Measurement of conditional phase shifts for quantum logic

Measurements of the birefringence of a single atom strongly coupled to a high-(cid:12)nesse optical resonator are reported, with nonlinear phase shifts observed for intracavity photon number much less than one. A proposal to utilize the measured conditional phase shifts for implementing quantum logic via a quantum-phase gate (QPG) is considered. Within the context of a simple model for the (cid:12)eld transformation, the parameters of the \truth table" for the QPG are determined.