  A suitable sequence of sharp pulses applied to a spin coupled to a bosonic
bath can cool its state, i.e., increase its polarization or ground state
occupation probability. Starting from an unpolarized state of the spin in
equilibrium with the bath, one can reach very low temperatures or sizeable
polarizations within a time shorter than the decoherence time. Both the bath
and external fields are necessary for the effect which comes from the
backreaction of the spin on the bath. This method can be applied to cool at
once a disordered ensemble of spins. Since the bath is crucial for this
mechanism, the cooling limits are set by the strength of its interaction with
the spin(s).
