Precision quantum simulation of magnon spectra and interactions

Understanding how microscopic atomic interactions give rise to macroscopic magnetic phenomena remains a fundamental challenge in condensed matter physics. This study investigates the dynamics of magnons—quantized spin waves—in a 2D XY spin-1/2 magnet. The researchers aimed to resolve the interacting dynamics of these quasi-particles by measuring both linear and non-linear response functions, which are often difficult to probe with high precision in traditional experimental or classical simulation settings.

The team utilized a 97-qubit hybrid analog-digital superconducting processor (the Willow architecture). They employed a two-pronged strategy: first, they used Hamiltonian learning to characterize the processor's evolution with high precision (approximately 0.1% error). Second, they interleaved digital gates with analog evolution to selectively excite specific magnon modes at tunable energy densities. This allowed them to measure retarded Green's functions and extract temperature-dependent spectra, lifetimes, and non-linear scattering mechanisms that are typically inaccessible in bulk spectroscopy experiments.

The experiments revealed that magnon decay rates are highly dependent on the mode structure. In rectangular geometries, decay rates were enhanced near van Hove singularities, while in diamond geometries, edge-localized modes showed suppressed decay. Moving beyond the linear regime, the researchers characterized non-linear self-scattering pathways. They discovered that magnon decay rates follow a power-law dependence on energy density, with an exponent that varies across the spectrum. Crucially, they found that these dynamics are driven by self-consistent broadening effects, where the linewidths of scattering partners feedback into the decay process, a phenomenon that explains why higher-energy modes exhibit sublinear scaling of decay rates with temperature.