We predict a spatially varying mode population to appear in a ballistic quantum channel formed in the two-dimensional electron gas of a gated GaAs/AlGaAs heterostructure due to pumping by a THz field. If a resonant coupling between two modes is suddenly switched on at the entrance, Rabi oscillations in the mode population will arise. We propose to use an array of gates in order to simulate a moving quantum point contact for detecting mode population oscillations since they discriminate between different modes. By consecutively activating them we expect to see both photovoltaic effects and photoconductive effects that can easily be distinguished from noise. ©1997 American Institute of Physics.