How are entangled photons created? From what I understand the prediction of entanglement is based the conservation of momentum. In that case, you can’t leave behind any momentum in the device that creates the pair. That must be hard to do. Any mistake would make the photons have different wavelengths or different spins (polarization, whatever). I understand stats and averages are important. Still the device could be consistently bad. How can you get the wavelength and spin consistent between the 2 photons?