  We show that three-level atoms excited by two cavity modes in a $\Lambda$
configuration close to electromagnetically induced transparency can produce
strongly squeezed bright beams or correlated beams which can be used for
quantum non demolition measurements. The input intensity is the experimental
"knob" for tuning the system into a squeezer or a quantum non demolition
device. The quantum correlations become ideal at a critical point characterized
by the appearance of a switching behavior in the mean fields intensities. Our
predictions, based on a realistic fully quantum 3-level model including cavity
losses and spontaneous emission, allow direct comparison with future
experiments.
