We study theoretically and experimentally the quantum properties of a type II frequency degenerate optical parametric oscillator below threshold with a quarter-wave plate inserted inside the cavity which induces a linear coupling between the orthogonally polarized signal and idler fields. This original device provides a good insight into general properties of two-mode gaussian states, illustrated in terms of covariance matrix. We report on the experimental generation of two-mode squeezed vacuum on non-orthogonal quadratures depending on the plate angle. After a simple operation, the entanglement is maximized and put into standard form, \textit{i.e.} quantum correlations and anti-correlations on orthogonal quadratures. A half-sum of squeezed variances as low as 0.33±0.02, well below the unit limit for inseparability, is obtained and the entanglement measured by the entropy of formation.
@article{arxiv.quant-ph/0410081,
title = {Experimental Generation of Large Quadrature EPR Entanglement with a Self-Phase-Locked Type II OPO Below Threshold},
author = {Julien Laurat and Thomas Coudreau and Gaelle Keller and Nicolas Treps and Claude Fabre},
journal= {arXiv preprint arXiv:quant-ph/0410081},
year = {2009}
}