Parity-protected superconducting diode effect in topological Josephson junctions

In bulk superconductors or Josephson junctions formed in materials with spin-orbit interaction, the critical current can depend on the direction of current flow or of applied magnetic field, an effect known as the superconducting (SC) diode effect. Here, we consider the SC diode effect in Josephson junctions in nanowire devices. We find that the $4\ensuremath{\pi}$-periodic contribution of Majorana bound states (MBSs) to the current phase relation (CPR) of single junctions results in a significant enhancement of the SC diode effect when the device enters the topological phase and can therefore be used as an indicator of the phase transition. Crucially, this enhancement of the SC diode effect is independent of the parity of the junction and therefore protected from parity-altering events, such as quasiparticle poisoning, which have hampered efforts to directly observe the $4\ensuremath{\pi}$-periodic CPR of MBSs. We show that this effect can be generalized to SQUIDs and that, in such devices, the parity-protected SC diode effect can provide a highly controllable probe of the topology in a Josephson junction.