Entangling photons via the quantum Zeno effect
  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. 
Nicolai ten Brinke, Andreas Osterloh, Ralf Sch\"utzhold
quant-ph