Resolving quantum Cooper-pair tunneling in graphene Josephson junctions under near-infrared illumination

We report the observation of discrete supercurrent suppression steps arising from quantum Cooper-pair tunneling in superconductor-graphene-superconductor (SGS) Josephson junctions under near-infrared illumination. This quantum phenomenon remains robust against variations in photon energy, optical power, bath temperature, and dc bias. The nearly identical step spacing observed in SGS Josephson junctions with different graphene channel lengths indicates the dominant role of the superconductor-graphene interface in governing Cooper-pair transport. In contrast, the discrete steps vanish in the negative differential resistance regime, where quasiparticle transport dominates, confirming their microscopic Cooper-pair tunneling origin.