  Because of the ultra-low photon energies in the mid-infrared and terahertz
frequencies, in these bands photodetectors are notoriously underdeveloped, and
broadband single photon detectors (SPDs) are non-existent. Advanced SPDs
exploit thermal effects in nano-structured superconductors, and their
performance is currently limited to the more energetic near-infrared photons
due to their high electronic heat capacity. Here, we demonstrate a
superconducting magic-angle twisted bilayer graphene (MAG) device that is
capable of detecting single photons of ultra-low energies by utilizing its
record-low heat capacity and sharp superconducting transition. We theoretically
quantify its calorimetric photoresponse and estimate its detection limits. This
device allows the detection of ultra-broad range single photons from the
visible to sub-THz with response time around 4 ns and energy resolution better
than 1 THz. These attributes position MAG as an excep-tional material for
long-wavelength single photon sensing, which could revolutionize such disparate
fields as quantum information processing and radio astronomy.
