Spin squeezing in two-level systems

Using a quantum theory for an ensemble of two-level atoms driven by a field in an optical cavity, we show that the spin associated with the atomic ensemble can be squeezed. Two kinds of squeezing are obtained: on the one hand, self-squeezing of the spin when the input field is a coherent one and the atomic ensemble exhibits a large nonlinearity; and on the other hand, squeezing transfer from an incoming squeezed field when the atomic ensemble has a quasilinear behavior.