Gate-Tunable Photoresponse of Graphene Josephson Junctions at Terahertz Frequencies

Graphene Josephson junctions (JJs) are promising platforms for broadband quantum sensing because graphene combines frequency-independent absorption, ultralow electronic heat capacity, and weak electron-phonon coupling. While previous studies focused on microwave and infrared regimes, the terahertz (THz) range─where highly sensitive quantum detectors remain scarce─has largely remained unexplored. Here, we demonstrate a gate-tunable THz photoresponse in graphene JJs. Low-intensity THz illumination strongly suppresses the critical current, generating a pronounced photovoltage under current bias. From photovoltage measurements and independent electron thermometry, we extract a responsivity of 88 kV W^-1 and a noise-equivalent power of 45 aW Hz^-1/2 at 1.7 K. In addition, the hysteretic regime that persists up to 0.9 K suggests a possible route toward single-photon THz detection above millikelvin temperatures. Our results establish graphene JJs as promising candidates for cryogenic THz quantum sensing.