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Out of equilibrium quasiparticles (QPs) are one of the main sources ofdecoherence in superconducting quantum circuits, and are particularlydetrimental in devices with high kinetic inductance, such as high impedanceresonators, qubits, and detectors. Despite significant progress in theunderstanding of QP dynamics, pinpointing their origin and decreasing theirdensity remain outstanding tasks. The cyclic process of recombination andgeneration of QPs implies the exchange of phonons between the superconductingthin film and the underlying substrate. Reducing the number of substratephonons with frequencies exceeding the spectral gap of the superconductorshould result in a reduction of QPs. Indeed, we demonstrate that surroundinghigh impedance resonators made of granular aluminum (grAl) with lower gappedthin film aluminum islands increases the internal quality factors of theresonators in the single photon regime, suppresses the noise, and reduces therate of observed QP bursts. The aluminum islands are positioned far enough fromthe resonators to be electromagnetically decoupled, thus not changing theresonator frequency, nor the loading. We therefore attribute the improvementsobserved in grAl resonators to phonon trapping at frequencies close to thespectral gap of aluminum, well below the grAl gap.

Phonon traps reduce the quasiparticle density in superconducting circuits