Direct production of three entangled fields at different wavelengths

This paper demonstrates the nonlinear system of the smallest order for direct tripartite entanglement production. An optical parametric oscillator operating above threshold produces an inseparable state of pump, signal and idler modes. Previous attempts had been frustrated by the presence of an extra noise source, which is shown to be generated by photon scattering of light due to thermal phonons of the crystal. Reduction of this noise is achieved by cooling the KTP crystal of OPO down to -10deg C. Reflected pump field, signal and idler fields are measured after reflection by empty cavities (enabling the measurement of amplitude or phase quadrature) using efficient photodiodes, allowing the complete measurement of the covariance matrix. The photocurrent component at 21 MHz, used in the analysis, was registered with the help of an analog-to-digital card and a computer.