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The possibility to generate multipartite entanglement in the laboratory allows further investigations of the structure of quantum correlations among many parties. We employ the optical parametric oscillator to directly produce three entangled bright beams of light and study their properties. In this tripartite continuous system, we observe a mixture of behaviours common to discrete systems pertaining to both the GHZ and the W classes of tripartite entangled states.

Recent experiments with Optical Parametric Oscillators have shown [1–3] that an unpredicted extra noise on the phase of the fields reduces the level of squeezing or quantum correlations. In some cases, it can hinder the attempts to observe entanglement in those systems [4]. It is probably the main reason why signal and idler entanglement above threshold remained unobserved by almost 20 years after its prediction [5].

In systems described by continuous variables (CV), multipartite entanglement can be produced by combination of squeezed states of electromagnetic fields in beam splitters [1]. Entanglement in this case is limited to the same frequency for all parties involved, owing to its interferometric generation. So far, experimental entanglement between more than two systems of continuous variables has been restricted to this method. In order to generate multi-wavelength entanglement, nonlinear processes are required to couple different modes of the field, e.g. the bipartite entanglement demonstration in ref. [2]. We presently demonstrate the nonlinear system of the smallest order for direct tripartite entanglement production. As described in [3], an Optical Parametric Oscillator operating above threshold produces an inseparable state of pump, signal and idler modes. Previous attempts [4] had been frustrated by the presence of an extra noise source, which we have shown to be generated by photon scattering of light due to thermal phonons of the crystal. We have achieved the reduction of this noise by cooling the KTP crystal of our OPO (described in ref. [4]) down to −10° 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.

This paper presents an experimental demonstration of three-color quantum correlations between the bright pump, signal, and idler beams produced by an optical parametric oscillator (OPO). This paper shows that the degree of entanglement between the twin beams can be increased by using information from the pump beam.

In this work, first measurements that indicate entanglement between non-degenerate signal and idler beams above threshold are presented. These curves are obtained synchronously varying analysis cavities' lengths around their resonances.