Quantum Circuits in Diffusion Models: A Fair-Comparison Study and a Mechanistic Analysis of Angle-Embedding Failures
This study investigates the efficacy of variational quantum circuits—such as EfficientSU2—in diffusion generative models, with a focus on the failure mechanism of angle embedding under unbounded score-matching objectives due to phase aliasing. To enable a fair evaluation, the authors propose a protocol that integrates quantum modules into DDPM and latent diffusion frameworks via squeeze-and-excitation structures, and conduct systematic assessments using NCSN-based score models, FID metrics, and multiple subsampling significance tests. Experiments show that quantum modules achieve average FID scores comparable to classical baselines despite using 4.5–9 times fewer parameters. Furthermore, applying a π·tanh(·) bounded input transformation effectively mitigates phase aliasing and significantly improves performance; however, the anticipated parameter efficiency advantage of quantum circuits is not realized.