Is it possible for two photons to join their energy to structure a one photon of higher frequency? Let's assume that the two incoming (IR) photons are prepared specially: coming in the same time, same wavelength, phase, appropriate (complementer) spins and angle, etc. They arrive the appropriate atom and giving the energy at the same time. The atom gets energized and releases one single photon (UV) with the double energy. 1. This is possible... OK, but why this is not happening in case of the photoelectric effect? 2. This is not possible, because of ... (the energy-time uncertainty?). What do you think? Thanks for the anwsers in advance.