Die separation for mitigation of phonon bursts in superconducting circuits

Cosmic rays and background radioactive decay can deposit significant energy into superconducting quantum circuits on planar chips. This energy converts into pair-breaking phonons that travel across the substrate and generate quasiparticles, leading to correlated energy and phase errors in nearby qubits. To mitigate this, we fabricated two separate dies and placed them adjacently without a galvanic connection between them. This blocks phonon propagation from one die to the other. Using microwave kinetic inductance detectors on both dies, we detected high-energy bursts and demonstrated the blocking effect. However, we also observed simultaneous events in both dies, likely originating from a single cosmic particle traversing both dies.