In this question on Quora: What is meant by "the photon is its own antiparticle"?, an answer is given which states:
In quantum theory, we have a procedure for transforming the wave function of a particle into that of an antiparticle. It is called the "CP" operation.
Unfortunately, the author of the answer does not explain what the "CP" operation is.
What is the "CP" Operation in the context of anti-particles?