The DUNE photon detection system.

DUNE is the US flagship project in neutrino physics and is currently under construction at Fermilab and South Dakota. The DUNE far detector is based on the liquid-argon TPC technology proposed by C. Rubbia in 1977. Liquid argon is also a high-yield scintillator, which produces about 25000 $128 {nm}$ photons per MeV. In this contribution, I will present the DUNE photon detection system (PDS) to shift, trap, and record the scintillator light, and provide the interaction time of neutrinos in DUNE together with a calorimetric energy measurement (light-charge anticorrelation). I discuss the performance of the system measured in the DUNE laboratories of Milano-Bicocca using both cosmic rays and alpha sources with custom cryogenic SiPMs. Special emphasis will be given to cryo-reliability of the semiconductor detectors and the light trapping system (coated dichroic filters coupled with WLS bars) in liquid nitrogen and liquid argon. The tests demonstrated the long-term reliability of the setup and initiated the assembly of the PDS in the ProtoDUNE-SP detector at CERN. The status of the construction and perspectives for the Run II of ProtoDUNE-SP in 2022 will also be reviewed.