Entangling photons via the quantum Zeno effect

The quantum Zeno effect describes the inhibition of quantum evolution by frequent measurements. In this talk, 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 ξ_2γ exceeds the one-photon loss rate ξ_1γ. In order to reach an error probability P_error, we need ξ_1γ/ξ_2γ < 2P_error²/π², which is a factor of 64 better than previous approaches. Since typical media have ξ_2γ < ξ_1γ, we discuss mechanisms for enhancing two-photon absorption as compared to one-photon loss. In particular, we present a mechanism which envisages three-level systems where the middle level is meta-stable (Λ-system). In this case, ξ_1γ is more strongly reduced than ξ_2γ and thus it should be possible to achieve ξ_2γ/ξ_1γ>>1. In conclusion, although our scheme poses serious experimental challenges, we find that a two-photon gate with an error probability P_error below 25 % might be feasible using present-day technology.