  Motivated by the recent experiment [V.A. Sautenkov, Yu.V. Rostovtsev, and
M.O. Scully, Phys. Rev. A 72, 065801 (2005)], we develop a theoretical model in
which the field intensity fluctuations resulted from resonant interaction of a
dense atomic medium with laser field having finite bandwidth. The
intensity-intensity cross correlation between two circular polarized beams can
be controlled by the applied external magnetic field. A smooth transition from
perfect correlations to anti-correlations (at zero delay time) of the outgoing
beams as a function of the magnetic field strength is observed. It provides us
with the desired information about decoherence rate in, for example, $^{87}$Rb
atomic vapor.
