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It turns out that the phases of the twin beams are quantum correlated as well, leading to [[Quantum entanglement|entanglement]], theoretically predicted in 1988.<ref>{{cite journal|author1=M. D. Reid |author2=P. D. Drummond |lastauthoramp=yes |journal=Phys. Rev. Lett. |volume=60|page=2731|doi=10.1103/PhysRevLett.60.2731 |year=1988|title=Quantum Correlations of Phase in Nondegenerate Parametric Oscillation|issue=26|bibcode=1988PhRvL..60.2731R |pmid=10038437}}</ref> Below threshold, entanglement was measured for the first time in 1992,<ref>{{cite journal|author1=Z. Y. Ou |author2=S. F. Pereira |author3=H. J. Kimble |author4=K. C. Peng |last-author-amp=yes |journal=Phys. Rev. Lett. |volume=68|page=3663|doi=10.1103/PhysRevLett.68.3663 |year=1992|title=Realization of the Einstein-Podolsky-Rosen paradox for continuous variables|issue=25|bibcode=1992PhRvL..68.3663O |pmid=10045765}}</ref> and in 2005 above threshold.<ref>{{cite journal|author1=A. S. Villar |author2=L. S. Cruz |author3=K. N. Cassemiro |author4=M. Martinelli |author5=P. Nussenzveig |last-author-amp=yes |journal=Phys. Rev. Lett. |volume=95|page=243603|doi=10.1103/PhysRevLett.95.243603 |year=2005|title=Generation of Bright Two-Color Continuous Variable Entanglement|issue=24|bibcode=2005PhRvL..95x3603V|arxiv = quant-ph/0506139 |pmid=16384378}}</ref>
Above threshold, the pump beam depletion makes it sensitive to the quantum phenomena happening inside the crystal. The first measurement of squeezing in the pump field after parametric interaction was done in 1997.<ref name="KasaiJiangrui1997">{{cite journal|last1=Kasai|first1=K|last2=Jiangrui|first2=Gao|last3=Fabre|first3=C|title=Observation of squeezing using cascaded nonlinearity|journal=Europhysics Letters (EPL)|volume=40|issue=1|year=1997|pages=25–30|issn=0295-5075|doi=10.1209/epl/i1997-00418-8|bibcode=1997EL.....40...25K|citeseerx=10.1.1.521.1373}}</ref>
It has been recently predicted that all three fields (pump, signal and idler) must be entangled,<ref>{{cite journal|author1=A. S. Villar |author2=M. Martinelli |author3=C Fabre |author4=P. Nussenzveig |last-author-amp=yes |journal=Phys. Rev. Lett. |volume=97|page=140504|doi=10.1103/PhysRevLett.97.140504 |year=2006|title=Direct Production of Tripartite Pump-Signal-Idler Entanglement in the Above-Threshold Optical Parametric Oscillator|issue=14|bibcode=2006PhRvL..97n0504V|arxiv = quant-ph/0610062 |pmid=17155232}}</ref> a prediction which was experimentally demonstrated by the same group.<ref>{{cite journal | last1 = Coelho | first1 = A. S. | last2 = Barbosa | first2 = F. A. S. | last3 = Cassemiro | first3 = K. N. | last4 = Villar | first4 = A. S. | last5 = Martinelli | first5 = M. | last6 = Nussenzveig | first6 = P. | year = 2009 | title = Three-Color Entanglement | url = http://www.sciencemag.org/content/326/5954/823.abstract | journal = Science | volume = 326 | issue = 5954| pages = 823–826 | doi=10.1126/science.1178683|arxiv = 1009.4250 |bibcode = 2009Sci...326..823C }}</ref>
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