  Josephson junctions (JJs) are ubiquitous superconducting devices, enabling
high sensitivity magnetometers and voltage amplifiers, as well as forming the
basis of high performance cryogenic computer and superconducting quantum
computers. While JJ performance can be degraded by quasiparticles (QPs) formed
from broken Cooper pairs, this phenomenon also opens opportunities to
sensitively detect electromagnetic radiation. Here we demonstrate single
near-infrared photon detection by coupling photons to the localized surface
plasmons of a graphene-based JJ. Using the photon-induced switching statistics
of the current-biased JJ, we reveal the critical role of QPs generated by the
absorbed photon in the detection mechanism. The photon-sensitive JJ will enable
a high-speed, low-power optical interconnect for future JJ-based computing
architectures.
