  The pure quantum correlations totally independent of the classical coherence
of light have been experimentally demonstrated. By measuring the visibility of
the interference fringes and the correlation variances of amplitude and phase
quadratures between a pair of bright twin optical beams with different
frequencies produced from a non-degenerate optical parametric oscillator, we
found that when classical interference became worse even vanished, the
quadrature quantum correlations were not influenced, completely. The presented
experiment obviously shows the quantum correlations of light do not necessarily
imply the classical coherence.
