  Energy dissipation is a fundamental process governing the dynamics of
physical, chemical, and biological systems. It is also one of the main
characteristics distinguishing quantum and classical phenomena. In condensed
matter physics, in particular, scattering mechanisms, loss of quantum
information, or breakdown of topological protection are deeply rooted in the
intricate details of how and where the dissipation occurs. Despite its vital
importance the microscopic behavior of a system is usually not formulated in
terms of dissipation because the latter is not a readily measureable quantity
on the microscale. Although nanoscale thermometry is gaining much recent
interest, the existing thermal imaging methods lack the necessary sensitivity
and are unsuitable for low temperature operation required for study of quantum
systems. Here we report a superconducting quantum interference nano-thermometer
device with sub 50 nm diameter that resides at the apex of a sharp pipette and
provides scanning cryogenic thermal sensing with four orders of magnitude
improved thermal sensitivity of below 1 {\mu}K/Hz1/2. The non-contact
non-invasive thermometry allows thermal imaging of very low nanoscale energy
dissipation down to the fundamental Landauer limit of 40 fW for continuous
readout of a single qubit at 1 GHz at 4.2 K. These advances enable observation
of dissipation due to single electron charging of individual quantum dots in
carbon nanotubes and reveal a novel dissipation mechanism due to resonant
localized states in hBN encapsulated graphene, opening the door to direct
imaging of nanoscale dissipation processes in quantum matter.
