We propose a linear optical scheme for direct realization of a nondestructive $N$-qubit controlled phase gate without resorting to a sequence of single- and two-qubit gates. The proposed setup involves linear optical elements, nonmaximally entangled pairs of photons, and multiphoton coincidence detection. To implement the scheme, physical qubits represented by the polarization states of single photons are probabilistically encoded into the two-photon states. Based on such encoding operation, a nondestructive $N$-qubit phase gate can be directly achieved by $N$-photon interference on an array of polarization beam splitters and $N$-photon coincidence detection.

• Received 6 August 2006

DOI:https://doi.org/10.1103/PhysRevA.74.044305