  Optical nonlinearities sensitive to individual photons may be extremely
useful as elements in quantum logic circuits for photonic qubits. A much cited
example is the work of Turchette et al. [Phys. Rev. Lett. 75, 4710 (1995)], in
which a phase shift of about 10 degrees was reported. To improve this result,
we propose a single sided cavity geometry with minimal cavity losses. It should
then be possible to achieve a nonlinear phase shift of 180 degrees.
