Quantized vortex dynamics in quantum fluids

Quantum fluids—such as superfluid helium-4 (He II) and atomic Bose–Einstein condensates—exhibit macroscopic quantum coherence that enables dissipationless flow. A defining consequence of this coherence is that vorticity cannot be continuously distributed as in classical fluids. Instead, it is confined to line-like topological defects called quantized vortices, each carrying a fixed circulation κ = h/m (for ⁴He, κ ≈ 9.97 × 10⁻⁸ m²/s) and possessing an atomic-scale core. In quantum turbulence, a dynamic tangle of interacting quantized vortices evolves through self-induced motion, long-range vortex–vortex interactions, and frequent reconnection events that abruptly change vortex topology and transfer energy across scales. This quantized, topological nature of vorticity makes He II an ideal model system for addressing fundamental questions in nonequilibrium physics, turbulence, and dissipation, where microscopic vortex-scale processes can be directly connected to macroscopic flow behavior.

Beyond helium, quantized vortex dynamics is broadly relevant across physics. In type-II superconductors, Abrikosov vortices (quantized flux lines) control dissipation and critical currents; in atomic condensates, vortex nucleation and interactions govern coherence and transport; and in neutron stars, quantized vortices in the neutron superfluid are widely believed to underlie sudden rotation “glitches” via vortex pinning, creep, and collective depinning events. In all these platforms, the central challenge is to understand how vortex motion, reconnections, and vortex–excitation interactions produce observable dissipation and large-scale dynamics. Our research program tackles this challenge by developing quantitative visualization tools in He II that directly reveal vortex geometry, motion, and statistics—enabling stringent tests of theory and uncovering new scaling laws in quantum turbulence.

Direct imaging of vortices using frozen-particle tracers

Our first method uses frozen particle tracers (e.g., solidified D₂) to decorate vortex cores. Under suitable conditions, particles become trapped on quantized vortices and are illuminated in a thin laser sheet, allowing real-time imaging of vortex structures and their dynamics. This approach enables us to track vortex motion in turbulent tangles and to quantify transport statistics. Using particle-based vortex tracking, we discovered robust superdiffusive vortex motion in quantum turbulence and showed that it originates from power-law temporal correlations of vortex velocity rather than rare Lévy-flight events. These measurements provide direct experimental access to vortex-trajectory statistics—critical inputs for predictive models of quantum turbulence and defect-mediated dissipation.

Figure 1: (a) Simulated normal-fluid velocity field u_n around a quantized vortex ring in He II using different mutual friction models. The normal-fluid vortex rings are rendered in reddish half circles. (b) Schematic diagram of the experimental setup. (c) Images showing the D₂ particles (white dots) trapped on a moving vortex ring in quiescent He II. The dashed ellipse is a fit to the trapped particles’ positions. (d) Obtained vortex-ring profile with the trapped particles (red dots) at different times. (e) Comparison of the observed ring radius R(t) evolution with model predictions.

Particle decoration also allows us to resolve key elementary processes, including vortex reconnections and the propagation of individual quantized vortex rings. Reconnections are fundamental topological events that rapidly reconfigure vortex tangles and seed energy transfer to smaller scales. By combining experiments with modeling, we have verified universal reconnection scaling of the minimum vortex separation near the reconnection time, and we observe clear asymmetry between pre- and post-reconnection dynamics, revealing intrinsic time-irreversibility in real quantum fluids. In vortex-ring experiments, a single ring decorated by discrete particles appears as an ellipse (the projection of a tilted circle). By fitting particle positions, we extract the ring radius R(t) and its orientation, enabling quantitative measurements of ring shrinkage due to dissipation and direct comparison with competing models of mutual friction and vortex–normal-fluid coupling.

He₂^* excimer molecular tracers for two-fluid diagnostics and low-temperature vortex imaging

Our second visualization strategy uses metastable He₂^* excimer molecules, which can be imaged via laser-induced fluorescence (LIF) and provide minimally invasive flow diagnostics. At temperatures above about 1 K, He₂^* tracers are completely entrained by the viscous normal-fluid component, making them ideal for unambiguous measurements of the normal-fluid velocity field in the two-fluid regime. This capability is essential for studying coupled superfluid–normal-fluid dynamics, for identifying flow structures and large-scale patterns, and for connecting vortex evolution to the surrounding normal-fluid motion. In contrast, at sufficiently low temperatures (typically T ≲ 0.5 K), He₂^* molecules can bind to quantized vortices, providing an opportunity to visualize vortex dynamics in the nearly pure-superfluid limit where the normal fraction becomes vanishingly small and conventional particle methods become challenging. This dual functionality—normal-fluid velocimetry at higher T and vortex-sensitive tagging at low T—offers a unique pathway toward unified diagnostics across the full He II phase diagram.

Together, these complementary tools establish a comprehensive experimental platform for quantized vortex dynamics: (i) direct imaging of individual vortex structures (rings, reconnections), (ii) quantitative extraction of dissipation and irreversibility at the vortex level, and (iii) statistical characterization of vortex motion in turbulent tangles to uncover universal scaling behavior. The resulting insights strengthen our fundamental understanding of quantum turbulence and provide broadly transferable physics relevant to vortex-mediated dissipation and defect dynamics in superconductors, ultracold gases, and astrophysical superfluids.