  The first evidence of the dc Josephson effect dates back to 1963 when J. S.
Rowell measured the diffraction pattern of the critical current flowing through
a single superconducting tunnel junction subjected to an in-plane magnetic
field. Interference of Josephson currents through two tunnel junctions
connected in parallel was achieved one year later leading to the first ever
superconducting quantum interferometer. The latter, together with Rowell's
observations, constituted the unequivocal demonstration of the Josephson
supercurrent-phase relation. Yet, the Josephson effect has further profound
implications going beyond electrical transport, as the interplay between the
Cooper condensate and unpaired electrons provides thermal flow through the
junction with phase coherence as well. Here we report the first demonstration
of quantum diffraction of a heat flux showing that a temperature-biased single
Josephson junction is exploited as a diffractor for thermal currents.
Specifically, thermal diffraction manifests itself with a peculiar modulation
of the electron temperature in a small metallic electrode nearby-contacted to
the junction when sweeping the magnetic flux $\Phi$. Remarkably, the observed
temperature dependence exhibits $\Phi$-symmetry and a clear reminiscence with a
Fraunhofer-like modulation pattern, as expected fingerprints for a quantum
diffraction phenomenon. Our results confirm a pristine prediction of quantum
heat transport and, joined with double-junction heat interferometry
demonstrated in Nature 492, 401 (2012), exemplify the complementary and
conclusive proof of the existence of phase-dependent thermal currents in
Josephson-coupled superconductors. This approach combined with well-known
methods for phase-biasing superconducting circuits provides with a novel tool
for mastering heat fluxes at the nanoscale.
