Phase signature of topological transition in Josephson junctions*

Topological transition transforms common superconductivity into an exotic phase of matter, which holds promise for fault-tolerant quantum computing. A hallmark of this transition is the emergence of Majorana states. While two-dimensional semiconductor/superconductor heterostructures are desirable platforms for topological superconductivity, direct phase-measurements as the fingerprint of the underlying topological transition have been missing.
On gate tunable Josephson junctions made on epitaxial Al/InAs, we observe a closing and a reopening of the superconducting gap with increasing in-plane magnetic field. Since our junctions are embedded into a phase-sensitive SQUID, we are able to measure a π-jump in the superconducting phase across the junction coincident with the closing and reopening of the superconducting gap. Theoretical simulations confirm this transition is topological and compatible with the emergence of Majorana states while the magnetic field angle dependence of the transition further constrain this scenario. Remarkably, in each junction, this topological transition can be controlled by changing the gate voltage. These findings reveal versatile two-dimensional platforms for scalable topological quantum computing.