  It is controversial what is the true role of entanglement in two-photon
virtual-state spectroscopy [Saleh et al, Phys. Rev. Lett. 80, 3483, 1998], a
two-photon absorption spectroscopic technique that can retrieve information
about the energy level structure of an atom or a molecule. The consideration of
closely related techniques, such as multidimensional pump-probe spectroscopy
[Roslyak et al, Phys. Rev. A 79, 063409, 2009], might suggest that
spectroscopic information retrieved in the two-photon absorption process is the
same regardless of the classical or quantum nature of the light source. Here,
we solve this debate by making use of a full quantum formalism to show that the
ability to obtain information about the energy level structure of a medium
requires the existence of temporal (frequency) correlations between the
absorbed photons. Moreover, we show that these correlations are not the only
requisite for retrieving such information. In fact, it is a combination of
both, the presence of frequency correlations and its specific spectral shape,
which makes the realization of two-photon virtual-state spectroscopy possible.
This result helps clarifying the discussion whether entanglement is needed or
not, and also, to specify the type of two-photon source that needs to be used
in order to experimentally perform the two-photon virtual-state spectroscopy
technique.
