ABSTRACT Particle plasmons, the collective oscillations of conduction electrons in metal nanoparticles, are central to nanophotonics and quantum technologies. Their quantum coherence is degraded by both energy relaxation and pure dephasing. While the former is well described by classical and semiclassical models, a fundamental microscopic understanding of pure dephasing, which destroys phase coherence without energy exchange, remains elusive. This work establishes a microscopic theory by identifying stochastic resonance frequency fluctuations as the physical origin of pure dephasing in particle plasmons. Starting from a many‐body electron Hamiltonian, we map the plasmon onto a quantum harmonic oscillator via collective coordinates and the random phase approximation. We demonstrate that environmental perturbations commuting with the plasmon number operator induce random frequency shifts, leading to phase diffusion. We then microscopically derive contributions from three dominant mechanisms: electron‐phonon scattering, defect scattering, and surface roughness scattering. These contributions are unified into a general scaling law that predicts the pure dephasing rate as a function of nanoparticle size and geometry, revealing a crossover between dominant mechanisms. This law provides a microscopic foundation for empirical damping formulas and clarifies the fundamental distinction between energy relaxation and pure dephasing, thereby completing the quantum mechanical picture of plasmon decoherence.
Weixiang Ye (Sun,) studied this question.