Vortex-Controlled Quasiparticle Multiplication and Self-Growth Dynamics in Superconductors

Even in the quantum limit, non-equilibrium quasiparticle (QP) populations induce QP poisoning that irreversibly relaxes the quantum state and significantly degrades the coherence of transmon qubits. A particularly detrimental yet previously unexplored mechanism arises from QP multiplication facilitated by vortex trapping in superconducting quantum circuits, where a high-energy QP relaxes by breaking additional Cooper pairs and amplifying the QP population due to the locally reduced excitation gap and enhanced quantum confinement within the vortex core. Here we directly uncover this elusive QP multiplication process by revealing vortex-controlled QP self-generation in a highly nonequilibrium regime preceding the phonon bottleneck of QP relaxation. At sufficiently low fluence, femtosecond-resolved magneto-reflection spectroscopy directly reveals a continuously increasing QP population that is strongly dependent on magnetic-field-tuned vortex density and absent at higher excitation fluences. Quantitative analysis of the emergent QP pre-bottleneck dynamics further reveals that, although the phonon population saturates within ≃10 ps, both free and trapped QPs continue to grow in a self-sustained manner–hallmarks of direct QP-vortex interactions. Our findings establish a powerful spectroscopic tool for uncovering quasiparticle multiplication and reveal vortex-assisted quasiparticle relaxation as a critical materials bottleneck for highly coherent quantum devices.