In the mission of understanding the origins of life on earth and in the universe, astronomers are looking to exoplanets for signs of life. The spectra of light that has passed through an exoplanet's atmosphere is analysed for biomarkers. Until now most exoplanets have been found and characterised using the transit method. In the coming decade the aim is to look for planets using direct detection methods. In part for this purpose the European Extremely Large Telescope (EELT) is currently under construction and in future the Habitable World Observatory (HWO) is planned as a space-based telescope. To make full use of the EELT and the WHO extreme adaptive optics (XAO) systems using wave front sensors and deformable mirrors are needed. Contemporary CCD cameras utilizing semiconductor band gap energies are not sufficiently sensitive for wavefront sensing in direct exoplanet detection, let alone perform measurements to characterise their atmospheres. To this end different detectors are needed.

Kinetic inductance detectors (KIDs) are promising superconducting, energy resolving devices capable of single photon detection in the near-infrared and visible regimes. Another often cited advantage of KIDs is that, in theory, many detectors can easily be coupled to a single set of readout electronics. To make devices capable of producing images comparable to those achieved with CCDs we need a similar amount of pixels. The largest KID arrays to date are in the order of a thousand pixels on one readout line, whereas CCDs have in the order of millions of pixels. In this thesis some theoretical and practical limits on KID multiplexability while retaining high pixel yield are explored.

Conventional semiconductor diodes dissipate energy in the form of heat when current passes through them. This is unwanted in, for example, cryogenic environments. Using a superconducting diode could mitigate this problem. These have been made by using special materials or combining multiple different circuit elements. We provide a systematic method of designing a tunable superconducting diode using a circuit of solely Josephson tunnel junctions. We show that even for a small number of Josephson junctions a strong diode effect can be achieved and that this method is stable under manufacturing tolerances. This method involves solving computationally inexpensive linear least squares problems to tune the Josephson energies of the junctions used.