  Topological defects are ubiquitous from solid state physics to cosmology,
where they drive phase transitions by proliferating as domain walls, monopoles
or vortices. As quantum excitations, they often display fractional charge and
anyonic statistics, making them relevant to topologically protected quantum
computation, but realizing a controlled physical resource for topological
excitations has been difficult. Here we report evidence of topological
excitations during the localization transition in strongly interacting
two-dimensional electron systems (2DESs) in GaAs/AlGaAs heterostructures. We
find the electrical conductivity at low electron densities to follow a
Berezinskii-Kosterlitz Thouless (BKT)-like order-disorder transition implying a
gate-tunable proliferation of charged topological defects. At low temperatures,
a weakening in the temperature dependence of conductivity was observed, and
linked to the zero point fluctuations and delocalization of the defects. Our
experiments also cast crucial insight on the nature of the ground state in
strongly interacting 2DESs in presence of disorder.
