  We propose a method to measure the quantum state of a single mode of the
electromagnetic field. The method is based on the interaction of the field with
a probe qubit. The qubit polarizations along coordinate axes are functions of
the interaction time and from their Fourier transform we can in general fully
reconstruct pure states of the field and obtain partial information in the case
of mixed states. The method is illustrated by several examples, including the
superposition of Fock states, coherent states, and exotic states generated by
the dynamical Casimir effect.
