Magnetic flux locking in two weakly coupled superconducting rings

TL;DR

This paper investigates quantum interference in two weakly coupled superconducting rings, revealing flux-locked states and transitions that enhance sensitivity to magnetic field gradients.

Contribution

It introduces a detailed analysis of flux locking and state transitions in a macroscopic superconducting molecule with weak Josephson coupling.

Findings

Identification of flux-locked states with zero total flux

Discovery of sudden transitions between states influenced by magnetic flux differences

Enhanced magnetic field gradient sensitivity due to flux locking behavior

Abstract

We have analyzed the quantum interference effects in the macroscopic ''superconducting molecule''. The composite system consists of two massive superconducting rings, each interrupted by a Josephson junction, which are at the same time weakly coupled with one another. The special case of coupling via the Josephson four-terminal junction is considered. The structure of the macroscopic quantum states in an applied magnetic field is calculated. It is shown, that depending on the values of the magnetic fluxes through each ring, the system displays two groups of states, the ''orthostates'' with both induced currents going in the same direction, and the ''parastates'' with the opposite currents and with the total induced flux locked to zero value. The transition to the flux locked state with changing of the total applied flux is sudden and is preserved in a certain interval which is determined by the difference of the fluxes applied through each ring. It makes the system sensitive to small gradients of the external magnetic field.