  We describe a scheme for producing an optical nonlinearity using an
interaction with one or more ancilla two-level atomic systems. The
nonlinearity, which can be implemented using high efficiency fluorescence
shelving measurements, together with general linear transformations is
sufficient for simulating arbitrary Hamiltonian evolution on a Fock state
qudit. We give two examples of the application of this nonlinearity, one for
the creation of nonlinear phase shifts on optical fields as required in single
photon quantum computation schemes, and the other for the preparation of
optical Schrodinger cat states.
