# Virtual-State Spectroscopy with Frequency-Tailored Intense Entangled Beams. (arXiv:1801.05871v1 [quant-ph])

In this contribution we analyze virtual-state spectroscopy --- a unique tool
for extracting information about the virtual states that contribute to the
two-photon excitation of an absorbing medium --- as implemented by means of
intense entangled beams with tunable spectral correlations. We provide a
thorough description of all contributing terms (classical and quantum) in the
two-photon absorption signal, as well as the limits imposed by the power of the
pump that produces the entangled beams on the observability of the spectral
lines of the virtual transitions. We find that virtual-state spectroscopy may
be implemented with entangled twin beams carrying up to $10^4$ photon pairs.
This implies that, in principle, one might be able to detect two-photon
absorption signals up to four orders of magnitude larger than previously
reported, thus paving the way towards the first experimental realization of the
virtual-state spectroscopy technique.