  It is shown, by considering the case of the harmonic oscillator, that quantum
fluctuations may be the most significant contribution to the random walk of a
single molecule. From this point, the controversy on the existence of a
standard quantum limit (SQL) is addressed and settled on the experimental
ground. Comparisons to the experimental data yet avalaible in the literature
provide a new estimate of the reduced Planck constant yielding \hbar = (1.1 \pm
0.2) ~ 10^{-34} J.s.
