Parametric down conversion

[Demystifier]

Do the two photons (signal and idler) created by PDC always have equal energy?
(Of course, it depends on the Lorentz frame, but I mean in the frame in which the nonlinear crystal is at rest.)

[DrChinese]

Do the two photons (signal and idler) created by PDC always have equal energy?
(Of course, it depends on the Lorentz frame, but I mean in the frame in which the nonlinear crystal is at rest.)

They are close to equal, but that is not a requirement of the PDC crystal output stream. It actually follows conservation of energy (you would expect that) and there is a spread of outputs. (The sum of the output frequencies is equal to the input frequency.)

Here is a good reference that shows the spread of output wavelengths/frequencies in Figure 1:

Entangled photon apparatus for the undergraduate laboratory (http://arxiv.org/abs/quant-ph/0205172)