  Dissipation-free photon-photon interaction at the single photon level is
studied in the context of cavity electromagnetically induced transparency
(EIT). For a single multilevel atom exhibiting EIT in the strong
cavity-coupling regime, the anharmonicity of the atom-cavity system has an
upper bound determined by single atom-photon coupling strength. Photon blockade
is inferred to occur for both single and multi-atom cases from the behaviour of
transition rates between dressed states of the system. Numerical calculations
of the second order coherence function indicate that photon antibunching in
both single and two-atom cases are strong and comparable.
