Spontaneous parametric down-conversion
The degenerate portion of the output of a Type I down converter is a squeezed vacuum that contains only even photon number terms.
Due to the conservation of momentum, the two photons are always symmetrically located on the sides of the cones, relative to the pump beam.
In particular, the trajectories of a small proportion of photon pairs will lie simultaneously on the two lines where the surfaces of the two cones intersect.
[6] Some of the characteristics of effective parametric down-converting nonlinear crystals include: SPDC was demonstrated as early as 1967 by S. E. Harris, M. K. Oshman, and R. L. Byer,[7] as well as by D. Magde and H.
[8] It was first applied to experiments related to coherence by two independent pairs of researchers in the late 1980s: Carroll Alley and Yanhua Shih, and Rupamanjari Ghosh and Leonard Mandel.
SPDC is widely used to create pairs of entangled photons with a high degree of spatial correlation.
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.
[15] 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.
