  A new method is described for determining the quantum state of correlated
multimode radiation by interfering the modes and measuring the statistics of
the superimposed fields in four-port balanced homodyne detection. The full
information on the $N$-mode quantum state is obtained by controlling both the
relative amplitudes and the phases of the modes, which simplifies the
reconstruction of density matrices to only $N+1$ Fourier transforms. In
particular, this method yields time-correlated multimode density matrices of
optical pulses by superimposing the signal by a sequence of short
local-oscillator pulses.
