  We have realized the nonlinear sign shift (NS) operation for photonic
qubits.This operation shifts the phase of two photons reflected by a beam
splitter using an extra single photon and measurement. We show that the
conditional phase shift is $(1.05\pm 0.06) \pi $ in clear agreement with
theory. Our results show that by using an ancilla photon and conditional
detection, nonlinear optical effects can be implemented using only linear
optical elements. This experiment represents an essential step for linear
optical implementations of scalable quantum computation.
