Direct comparison of three methods for measuring the Current–Phase Relationship of Nb-based Josephson junctions

Measurement of the current–phase relationship (CΦR) of Josephson junctions (JJs) is critical for understanding the operation of superconducting circuits such as those used for the voltage standard, SFQ digital logic, and quantum information devices. Numerous techniques have been developed to measure the CΦR, and recent studies have shown that the extracted relationships can depend sensitively on the chosen measurement method [I. Babich et al., Nano Lett. 23, 6713 (2023)]. In this presentation, we investigate and compare several experimental circuit configurations for measuring the CΦR of JJs. Each configuration in our study embeds the junction in an RF SQUID and couples it inductively to a readout scheme to extract the CΦR. The three readout configurations employ: (1) a DC SQUID magnetometer, (2) a scanning SQUID microscope for imaging, and (3) a microwave resonator. We present 4 K measurements and extracted CΦRs for junctions from the same wafer using all three measurement methods, enabling a direct comparison of the results. The JJs studied had niobium-doped amorphous silicon barriers with niobium electrodes (Nb/Nb_xSi_1-x/Nb with x≈13%); two different barrier thicknesses were measured.