Quantum correlations of two optical fields close to electromagnetically induced transparency.

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 nondemolition measurements. The input intensity is the experimental "knob" for tuning the system into a squeezer or a quantum nondemolition 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 three-level model including cavity losses and spontaneous emission, allow direct comparison with future experiments.