Questions

From one side we know the rule (Fresnel-Arago law) that orthogonal states do not interfere (like in quantum erasure experiment). For instance, orthogonal states of photon do not interfere. On the other side, the interference must happen when we join two streams of electrons in Stern-Gerlach experiment. How to resolve this collision?

If a photon can only (?) interfere with itself, can two coherent laser beams interfere? How?

Can a photon decay to two parallel photons?

Is two photon state "naturally" entangled? It least, it looks to be non-separable.

How to consider coordinate delta-function vs Hilbert space with countable basis?