A Josephson phase battery

A phase battery is a quantum device that provides a persistent phase bias to the wave function of a quantum circuit. A hybrid superconducting and magnetic circuit containing two anomalous Josephson junctions can provide a tunable Josephson phase that persists in the absence of external stimuli. A classical battery converts chemical energy into a persistent voltage bias that can power electronic circuits. Similarly, a phase battery is a quantum device that provides a persistent phase bias to the wave function of a quantum circuit. It represents a key element for quantum technologies based on phase coherence. Here we demonstrate a phase battery in a hybrid superconducting circuit. It consists of an n-doped InAs nanowire with unpaired-spin surface states, that is proximitized by Al superconducting leads. We find that the ferromagnetic polarization of the unpaired-spin states is efficiently converted into a persistent phase bias phi(0)across the wire, leading to the anomalous Josephson effect(1,2). We apply an external in-plane magnetic field and, thereby, achieve continuous tuning of phi(0). Hence, we can charge and discharge the quantum phase battery. The observed symmetries of the anomalous Josephson effect in the vectorial magnetic field are in agreement with our theoretical model. Our results demonstrate how the combined action of spin-orbit coupling and exchange interaction induces a strong coupling between charge, spin and superconducting phase, able to break the phase rigidity of the system.