Generation of Bright Two-Color Continuous Variable Entanglement

We present the first measurement of squeezed-state entanglement between the twin beams produced in an optical parametric oscillator operating above threshold. In addition to the usual squeezing in the intensity difference between the twin beams, we have measured squeezing in the sum of phase quadratures. Our scheme enables us to measure such phase anticorrelations between fields of different frequencies. In the present measurements, wavelengths differ by $\approx 1\mathrm{nm}$. Entanglement is demonstrated according to the Duan et al. criterion [ Phys. Rev. Lett. 84, 2722 (2000)] ${\Delta }^2{\widehat{p}}_{-}+{\Delta }^2{\widehat{q}}_{+}=1.41(2)<2$. This experiment opens the way for new potential applications such as the transfer of quantum information between different parts of the electromagnetic spectrum.