Generation of two-color polarization-entangled optical beams with a   self-phase-locked two-crystal Optical Parametric Oscillator
  A new device to generate polarization-entangled light in the continuous variable regime is introduced. It consists of an Optical Parametric Oscillator with two type-II phase-matched non-linear crystals orthogonally oriented, associated with birefringent elements for adjustable linear coupling. We give in this paper a theoretical study of its classical and quantum properties. It is shown that two optical beams with adjustable frequencies and well-defined polarization can be emitted. The Stokes parameters of the two beams are entangled. The principal advantage of this setup is the possibility to directly generate polarization entangled light without the need of mixing four modes on beam splitters as required in current experimental setups. This device opens new directions for the study of light-matter interfaces and generation of multimode non-classical light and higher dimensional phase space. 
Julien Laurat (CALTECH), Ga\"elle Keller (LKB - Jussieu), Claude Fabre   (LKB - Jussieu), Thomas Coudreau (LKB - Jussieu, MPQ)