In contrast to ensemble-averaged
measurements, single-molecule
experiments directly display the heterogeneity of molecular properties
in space and time. In many complex systems, spatial heterogeneity
is regularly accompanied by temporal or dynamic heterogeneity; if
a property differs from molecule to molecule, it will often vary in
time for one and the same molecule. In this short paper, we discuss
a few examples of complex systems where dynamical heterogeneity was
observed in single-molecule or single-particle optical signals. For
single biomolecules, the first demonstration of dynamic heterogeneity
in a single enzyme was provided by Xie and colleagues. Other examples
are found in glassy systems, and very recently in the magnetic relaxation
of single superparamagnetic nanoparticles. The ubiquity of this phenomenon
suggests that, rather than an exception, dynamic heterogeneity is
the rule in complex systems with multiple degrees of freedom.