  We use the quantum correlations of twin-beams of light to probe the added
noise when one of the beams propagates through a medium with anomalous
dispersion. The experiment is based on two successive four-wave mixing
processes in rubidium vapor, which allow for the generation of bright
two-mode-squeezed twin-beams followed by a controlled advancement while
maintaining the shared quantum-correlations between the beams. The demonstrated
effect allows the study of irreversible decoherence in a medium exhibiting
anomalous dispersion, and for the first time shows the advancement of a bright
nonclassical state of light. The advancement and corresponding degradation of
the quantum correlations are found to be operating near the fundamental quantum
limit imposed by using a phase-insensitive amplifier.
