A Simple Scheme for Directly Measuring the Wigner Functions of Cavity Fields

We propose a scheme for directly measuring the Wigner functions of cavity fields. The scheme is based on the Raman interaction between atoms and cavity fields. We find a simple and direct relation between the Wigner function and the atomic population difference. By suitably choosing the interaction time, we find that the Wigner function is just two times of the atomic population difference. Thus, one can obtain the Wigner function by measuring the atomic populations and calculating the population difference.