Nondestructive two-photon parity detector with near unity efficiency

It has been shown that two-photon parity check has widespread applications in optical quantum information processing. In recent years much effort has been made to implement nondestructive two-photon parity detection using cross-Kerr nonlinearity (XKNL) effect. We here put forward an XKNL-based scheme for nearly deterministic two-photon parity detection without destroying the photons, in which the required cross-Kerr phase shift could be much (even several orders of magnitude) smaller than that required for previous schemes. The scheme utilizes a ring cavity fed by a coherent state beam as a quantum information bus which interacts with one of the two path modes of two polarized photons through a cross-Kerr medium. The measurement outcome of the bus mode reveals the polarization-parity of the two photons.