  Quantum tomography is a procedure to determine the quantum state of a
physical system, or equivalently, to estimate the expectation value of any
operator. It consists in appropriately averaging the outcomes of the
measurement results of different observables, obtained on identical copies of
the same system. Alternatively, it consists in maximizing an appropriate
likelihood function defined on the same data. The procedure can be also used to
completely characterize an unknown apparatus. Here we focus on the
electromagnetic field, where the tomographic observables are obtained from
homodyne detection.
