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== Two photon interference ==

The interference of light was first studied by Thomas Young with his famous double slit experiment. This experiment showed first-order interference. Showed second-order interference of light and the first time experiment is by Hanbury Brown and Twiss using an intensity interferometer.<ref>R.Hanbury Brown and Twiss, Nature (London) 177, 27-32 (1956)</ref>
 
Another type of interferometer which use the photon pairs from the spontaneous parametric down-conversion(SPDC) was built and used to show the quantum nature of light in 1987, which is called the 'Hong-Ou-Mandel dip' experiment.<ref>CK Hong, ZY Ou, L Mandel, Phys. Rev. Lett 59, 2044 (1987))[http://prola.aps.org/pdf/PRL/v59/i18/p2044_1 url]</ref> 
[[Image:setup.jpg|frame|right|Fig.1. Geometry of the experiment]]





As shown in Fig.1, Generation of a photon pair is realized via parametric down conversion.Two photons where overlapped on a beam splitter spatially and temporally. The coincidence counts dropped down when the path lengths of the photons became identical.(Fig.2)

[[Image:coincident.jpg|frame|center|Fig.2. Single rate and coincident rate are measured for path delay]]


References: <references/>


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== Photon bunching ==

In recent years, there is an interesting subject of the study of the thermal nature in the process of spontaneous parametric down-conversion (SPDC)<ref>P.R.Tapster and J.G.Rarity, J.Mod.Optic,45,595(1998)</ref><ref>B. Blauensteiner, I. Herbauts,S. Bettelli, etc. ArXiv: 0810.4785v1 [quant-ph], 27, 2008.</ref>. Since in this process photons from a pump laser beams are randomly converted into pairs of photons at lower frequency. Bunched light consists of a stream of photons with the photons all clumped together in bunches. In 1987, Yurke and Potasek have investigated the thermal nature of the field in one arm of a SPDC source)<ref>B. Yurke and M. Potasek, Phys. Rev. A 36, 3464 (1987).</ref>. We will measure the photon statistics of weak light pulse emitted in the process of pulsed parametric down-conversion.


Fig.3 is the experimental setup

[[Image:experimental setup.jpg|frame|center|Fig.3. experimental setup]]





References: <references/>