In many experiments like the Delayed Choice Quantum Eraser Experiment, scientists use entangled photons.
There are many ways of creating entangled photons, the mainly used methods are:
In this case they use a special crystal, and input a single (pump) photon, and the output is a pair (or more) photons, whose total energy equals the input photons'. Momentum is conserved as well, the total input photon momentum and the output photons' and crystal lattice's momentum is equal.
if the signal and idler photons share the same polarization to each other, but are orthogonal to the pump polarization, it is Type-I SPDC. If the signal and idler photons have perpendicular polarizations, it is deemed Type II SPDC[2]
https://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion
photons emitted from decay cascade of the bi-exciton in a quantum dot,[94] the use of the Hong–Ou–Mandel effect
When light (UV) shines on a quantum dot, the electron can be excited to a higher energy level.
We demonstrate the on-demand emission of polarisation-entangled photon pairs from the biexciton cascade of a single InAs quantum dot embedded in a GaAs/AlAs planar microcavity.
https://arxiv.org/ftp/quant-ph/papers/0601/0601187.pdf
https://en.wikipedia.org/wiki/Quantum_entanglement
In the earliest tests of Bell's theorem, the entangled particles were generated using atomic cascades.
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] Other than SPDC-generated photon pairs, the photons of a semiconductor-emitted pair usually are not identical but have different energies.
