  While unusual normal state properties, such as non-Fermi liquid behavior of
the resistivity, are commonly associated with strong quantum fluctuations,
evidence for its presence inside the superconducting dome are much scarcer. In
this paper, we use sensitive and minimally invasive optical magnetometry based
on NV-centers in diamond to probe the doping evolution of the $T=0$ penetration
depth in the electron-doped iron-based superconductor
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. A non-monotonic evolution with a pronounced
peak in the vicinity of the putative magnetic QPT is found. This behavior is
reminiscent to that previously seen in isovalently-substituted
BaFe$_2$(As$_{1-x}$P$_x$)$_2$ compounds, despite the notable differences
between these two systems. Whereas the latter is a very clean system that
displays nodal superconductivity and a single simultaneous first-order
nematic-magnetic transition above, and even somewhat below, $T_c$, the former
is a significantly dirtier system with fully gapped superconductivity and split
second-order nematic and magnetic transition above $T_c$. Thus our observation
that such distinct systems display remarkably similar penetration depth peaks,
combined with the theoretical result that a QPT alone does not ensure the
existence of a peak, unveils a puzzling and seemingly universal manifestation
of quantum fluctuations in the iron pnictides.
