What would happen if you could fuse two photons together? How do you use anti matter's potential energy?

Answer 1

I'll answer the second question first, since it relates to the first question. When a particle and its corresponding antiparticle come into contact, they annihilate, typically producing two gamma ray photons. Those photons can then interact with matter to produce energy in more usable form, like heat for instance. As to the first question, there is something called CPT invariance in physics which implies that the reverse process is possible. That is, theoretically two photons can inelastically collide and produce a particle-antiparticle pair if their initial energy is high enough. That's not exactly "fusing", though, since the photons disappear altogether instead of getting squeezed together into a stable bound state, like nuclear fusion.

Answer 2

The first question doesn't make much sense. The answer to the second one is that you release the rest energy of anti-matter by annihilating it with its antiparticle.