  We present a model to realize a probabilistic conditional sign flip gate
using only linear optics. The gate operates in the space of number state qubits
and is obtained by a nonconventional use of the teleportation protocol. Both a
destructive and a nondestructive version of the gate are presented. In the
former case an Hadamard gate on the control qubit is combined with a projective
teleportation scheme mixing control and target. The success probability is 1/2.
In the latter case we need a quantum encoder realized via the interaction of
the control qubit with an ancillary state composed of two maximally entangled
photons. The success probability is 1/4.
