I have been getting conflicting information so here are my questions, re: 1. Single photon interference 2. Interference between two photons 3. Two-photon interference 1. Some literature says that only single photon/particle interference exists. Can two photons interfere with each other? 2. Can the "waves" of two photons interfere with each other? By waves I mean as posited by the DeBroglie-Bohm (DBB) interpretation. i.e. if two photons are sent separately through their own set of double slits; as we bought the double slits closer would there be interference between the "waves" of photon A with that of photon B? we are talking about - a total of 4 slits i.e. two double slits being brought closer. 3. Does two-photon = entangled photons/particles? 4. Do two-photon interfere? 5. Can they do so non-locally? 6. What is the relation between relation between two-photon interference and coherence?