  We demonstrate coherent control of cyclotron resonance (CR) in a
two-dimensional electron gas (2DEG). We use a sequence of terahertz pulses to
control the amplitude of CR oscillations in an arbitrary fashion via
phase-dependent coherent interactions. We observe a self-interaction effect,
where the 2DEG interacts with the terahertz field emitted by itself within the
decoherence time, resulting in a revival and collapse of quantum coherence.
These observations are accurately describable using {\em single-particle}
optical Bloch equations, showing no signatures of electron-electron
interactions, which verifies the validity of Kohn's theorem for CR in the
coherent regime.
