  Transition Edge Sensors (TES) are superconducting microcalorimeters that can
be used for single-photon detection with extremely low backgrounds. When they
are within their superconducting transition region, small temperature
fluctuations - like the energy deposited by single photons - lead to large
resistance variations. These variations can be measured using Superconducting
Quantum Interference Devices (SQUIDs). This technology is planned to be used as
a single-photon detector for later runs of the ALPS II experiment, a
light-shining-through-walls experiment at DESY Hamburg, searching for
Axion-Like Particles (ALPs), which are possible Dark Matter (DM) candidates.
Due to the very low dark count rates in our setup, our TES system might be
viable for direct DM searches at sub-MeV masses through electron-scattering of
DM in the superconducting chip, as well. Simulations concerning background
rejection and calibration methods demonstrate the needed sub-eV sensitivity
already.
