  We experimentally study the motion of light-activated colloidal microswimmers
in a viscoelastic fluid. We find that, in such a non-Newtonian environment, the
active colloids undergo an unexpected transition from enhanced angular
diffusion to persistent rotational motion above a critical propulsion speed,
despite their spherical shape and stiffness. We observe that, in contrast to
chiral asymmetric microswimmers, the resulting circular orbits can
spontaneously reverse their sense of rotation and exhibit an angular velocity
and a radius of curvature that non-linearly depend on the propulsion speed. By
means of a minimal non-Markovian Langevin model for active Brownian motion, we
show that these non-equilibrium effects emerge from the delayed response of the
fluid with respect to the self-propulsion of the particle without counterpart
in Newtonian fluids.
