  Crystallization and vitrification are two different routes to form a solid.
Normally these two processes suppress each other, with the glass transition
preventing crystallization at high density (or low temperature). This is even
true for systems of colloidal hard spheres, which are commonly used as building
blocks for novel functional materials with potential applications, e.g.
photonic crystals. By performing Brownian dynamics simulations of glassy
systems consisting of mixtures of active and passive hard spheres, we show that
the crystallization of such hard-sphere glasses can be dramatically promoted by
doping the system with small amounts of active particles. Surprisingly, even
hard-sphere glasses of packing fraction up to $\phi = 0.635$ crystallize, which
is around $0.5\%$ below the random close packing at $\phi \simeq 0.64$. Our
results suggest a novel way of fabricating crystalline materials from
(colloidal) glasses. This is particularly important for materials that get
easily kinetically trapped in glassy states, and crystal nucleation hardly
occurs.
