Correlated photons are created in the χ⁽²⁾ nonlinear process of spontaneous parametric downconversion (SPDC), when a high energy pump photon ω_p interacts with a non-linear medium such that it decays into two lower energy photons under the constraints of energy and momentum conservation^[1]. Photons are created almost simultaneously^[2] in the other two fields at frequencies ω₁ and ω₂, although the time of emission t of the photon pairs is random^[3] (see Figure 1):

Figure 1:  Schematic of Type I downconversion

Detection of one of these photons indicates the existence of its twin, and if its direction and/or energy are also measured, the conservation rules allow the direction and/or energy of its twin to be inferred. These ‘twin’ photons are said to be correlated. The polarisations of the twin photons are also correlated, and are either the same or orthogonal, depending on the particular downconversion process employed.

Correlated photons have been used to demonstrate fundamental quantum mechanics and developed to carry out quantum-enhanced measurements. Click on the images below for more detail:

References

1. Y. B. Zel'dovich and D. N. Klyshko, JETP Letters, 9 (1969)
2. C. K. Hong, Z. Y. Ou and L. Mandel, Physical Review Letters, 59 18 (1987)
3. J. D. Franson, Physical Review Letters, 62 19 (1989)

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