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arXiv:2608.12454

Floquet Quasiparticle Poisoning of Frozonium

Haoyu Guo, Debanjan Chowdhury

cond-mat.supr-concond-mat.mes-hallquant-phSubmitted Aug 12, 2026• Announced Aug 14, 2026• 0 citations
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Abstract

Periodic driving can suppress the Josephson nonlinearity of a fluxonium superconducting circuit, producing a nearly harmonic Floquet spectrum at isolated freezing points [K. Lewellen et al., Newton 2, 100434 (2026)]. Here we show that this dynamically frozen behavior does not generically suppress quasiparticle-induced dissipation in the resulting frozonium circuit. We formulate quasiparticle processes in the frozonium using a Floquet framework and analyze both drive-assisted Cooper-pair breaking and tunneling of pre-existing quasiparticles. Pair generation is controlled by gap-breaking thresholds at high drive frequencies, while multiphoton resonances produce pronounced rate enhancements at lower frequencies. Quasiparticle tunneling exhibits connected resonance structures organized by the harmonic Floquet-Magnus spectrum near the freezing point, with resonant hybridization generating characteristic avoided crossings. Our results show that suitable operating regimes must balance dynamical freezing against quasiparticle loss and provide a framework for identifying experimental drive parameters away from harmful resonances.

Comments
Main text: 10 pages, 4 figures. Supplementary material: 4 pages

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Periodic driving can suppress the Josephson nonlinearity of a fluxonium superconducting circuit, producing a nearly harmonic Floquet spectrum at isolated freezing points [K. Lewellen et al., Newton 2, 100434 (2026)]. Here we show that this dynamically frozen behavior does not generically suppress quasiparticle-induced dissipation in the resulting frozonium circuit. We formulate quasiparticle processes in the frozonium using a Floquet framework and analyze both drive-assisted Cooper-pair breaking and tunneling of pre-existing quasiparticles. Pair generation is controlled by gap-breaking thresholds at high drive frequencies, while multiphoton resonances produce pronounced rate enhancements at lower frequencies. Quasiparticle tunneling exhibits connected resonance structures organized by the harmonic Floquet-Magnus spectrum near the freezing point, with resonant hybridization generating characteristic avoided crossings. Our results show that suitable operating regimes must balance dynamical freezing against quasiparticle loss and provide a framework for identifying experimental drive parameters away from harmful resonances.
Floquet engineering is a powerful method that can be used to modify the properties of interacting many-body Hamiltonians via the application of periodic time-dependent drives. Here, we consider the physics of an inductively shunted superconducting Josephson junction in the presence of Floquet drives in the fluxonium regime and beyond, which we dub the frozonium artificial atom. We find that in the vicinity of special ratios of the drive amplitude and frequency, the many-body dynamics can be tuned to that of an effectively linear bosonic oscillator, with additional nonlinear corrections that are suppressed in higher powers of the drive frequency. By analyzing the inverse participation ratios between the time-evolved frozonium wavefunctions and the eigenbasis of a linear oscillator, we demonstrate the ability to achieve a novel dynamical control using a combination of numerical exact diagonalization and Floquet-Magnus expansion. We discuss the physics of resonances between quasi-energy states induced by the drive and ways to mitigate their effects. We also highlight the enhanced protection of frozonium against external sources of noise present in experimental setups. This work lays the foundation for future applications in quantum memory and bosonic quantum control using superconducting circuits.
Synopsis High-order nonlinear multiphoton dynamics of closed and open periodically driven quantum systems are studied within the semiclassical Floquet formalism. Multiphoton up/down frequency conversion in a super-conducting circuit is realized and faithfully reproduced with calculations. The absorption spectra are suppressed at sharp avoided crossings of the Floquet states. We include the effect of coupling to cavity by time evolving the master equation. The characteristics of the time-averaged absorption spectra are investigated by probing details near and around the exceptional points where suppression occurs.
Microwave drives play a central role in the control of superconducting quantum circuits, enabling qubit gates, readout, and parametric interactions. As the drive frequencies are typically an order of magnitude smaller than (twice) the superconducting gap, it is generally assumed that such drives do not disturb the BCS ground state. However, sufficiently strong drives can activate multiphoton pair-breaking processes that generate quasiparticles (QPs) and result in qubit errors. In this work, we present a theoretical framework for calculating the rates of multiphoton-assisted pair-breaking transitions induced by charge- or flux-coupled microwave drives. Through illustrative examples, we show that photon-assisted QP generation may affect novel high-frequency dispersive readout architectures, as well as Floquet-engineered superconducting circuits operating under strong driving.
Periodic driving enables the engineering of complex quantum matter, yet in interacting systems it generically leads to energy absorption, which limits the lifetime of the engineered states. To address this challenge, dynamical freezing has been proposed as a mechanism for stabilizing non-equilibrium states over parametrically long timescales. While theory predicts robust freezing under simplifying assumptions, realistic platforms inevitably include additional interaction processes that alter its stability. Here, we report the experimental observation of dynamical freezing in programmable Rydberg atom arrays of up to 100 atoms in one and two dimensions. We find that while single-frequency driving produces pronounced suppression of excitation dynamics, the freezing behavior is restricted to a narrow parameter regime due to interaction-induced heating channels present in realistic simulators. Using a perturbative Floquet analysis of the fully interacting atomic system, we identify the dominant microscopic heating processes responsible for this destabilization. Leveraging this understanding, we design a dual-parameter modulation of detuning and Rabi frequency that coherently cancels these absorption pathways and substantially broadens the freezing regime, making it also robust across different geometries. Our results reveal how heating processes shape the stability of dynamical freezing in interacting Floquet systems and demonstrates a route to control driven many-body dynamics in realistic experimental platforms.
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 μm2\mathrm{μm^{2}}μm2. 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.
Nonequilibrium quasiparticles (QPs) generated by stray infrared and ionizing radiation can limit the performance of superconducting quantum processors and present challenges for quantum error correction schemes. Models of QP-induced energy relaxation commonly assume that the characteristic energy of the QPs and the qubit transition energy are both small relative to the superconducting gap. Under these assumptions, certain qubits such as the fluxonium would exhibit protection against QP-induced dissipation at specific bias points. Here, we show that this is not necessarily the case, numerically analyzing the predicted rate of QP-induced dissipation in fluxonium qubits for different QP energy distributions and for QPs created via photon-assisted tunneling processes. We find that accounting for small numerical factors, existing theoretical models predict sensitivity to QP-induced errors at bias points previously thought to be protected. We find that inclusion of asymmetry in the superconducting gap energy across the junction can reintroduce suppression of QP-induced relaxation, as expected. Additionally, for QPs created by photon-assisted tunneling, we predict that T1T_1T1​ protection will only occur for a specific energy of pair-breaking radiation. This understanding of fluxonium sensitivity to QP-induced dissipation informs the development of fluxonium-based processors and future QP-mitigation strategies.