  Current flow in electronic devices can be asymmetric with bias direction, a
phenomenon underlying the utility of diodes and known as non-reciprocal charge
transport. The promise of dissipationless electronics has recently stimulated
the quest for superconducting diodes, and non-reciprocal superconducting
devices have been realized in various non-centrosymmetric systems. Probing the
ultimate limits of miniaturization, we have created atomic-scale Pb--Pb
Josephson junctions in a scanning tunneling microscope. Pristine junctions
stabilized by a single Pb atom exhibit hysteretic behavior, confirming the high
quality of the junctions, but no asymmetry between the bias directions.
Non-reciprocal supercurrents emerge when inserting a single magnetic atom into
the junction, with the preferred direction depending on the atomic species.
Aided by theoretical modelling, we trace the non-reciprocity to quasiparticle
currents flowing via Yu-Shiba-Rusinov (YSR) states inside the superconducting
energy gap. Our results open new avenues for creating atomic-scale Josephson
diodes and tuning their properties through single-atom manipulation.
