Editing Spontaneous parametric down-conversion (section)

== Alternatives ==
The newly observed effect of [[two-photon emission]] from electrically driven semiconductors has been proposed as a basis for more efficient sources of entangled photon pairs.<ref>{{cite journal | last1=Hayat | first1=Alex | last2=Ginzburg | first2=Pavel | last3=Orenstein | first3=Meir | title=Observation of two-photon emission from semiconductors | journal=Nature Photonics | publisher=Springer Science and Business Media LLC | volume=2 | issue=4 | date=2008-03-02 | issn=1749-4885 | doi=10.1038/nphoton.2008.28 | pages=238–241}}</ref> Other than SPDC-generated photon pairs, the photons of a semiconductor-emitted pair usually are not identical but have different energies.<ref>{{Cite journal|doi=10.1051/0004-6361:20053988|arxiv=astro-ph/0508144|title=Induced two-photon decay of the 2s level and the rate of cosmological hydrogen recombination|journal=Astronomy and Astrophysics|volume=446|issue=1|pages=39–42|year=2006|last1=Chluba|first1=J.|last2=Sunyaev|first2=R. A.|bibcode=2006A&A...446...39C|s2cid=119526307 }}</ref> Until recently, within the constraints of quantum uncertainty, the pair of emitted photons were assumed to be co-located: they are born from the same location. However, a new nonlocalized mechanism for the production of correlated photon pairs in SPDC has highlighted that occasionally the individual photons that constitute the pair can be emitted from spatially separated points.<ref>{{Cite journal|last1=Forbes|first1=Kayn A.|last2=Ford|first2=Jack S.|last3=Andrews|first3=David L.|date=2017-03-30|title=Nonlocalized Generation of Correlated Photon Pairs in Degenerate Down-Conversion|journal=Physical Review Letters|volume=118|issue=13|pages=133602|doi=10.1103/PhysRevLett.118.133602|pmid=28409956|bibcode=2017PhRvL.118m3602F|url=https://ueaeprints.uea.ac.uk/63173/1/PhysRevLett.118.133602.pdf}}</ref><ref>{{Cite journal|last1=Forbes|first1=Kayn A.|last2=Ford|first2=Jack S.|last3=Jones|first3=Garth A.|last4=Andrews|first4=David L.|date=2017-08-23|title=Quantum delocalization in photon-pair generation|journal=Physical Review A|volume=96|issue=2|pages=023850|doi=10.1103/PhysRevA.96.023850|bibcode=2017PhRvA..96b3850F|url=https://ueaeprints.uea.ac.uk/64664/1/Quantum_delocalization_in_photon_pair_generation.pdf}}</ref>