  Rydberg alkaline earth atoms are promising tools for quantum simulation and
metrology. When one of the two valence electrons is promoted to long-lived
circular states, the second valence electron can be optically manipulated
without significant autoionization. We harness this feature to demonstrate
laser slowing of a thermal atomic beam of circular strontium atoms. By driving
the main ion core 422 nm wavelength resonance, we observe a velocity reduction
of 50 m/s without significant autoionization. We also show that a superposition
of circular states undergoes very weak decoherence during the cooling process,
up to the scattering of more than thousand photons. This robustness opens new
perspectives for quantum simulations over long timescales with circular atoms,
while simultaneously cooling their motional state. It makes it possible to
mitigate the harmful effects of unavoidable heating due to spin-motion coupling
during a quantum simulation.
