We have experimentally demonstrated how two beams of light separated by an octave in frequency can become entangled after their interaction in a second-order nonlinear medium. The entangler consisted of a nonlinear crystal placed within an optical resonator that was strongly driven by coherent light at the fundamental and second-harmonic wavelengths. An inter-conversion between the fields created quantum correlations in the amplitude and phase quadratures, which were measured by two independent homodyne detectors. Analysis of the resulting correlation matrix revealed a wavefunction inseparability of 0.74(1) < 1 thereby satisfying the criterion of entanglement.
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