<p>The on-demand selective population transfer between states in multilevel quantum systems is a challenging problem with implications for a wide range of physical platforms including photon and non-equilibrium exciton–polariton condensates. Here we introduce a universal strategy for this selective transfer based on a strong time-periodic energy modulation, which is experimentally demonstrated by using a gigahertz acoustic wave to control the gain and loss of confined modes of exciton–polariton condensates in a microcavity. The harmonic acoustic field shifts the energy of the excitonic component relative to the photonic ones, which generates a dynamic population transfer within a multimode condensate that can be controlled by the acoustic amplitude. In this way, the full condensate population can be selectively transferred to the ground state to yield a single-level emission consisting of a spectral frequency comb with gigahertz repetition rates as well as picosecond-scale correlations. A theoretical model reproduces the observed time evolution and reveals a dynamical interplay between bosonic stimulation and the adiabatic Landau–Zener-like population transfer. Our approach provides a new avenue for the Floquet engineering of light–matter systems and enables tunable single- or multiwavelength ultrafast pulsed laser-like emission for information technologies.</p>