  We report the measurement of the entangled two photon absorption cross
section, $\sigma_E$, at 808 nm on organic chromophores in solution in a low
photon flux regime. We performed measurements on Zinc tetraphenylporphyrin
(ZnTPP) in Toluene and Rhodamine B (RhB) in Methanol. This is, to the best of
our knowledge, the first time that $\sigma_E$ is measured for RhB.
Additionally, we report a systematic study of the dependence of $\sigma_E$ on
the molecular concentration for both molecular systems. In contrast to previous
experiments, our measurements are based on detecting the pairs of photons that
are transmitted by the molecular system. By using a coincidence count circuit
it was possible to improve the signal to noise ratio. This type of work is
important for the development of spectroscopic and microscopic techniques using
entangled photons.
