Topological contribution of the nulltet supercurrent

Determine whether the dynamically stabilized nonlocal supercurrent (the "nulltet") that couples two contacts which are both voltage-biased relative to a common superconducting island in a three-junction Josephson circuit carries a topological contribution.

Background

The work studies a Josephson circuit where three superconducting electrodes are connected to a floating superconducting island via separate Josephson junctions. In addition to conventional supercurrents between each contact and the island, the authors observe multiplet resonances (including quartets) and a novel nonlocal supercurrent between two contacts that are both voltage-biased with respect to the island.

This novel branch, termed the "nulltet," arises from synchronization of the phase dynamics and exhibits distinctive Shapiro-step behavior. Given theoretical proposals that similar circuit-mediated multiplet phenomena can host topological features controlled by Josephson energy scales, the authors explicitly raise whether the observed nulltet supercurrent may also carry a topological contribution.