  We report on the observation of collective radiative decay, or superradiance,
of cyclotron resonance (CR) in high-mobility two-dimensional electron gases in
GaAs quantum wells using time-domain terahertz magnetospectroscopy. The decay
rate of coherent CR oscillations increases linearly with the electron density
in a wide range, which is a hallmark of superradiant damping. Our fully quantum
mechanical theory provides a universal formula for the decay rate, which
reproduces our experimental data without any adjustable parameter. These
results firmly establish the many-body nature of CR decoherence in this system,
despite the fact that the CR frequency is immune to electron-electron
interactions due to Kohn's theorem.
