  Josephson junctions enable dissipation-less electrical current through metals
and insulators below a critical current. Despite being central to quantum
technology based on superconducting quantum bits and fundamental research into
self-conjugate quasiparticles, the spatial distribution of super current flow
at the junction and its predicted evolution with current bias and external
magnetic field remain experimentally elusive. Revealing the hidden current
flow, featureless in electrical resistance, helps understanding unconventional
phenomena such as the nonreciprocal critical current, i.e., Josephson diode
effect. Here we introduce a platform for nanoscale, quantitative, and
nonperturbative visualization of the super current flow, overcoming the
long-standing challenge. Utilizing a scanning magnetometer based on nitrogen
vacancy centers in diamond, we uncover competing ground states electrically
switchable within the zero-resistance regime. The competition results from the
superconducting phase re-configuration induced by the Josephson current and
kinetic inductance of thin-film superconductors. We further identify a new
mechanism for the Josephson diode effect involving the Josephson current
induced phase. The nanoscale super current flow emerges as a new experimental
observable for elucidating unconventional superconductivity, and optimizing
quantum computation and energy-efficient devices.
