In the BULLKID project, rather than using such a circuit as a tank or acceptor filters, placed along the transmission line, which would affect the RF power transmitted past it in all frequencies, we use the circuit as a bridge between the transmission line and the ground, making it act as a thin notch filter instead. This allows us to both measure the changes in inductance more precisely, and to measure more of them in parallel, as long as each is tuned to filter a different frequency.
To achieve this behaviour, the BULLKID detectors are designed with a lumped element architecture, where the resonant circuit is created fully standalone, with both an inductor meander and a capacitor formed by interdigitated metal traces, and is placed beside the transmission line, thus allowing it to couple both to the line and to a ground plane, acting as a bridge between the two for signals at the circuit’s resonant frequency.
This architecture has several benefits: first of all, it allows the design to reach higher frequencies, and to tune the resonance characteristics (resonance frequency and resonator quality factor) with more accuracy.
It also allows for the optimisation of the design to minimise the magnetic form of inductance, thus to maximise the impact of changes in kinetic inductance on the resonance frequency, and the efficiency with which the energy of a particle interaction is collected in the sensitive parts of the superconductor, where most of the super-current is flowing in.
As the detectors are resonator circuits, the readout of such sensors is achieved by probing them with known RF signals transmitted past them, and then measuring the returning signal. By measuring the changes in phase and amplitude of this signal, we can measure the actual underlying changes in the resonance characteristics, linearly driven by changes in the impedance of the circuit.
This is another benefit of the technology: the readout system is fundamentally based on already established RF technology, and, as the information is not in fact directly accessible, but rather is encoded onto a carrier signal, the readout process is much simpler, and less sensitive to setup-dependent characteristics.
