Visualizing Supercurrent flow in Josephson junctions

Josephson junctions are building blocks useful in quantum sensing, quantum computing and fundamental research on strong correlation and superconductivity. However, the spatial distribution of super current flow at the junction and its predicted evolution with external bias remain experimentally elusive. Here, we report recent measurements using a scanning nitrogen-vacancy (NV) magnetometer to image the supercurrent flow in a Josephson junction. Revealing the supercurrent flow, which is featureless in electrical resistance, not only demonstrates the evolution of the current by the external bias but helps understand unconventional characteristics such as nonreciprocal critical current, i.e., Josephson diode effect. We also uncover competing ground states that are electrically switchable within the zero-resistance regime. The nanoscale supercurrent flow emerges as a new experimental observable for elucidating unconventional superconductivity and optimizing superconducting devices.