  Quantum spin Hall-superconductor hybrids are promising sources of topological
superconductivity and Majorana modes, particularly given recent progress on
HgTe and InAs/GaSb. We propose a new method of revealing topological
superconductivity in extended quantum spin Hall Josephson junctions supporting
`fractional Josephson currents'. Specifically, we show that as one threads
magnetic flux between the superconductors, the critical current traces an
interference pattern featuring sharp fingerprints of topological
superconductivity---even when noise spoils parity conservation.
