Magnetic Phase Control of a Thick SNS Weak Link: Proposed experimental scheme

Abstract

In contrast to the extensive literature on thin tunnel junctions and traditional SQUID geometries, there is almost no quantitative experimental data on magnetic control of the Josephson phase in thick SNS weak links. The standard view is that in such compact structures without macroscopic loops the local magnetic coupling to the phase is negligibly small, which in practice forces one to use bulky SQUID devices for phase control. We show that this view is overly restrictive. We consider a thick SNS bridge with an on-chip microcoil placed directly above it, which controls the Josephson phase via strong phase-flux coupling enhanced near the Josephson plasma resonance. In the proposed configuration realistic thick SNS weak links, with normal-layer thickness d of order xi, can achieve phase-flux efficiencies of order 30-60 percent of an ideal dc SQUID. Within a standard RSJ/RCSJ model and linear circuit theory we show that this unexpectedly strong coupling arises from the combination of a large kinetic inductance of the thick SNS bridge and resonant amplification of about 15-35 dB, rather than from any exotic microphysics. The proposed experiment, a comparative analysis of Shapiro steps driven by a direct RF signal and by the magnetic field of the same microcoil, provides a direct and quantitative method to measure the phase-flux response of a thick SNS junction. If confirmed experimentally, such structures may become compact phase elements capable of locally controlling the Josephson phase without a macroscopic loop and enabling dense, locally addressable phase control in superconducting quantum circuits.