We study the statistics of multiphoton absorption/emission by a mesoscopic ring thread by a time-dependent flux Φ(t). For a harmonic Φ(t)=Φcos(ωt) a probability of absorbing/emitting n quanta ℏω is computed. For a weak pumping and low frequencies where the random matrix theory (RMT) applies, the multiphoton processes with n>1 are negligible and the energy absorption is ultra-quantum. Increasing the pumping intensity RMT breaks down and multiphoton processes emerge. At strong pumping the typical n is large and is described by a classical Drude picture of continuous energy absorption given by the work done by an external electric field E(t)=-∂_{t}Φ(t) during a ballistic motion between successive scattering off impurities. This quantum to classical crossover in energy absorption is described within the Keldysh diagrammatic technique which leads to a phenomenology of Continuous Time Random Walks in the energy space with corrections due to interference effects and anisotropy of impurity scattering. |