Good evening. Usually we say that during light-matter interaction, energy of light should be transferred piece by piece, rather than continuously, with each piece equals hbar*ω. But in photon echo experiment, we say atoms are excited into a "superposition state" of ground state and 1st excited. For simplicity, we suggest a (1/sqrt(2)) (|0>+|1>) superposition state, thus expectation of energy of this state is 1/2*hbar*ω (incident light is on resonance). So it seems that each atom absorbs half photon, is that correct? Thanks a lot.