# Applications for Microwave Kinetic Induction Detectors in Advanced Instrumentation
## 2 Microwave Kinetic Inductance Detectors: Basic Operation Principle
As MKID \(LC\)-resonators are superconducting they have very little electrical losses and thus a very sharp resonance. Even a tiny change in resonant frequency produces a significant change in the phase signal (the sketch of Figure 1b for the amplitude signal is exaggerated for clarity, the change in amplitude is often much smaller). This phase shift can be quickly and very precisely measured, and every single photon that hits an MKID produces its own signal pulse. The pulse height is determined by the amount of broken Cooper pairs and thus the photon's energy. For sufficient photon energy this is used to not only count single photons but also to determine their individual energies. For longer wavelengths, single photons that can still break Cooper pairs no longer cause measurable signal pulses, but the photon flux can still be measured by monitoring the shift of the MKID's phase signal as the quasi-particle concentration will reach a flux dependent equilibrium between Cooper pair breaking and QP recombination. As they are superconducting detectors MKIDs do have to operate at mK temperatures in most cases. As available cooling power at below 1 K is small, the heat influx from the surrounding room temperature environment has to be minimized. It is therefore important to be able to monitor and read out as many pixels as possible on a single electrical line between room temperature and the detectors themselves. This is the unique strength of MKIDs among low temperature detectors: they offer build-in frequency domain multiplexing. Every MKID pixel in a detector array has its individual, lithographically defined resonant frequency, usually spaced evenly over the frequency range the readout electronics can cover. As superconducting \(LC\)-resonators have very sharp resonances they do barely attenuate signals that are even just a few hundred kHz off their resonance. Many MKID resonators can thus be coupled to the same microwave feedline (see Figure 1d) without significant crosstalk. If one detector is hit by a photon and shifts in frequency the others will not react. This way it is possible to couple to and monitor thousands of MKIDs with only a few feedlines, requiring just two electrical connections to room temperature each. Up to 2000 MKID pixels per feedline have already been demonstrated [2; 3] and MKID arrays with up to 20,440 individual detectors are being used at the time of writing [3]. The current limit of 2000 pixels per feedline is the result of the highest acceptable single detector resonant frequency (currently 8 GHz due to lack of very low noise amplifiers for higher frequencies), the fact that harmonic frequencies have to be avoided (resulting in a 4-8 GHz frequency window), and a minimum distance of 2 MHz between MKIDs in frequency space to optimize fabrication yield and time resolution. The goal of this chapter was to only give a very short overview of the operating principle of Microwave Kinetic Inductance Detectors. A more thorough discussion would go beyond the scope of this paper but can be found for example in several excellent reviews [4; 5; 6].