Vortex-Controlled Quasiparticle Multiplication and Self-Growth Dynamics in Superconducting Resonators

TL;DR

This work tackles quasiparticle poisoning in superconducting qubits by uncovering a vortex-assisted QP multiplication mechanism that operates in a pre-bottleneck regime. Using femtosecond-resolved magneto-pump–probe spectroscopy on Nb superconducting resonators and an extended three-fluid Rothwarf–Taylor framework incorporating trapped QPs $q_t$, free QPs $q_f$, and phonons $p$, the authors reveal field-tunable QP growth and self-sustained dynamics driven by QP–vortex interactions. At low excitation, they observe a ~34% increase in QP density near a vortex density of about $100~ ext{Φ}_0/ ext{μm}^2$, while higher fluences recover conventional hot-phonon–mediated relaxation with diminished magnetic-field sensitivity. The results provide a spectroscopic tool and a quantitative framework for mitigating QP poisoning by controlling vortex density and excitation, with direct implications for enhancing coherence in superconducting qubits and for exploring ultrafast nonequilibrium processes in vortex-structured superconductors.

Abstract

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 resolve 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 $\simeq$10~ps, both free and trapped QPs continue to grow in a self-sustained manner--hallmarks of the long-anticipated QP-vortex interactions in nonequilibrium superconductivity. We estimate a substantial increase of $\sim$34\% in QP density at vortex densities of $\sim$ 100 magnetic flux quanta per $\mathrm{μm^{2}}$. Our findings establish a powerful spectroscopic tool for uncovering QP multiplication and reveal vortex-assisted QP relaxation as a critical materials bottleneck whose mitigation will be essential for resolving QP poisoning and enhancing coherence in superconducting qubits.

Figures (5)

• Figure 1: (a) Schematic illustration of quasiparticle (QP) generation and recombination processes in the nonequilibrium superconducting (SC) state without vortices. Here, $q$ denotes the QP density and $p$ denotes the phonon density; $q_T$ and $p_T$ indicate their respective thermal equilibrium values. (b) Schematic illustration of quasiparticle trapping by a single magnetic vortex in the SC state. The dynamics are governed by the detailed balance among trapped QPs ($q_t$), free QPs ($q_f$), and phonon population ($p$) (main text). Here, $\Gamma$ denotes the vortex-induced QP trapping rate, while $\eta$ represents the QP detrapping rate. (c) Magnetic force microscopy (MFM) image of vortices in a Nb thin film under $\pm3$ mT magnetic fields. Each spot corresponds to a single magnetic vortex. Scale bar is 1 $\mu$m.
• Figure 2: Quasiparticle dynamics in the superconducting state of Nb under 0 T and 0.4 T magnetic fields, measured via the differential reflectivity signal $\Delta R / R_\text{SC}$ at various pump fluences: (a) 2.5 $\mu$J$/$cm$^2$; (b) 2 $\mu$J$/$cm$^2$; (c) 1.5 $\mu$J$/$cm$^2$; (d) 1 $\mu$J$/$cm$^2$. All measurements were done at temperature $T = 2.27$ K. All the $\Delta R / R_\text{SC}$ data measured at 0.4 T (blue circles) are scaled to the 0 T result at 50 ps for easier comparison of dynamics. The blue trace in (a) shows the unscaled raw data measured at 2.5 $\mu$J/cm$^2$, while the gray trace in (a) corresponds to the raw data taken under a magnetic field of 1 T which show $\Delta R / R_\text{SC}$ signals vanish after the complete quenching of superconductivity.
• Figure 3: Magnetic-field-dependent, photo-induced superconducting $\Delta R / R_\text{SC}$ components measure quasiparticle change at fixed delay times of 10 ps (dotted lines) and 50 ps (solid lines). Data were recorded at $T = 2.27$ K for pump fluences of 0.5 (red) and 1 $\mu$J/cm$^2$ (black) in (a) and 3 $\mu$J/cm$^2$ in (b). The inset in (b) shows the scaled data for the two time delays in (b).
• Figure 4: (a) Comparison between measured superconducting reflectivity transients $\Delta R / R_\text{SC}$ (dots) and fits (solid lines) based on the extended Rothwarf--Taylor model in the vortex states at various magnetic field strengths. The model captures the evolution of QP relaxation dynamics from phonon-bottleneck-dominated decay at zero field to vortex-controlled pre-bottleneck relaxation and QP multiplication at higher fields. (b) and (c): Model simulations showing the population dynamics of free quasiparticles ($q_f$, red), trapped quasiparticles ($q_t$, blue), and phonons ($p$, green) at 0 T and 0.2 T.
• Figure 5: Magnetic field dependence of the four characteristic rates extracted from the model fits using the extended R-T model in the vortex states: (a) Pair-breaking rate $\beta$ shows a decrease as the field approaches the critical field $H_\text{c}$, consistent with SC gap suppression and reduced pair-breaking efficiency. (b) QP recombination rate $R$ increases monotonically with field, reflecting enhanced recombination in vortex-core regions where the SC order parameter is suppressed. (c) Vortex-induced QP trapping rate $\Gamma$ increases with field, in line with the rising vortex density in the type-II SC state. (d) Phonon-mediated detrapping rate $\eta$ remains approximately constant up to $H \approx 0.4$ T, then declines near $H_\text{c}$, due to phonon softening and reduced detrapping efficiency. Together, these trends reveal a crossover from phonon-dominated to vortex-dominated relaxation dynamics as magnetic field increases.