  Realizing optical-nonlinear effects at a single-photon level is a highly
desirable but also extremely challenging task, because of both fundamental and
practical difficulties. We present an avenue to surmounting these difficulties
by exploiting quantum Zeno blockade in nonlinear optical systems. Considering
specifically a lithium-niobate microresonator, we find that a deterministic
phase gate can be realized between single photons with near-unity fidelity.
Supported by established techniques for fabricating and operating such devices,
our approach can provide an enabling tool for all-optical applications in both
classical and quantum domains.
