Strong reduction of quasiparticle fluctuations in a superconductor due to decoupling of the quasiparticle number and lifetime

In this letter, we address the quasiparticle dynamics in thin aluminium, which is critical to improve the sensitivity of superconducting detectors and the coherence time of qubits. We have measured temperature dependent quasiparticle fluctuations in a small Al volume, embedded in a NbTiN superconducting microwave resonator. When the quasiparticle lifetime saturates at low temperatures, we observe a noise level reduction of a factor $\sim$100, from which we deduce that the number of quasiparticles does not saturate, but continues to reduce exponentially with temperature. Comparing resonators on substrate and membrane, we show that the saturation lifetime does not depend on phonon trapping. Quasiparticle trapping is consistent with this behavior, when on-trap recombination limits the observed lifetime. Counter-intuitively, this effect improves the sensitivity of single-photon detectors.