  We experimentally demonstrate that the quantum entanglement between amplitude
and phase quadratures of optical modes produced from a non-degenerate optical
parametric amplifier (NOPA) can be enhanced and manipulated phase-sensitively
by means of another NOPA. When both NOPAs operate at de-amplification, the
entanglement degree is increased at the cavity resonance of the second NOPA.
When the first NOPA operates at de-amplification and the second one at
amplification, the spectral features of the correlation variances are
significantly changed. The experimental results are in good agreement with the
theoretical expectation.
