Using disordered superconductors, like β-Ta, TiN, Hf and PtSi, for MKIDs has the potential to improve the photon absorption efficiency, η, over a broad wavelength range. However, the resolving power, R, of these MKIDs is not higher than 20 at 1µm, while the fundamental Fano limit is around 65. To improve R, the signal-to-noise ratio must be increased. We measure the single photon pulse response of β-Ta MKIDs and show that the pulse duration is limited by disorder in two ways. First, the initial pulse decay is faster than exponential, which we attribute to slow quasiparticle diffusion. Second, the decay time of the pulse tail is faster due to low energetic, localized quasiparticles. Both these effects do not occur in conventional Al at the same experimental conditions. These results imply a trade-off between η and R and shows that improving MKIDs by using disordered superconductors is not straightforward.
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