  We investigate the dynamics of a single chiral active particle subject to an
external torque due to the presence of a gravitational field. Our computer
simulations reveal an arbitrarily strong increase of the long-time diffusivity
of the gravitactic agent when the external torque approaches the intrinsic
angular drift. We provide analytic expressions for the mean-square displacement
in terms of eigenfunctions and eigenvalues of the noisy-driven-pendulum
problem. The pronounced maximum in the diffusivity is then rationalized by the
vanishing of the lowest eigenvalues of the Fokker-Planck equation for the
angular motion as the rotational diffusion decreases and the underlying
classical bifurcation is approached. A simple harmonic-oscillator picture for
the barrier-dominated motion provides a quantitative description for the onset
of the resonance while its range of validity is determined by the crossover to
a critical-fluctuation-dominated regime.
