Role of quantum effects in the glass transition*

It is shown that quantum effects lead to a significant decrease of the glass transition temperature $T_{g}$ with respect to the melting temperature $T_{m}$, so that the ratio $T_{g}$/$T_{m}$ can be much smaller than the typical value of 2/3 in materials where $T_{g}$ is near or below $\sim$ 60 K. Furthermore, it is demonstrated that the viscosity or structural relaxation time in such low temperature glass-formers should exhibit highly unusual temperature dependence, namely a decrease of the apparent activation energy upon approaching $T_{g}$ (instead of traditional increase).