The first task of the project was to assemble the project team, that now comprises five laboratories (three departements of Observatoire de Paris, Université PSL: GEPI, LESIA, LERMA; one of the Université de Bordeaux: LAB; one of Université Paris Cité: APC) and about 25 people. The first milestone achieved was the completion of the conceptual design of the SPIAKID instrument, that was validated by the Preliminary Design Review conducted in March 2022 by a panel composed by scientists and engineers of the European Southern Observatory and of the Institut Néel. On the basis of this conceptual design we could launch the tender and sign the contract for the procurement of the main subsystem of the SPIAKID instrument: a cryostat capable of cooling down to 100 mK and large enough to accomodate a focal plane comprising a mosaic of four detectors of 20 000 pixels each. Thanks to the optics that couples our instrument with the NTT telescope this mosaic will cover a square with a side of 2' on the sky.
In parallel to the development of the SPIAKID instrument we are developing the detectors, exploring different pixel designs and also discovering some unexpected phenomena. The capacitor in the LC circuit can be manufactured either by deposing several superconducting strips on the substrate material (e.g. sapphire), in this case we speak of interdigitated capacitor, or by manufacturing two parallel plates of superconduting material separated by an insulator, in this case we talk about parallel plate capacitor or MIM (Metal-Insulator-Metal). The parallel plate capacitor has several attractive features, mainly the possibility of manufacturing smaller pixels, for a given value of the capacity, and a simpler design. However our tests indicated that the insulator was not totally blocking the possibility of a charge flow between the two plates. For this reason we decided to develop a pixel with a parallel plate capacitor and the ultimate insulator: vacuum. This proved successful, although the manufacture of such a pixel is difficult and was in fact object of a patent. For SPIAKID we shall use the simpler interdigitated capacitors and not parallel plate vacuum pixels, that are however well adapted to instruments working in the domain of the microwaves. Yet the SPIAKID project stimulated a major progress in our understanding of the MKID devices and in their manufacture. In the attempt to increase the quantum efficiency of the detector we explored the effect of placing a reflecting surface below the inductance (Nicaise et al. 2022), we found that the transparent material (Al2O3) used to space the reflector from the inductor introduces a parasitic capacitance and increses the noise. We are going to further develop this design to try to minimise the parasitic capacitance and the noise.
During our study of the detectors we came across an unexpected new phenomenon, that manifests as large ``inverse response'' of the detector, in the sense that the frequency of the oscillator shifts towards higher frequencies, while the normal response of the MKID is to shift the frequency to lower frequencies. This phenomenon is observed only at temperatures that are about 1/100
of the critical temperature and we interpret as the interaction of phonons, produced by light absorption, with the energy levels of the substrate (Two-Level-Systems). This result was published on a refereed paper (Hu et al. 2021) and we are exploring the possibility of using this inverse response as an alternative way to achieve photon detection.
Three major milestones in the detector development have been achieved during this period: the manufacture of a detector with 20 000 pixels, the manufacture of TiN thin films with sub-stoichiometric composition displaying a critical temperature around 1 K, and the operation of a detector in photon counting mode.
