  The quantum Zeno effect describes the inhibition of quantum evolution by
frequent measurements. Here, we propose a scheme for entangling two given
photons based on this effect. We consider a linear-optics set-up with an
absorber medium whose two-photon absorption rate $\xi_{2\gamma}$ exceeds the
one-photon loss rate $\xi_{1\gamma}$. In order to reach an error probability
$P_{\rm error}$, we need $\xi_{1\gamma}/\xi_{2\gamma}<2P_{\rm error}^2/\pi^2$,
which is a factor of 64 better than previous approaches (e.g., by Franson et
al). Since typical media have $\xi_{2\gamma}<\xi_{1\gamma}$, we discuss three
mechanisms for enhancing two-photon absorption as compared to one-photon loss.
The first mechanism again employs the quantum Zeno effect via self-interference
effects when sending two photons repeatedly through the same absorber. The
second mechanism is based on coherent excitations of many atoms and exploits
the fact that $\xi_{2\gamma}$ scales with the number of excitations but
$\xi_{1\gamma}$ does not. The third mechanism envisages three-level systems
where the middle level is meta-stable ($\Lambda$-system). In this case,
$\xi_{1\gamma}$ is more strongly reduced than $\xi_{2\gamma}$ and thus it
should be possible to achieve $\xi_{2\gamma}/\xi_{1\gamma}\gg1$. In conclusion,
although our scheme poses challenges regarding the density of active
atoms/molecules in the absorber medium, their coupling constants and the
detuning, etc., we find that a two-photon gate with an error probability
$P_{\rm error}$ below 25% might be feasible using present-day technology.
