Kinetic Inductance Detectors (KIDs) are a new detector technology based on superconducting resonators. They change their resonant frequency and quality factor (Q) when incident radiation is absorbed in the superconducting material. Each resonator is designed to have a distinct resonant frequency, typically in the GHz range. The low loss in superconducting resonators at these frequencies allows very high quality factors (Q>10^{6}), and large numbers (1000's) of resonators to be read out on a single coaxial cable.

A good way to understand this is to imagine each resonator (detector pixel) as a radio station. It's not hard to listen to one specific station, despite there being many stations at nearby frequencies. A single amplifier can boost the signal from an antenna, boosting the reception of all the stations simultaneously. Using frequency domain multiplexing, each resonator in an array is fabricated with a different resonant frequency, and is excited and read-out with a sine-wave at its unique resonant frequency. One amplifier boosts the signal from all the resonators, and the readout electronics do the work of separating the signals from the individual resonators.

As stated above, KIDs are superconducting detectors. Superconductivity is mediated by Cooper pairs —usually described as pairs of electrons—that move through the crystal lattice without resistance, but with a small amount of Inductance associated with their velocity: This is the Kinetic Inductance. In a superconductor at absolute zero temperature all charge carriers are Cooper pairs. At non-zero temperatures there is an exponential number of excitations, called quasiparticles, that behave to some extent as normal electrons, in that their motion is resistive. When an A-C signal is applied to a superconductor we therefore have a complex response. It is the combination of a resistive (real) term due to the quasiparticles and an inductive (imaginary) term due to the Cooper pairs. The complex surface impedance σ can be given by: σ=σ_{1} - iσ_{2}, where σ_{1} is due to the quasiparticles. One can measure the exact value of σ if the superconductor is embedded in a resonant circuit. The resonant frequency is affected by changes in σ_{2}, and the Q factor is affected by changes in σ_{1} and σ_{2}.

The KID detection principle. Image credit: B. Mazin. PhD Thesis, Caltech, 2004.

An incident photon with energy hν>2Δ (Δ is the superconducting energy gap) absorbed in a superconducting film cooled to T<<T_{c} (the superconducting transition temperature) will break Cooper pairs and create a number of quasiparticle excitations N_{qp}=ηhν/Δ. In panel (a) of the figure above, Cooper pairs (**C**) are shown at the Fermi level, and the density of states for quasiparticles is plotted as the shaded area, as a function of quasiparticle energy E.

Panel (b) represents the KID as a parallel LC circuit, capacitively coupled to a strip line. The increase in quasiparticle density changes the (mainly inductive) surface impedance of the film - represented as a variable inductor. The effect of the surface inductance is to increase the total inductance L, while the effect of the surface resistance R_{s} is to make the inductor slightly lossy.

Panel (c) shows that on resonance, the LC circuit loads the strip line, producing a dip in its transmission. The quasiparticles produced by an incident photon increase both L_{s} and R_{s}, which moves the resonance to lower frequency, and makes the dip broader and shallower. Both of these effects contribute to changing the amplitude (c) and phase (d) of a microwave probe signal transmitted past the circuit.

The theoretical sensitivity of a KID scales as exp(-Δ/k_{B}T) at temperatures well below the superconducting transition temperature.This concept takes advantage of recent dramatic advances in the performance of cryogenig HEMT amplifiers, which provide noise temperatures below 10K across multi-GHz bandwidths, and now operate up to several hundred GHz.

These devices are simple to fabricate, rugged, resistant to radiation and thermal cycling, highly multiplexable, and are ideal candidates for future large detector arrays for space applications.