# Applications for Microwave Kinetic Induction Detectors in Advanced Instrumentation
## 1 Introduction  
Sensors to detect and characterize particles or photons are key components for many areas in modern science. Currently, the most successful particle or photon detectors are semiconductor-based CCD and CMOS arrays. They are used intensively for high energy particles and photons but are especially dominant for light around the ultraviolet (UV) and optical where Si-based arrays have achieved impressive results and immense array sizes. As their detection principle is to excite charge carriers across the semiconductor band gap, this band gap defines the detector's basic characteristics and is responsible for significant limitations. For example, the band gap in Si is around 1.1 eV, thus any photon to be detected with a Si-based CCD needs at least this 1.1 eV of energy (equivalent to a 1088 nm wavelength) to excite one electron, and even a photon with just a little below twice this energy still results in the same signal. Semiconductor-based CCDs therefore can only achieve energy resolution on a single pixel basis at energies that are significantly higher than their band gap. For this reason, small band gap semiconductors like e.g., HgCdTe, are used for lower photon energies in the infrared (IR) but practical limitations for their smallest achievable band gaps still restrict their energy resolution and low energy cut-off. An intriguing option to build on and extend the tremendous success of semiconductor-based detectors is to instead use the much smaller band gaps found in superconductors. The well-known effect of superconductivity of a vanishing resistance below a critical temperature \(T_{c}\) is caused by the pairing of charge carrying electrons into Cooper pairs which then condense into the lowest possible state. The superconductor band gap \(\Delta\) separates this lowest state from higher up ones, and any Cooper pair that gets excited over this gap is broken into unpaired and dissipative electrons, forming a so-called quasi-particle (QP). Superconductor band gaps can be significantly smaller compared to typical semiconductor gaps and regarding detectors, superconductors with small \(T_{c}\) are preferred--as a small \(T_{c}\) also means a small band gap \(\Delta\). Aluminum, for example, can have a \(T_{c}\) of 1.2 K and a \(\Delta\) of 1.8 \(\times\) 10\({}^{-4}\) eV. As \(\Delta\) is defined per electron, an energy of at least 2\(\Delta\) isrequired to break a Cooper pair, but this is still almost four orders of magnitude below the Si band gap. This allows superconducting detectors to achieve stronger signals, higher sensitivities for lower energies, and much better energy resolution. Superconductors come with further advantages like the lack of dissipation and high charge carrier mobility, but they also have one obvious disadvantage: they have to operate at very low temperatures to stay superconductive. This comes with the added benefit of vanishing thermal noise but can present significant engineering challenges, especially if many pixels in a large detector array are desired. Cooling power at temperatures below 1 K is typically very limited, and every electrical connection required between the detector array and room temperature unavoidably contributes to the thermal load on the low temperature stage. If every single low temperature detector in an array requires two or more electrical connections this can severely restrict the possible number of pixels. (CCD arrays avoid millions of wires by applying a voltage across the array and shifting their pixel rows one by one downwards into the read-out row, combined with reading that row pixel by pixel by again stepwise shifting the individual signals. This cannot be done with superconductors as due to their vanishing resistance voltages can be applied). It is therefore important for a low temperature detector to be multiplexable, allowing many individual detectors to be monitored with only a few electrical connections.