Design of many-atom cavity QED systems for strong two-photon nonlinearity

We propose a new scheme for strong two-photon nonlinearity (TPN) by cavity QED systems with many three-level atoms, although the TPN due to the saturation effect of two-level atoms generally declines with an increase in the number of atoms. The reduction in anharmonicity by the increase in the number of atoms can be overcome if the system satisfies particular criteria. The new scheme is significant for realizing TPN devices using solid-state materials because it is generally difficult to control the number of atoms, which are few, introduced in the device.