  We calculate the rate of two-photon absorption for frequency entangled
photons in a tapered optical fiber whose diameter is comparable to the
wavelength of the light. The confinement of the electric field in the
transverse direction increases the intensity associated with a single photon,
while the two-photon absorption rate is further enhanced by the fact that the
sum of the frequencies of the two photons is on resonance with the upper atomic
state, even though each photon has a relatively broad linewidth. As a result,
the photons are effectively confined in all three dimensions and the two-photon
absorption rate for frequency-entangled photons in a tapered fiber was found to
be comparable to that for unentangled photons in a microcavity with a small
mode volume.
