  Based on a variant of the frequency function approach of Almgren([Al]), we
establish an optimal upper bound on the vanishing order of solutions to
stationary Schr\"odinger equations associated to sub-Laplacian on Carnot groups
of arbitrary step. Such bound provides a quantitative form of strong unique
continuation and can be thought of as an analogue of the recent results of
Bakri and Zhu for the standard Laplacian.
