  We report a quantitative experimental study of the crystallization kinetics
of supercooled quantum liquid mixtures of para-hydrogen (pH$_2$) and
ortho-deuterium (oD$_2$) by high spatial resolution Raman spectroscopy of
liquid microjets. We show that in a wide range of compositions the
crystallization rate of the isotopic mixtures is significantly reduced with
respect to that of the pure substances. To clarify this behavior we have
performed path-integral simulations of the non-equilibrium pH$_2$-oD$_2$ liquid
mixtures, revealing that differences in quantum delocalization between the two
isotopic species translate into different effective particle sizes. Our results
provide first experimental evidence for crystallization slowdown of quantum
origin, offering a benchmark for theoretical studies of quantum behavior in
supercooled liquids.
