Realization of the Einstein–Podolsky–Rosen paradox in the far-field of the optical parametric oscillator above threshold

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Abstract
We consider an optical parametric oscillator above threshold; under appropriate conditions and in the far-field, the emitted signal is composed by two spatially separate beams, highly correlated on the quantum level. We show that this system can realize the Einstein–Podolsky–Rosen paradox for continuous and macroscopic observables. The pair of non-commuting obserables considered are the intensity and the phase of the two separate beams; the spectral variances of the fluctuation in the intensity difference and in the phase sum of the two beams are calculated. The conditions for which the product of the spectral variances is below the limit settled by the Heisenberg uncertainty principle are studied.