  Topology is a powerful recent concept asserting that quantum states could be
globally protected against local perturbations. Dissipationless topologically
protected states are thus of major fundamental interest as well as of practical
importance in metrology and quantum information technology. Although
topological protection can be robust theoretically, in realistic devices it is
often fragile against various dissipative mechanisms, which are difficult to
probe directly because of their microscopic origins. By utilizing scanning
nanothermometry, we visualize and investigate microscopic mechanisms
undermining the apparent topological protection in the quantum Hall state in
graphene. Our simultaneous nanoscale thermal and scanning gate microscopy shows
that the dissipation is governed by crosstalk between counterpropagating pairs
of downstream and upstream channels that appear at graphene boundaries because
of edge reconstruction. Instead of local Joule heating, however, the
dissipation mechanism comprises two distinct and spatially separated processes.
The work generating process that we image directly and which involves elastic
tunneling of charge carriers between the quantum channels, determines the
transport properties but does not generate local heat. The independently
visualized heat and entropy generation process, in contrast, occurs nonlocally
upon inelastic resonant scattering off single atomic defects at graphene edges,
while not affecting the transport. Our findings offer a crucial insight into
the mechanisms concealing the true topological protection and suggest venues
for engineering more robust quantum states for device applications.
