements of eTPA, and explore three different two-photon
quantum interferometers in the context of assessing eTPA. We demonstrate that
the so-called N00N-state configuration is the only one amongst those considered
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.
