  In a Josephson junction, which is the central element in superconducting
quantum technology, irreversibility arises from abrupt slips of the
gauge-invariant quantum phase difference across the contact. A quantum phase
slip (QPS) is often visualized as the tunneling of a flux quantum in the
transverse direction to the superconducting weak link, which produces
dissipation. In this work, we detect the instantaneous heat release caused by a
QPS in a Josephson junction using time-resolved electron thermometry on a
nanocalorimeter, signaled by an abrupt increase of the local electronic
temperature in the weak link and subsequent relaxation back to equilibrium.
Beyond providing a cornerstone in experimental quantum thermodynamics in form
of observation of heat in an elementary quantum process, this result sets the
ground for experimentally addressing the ubiquity of dissipation, including
that in superconducting quantum sensors and qubits.
