Understanding anti-parity-time symmetric systems with a conventional heat transfer framework -- comment on "Anti-parity-time symmetry in diffusive systems"

Abstract: Inspired by non-Hermitian physics, Li et al. (Science 364, 170-173) theoretically predicted and experimentally demonstrated a stationary temperature profile in a diffusive heat transfer system - seemingly indicating that heat "stops" diffusing. By analogy to the wave physics framework, the motionless and moving temperature profiles are manifestations of the anti-parity-time APT symmetry and symmetry breaking states, respectively. Their experimental setup consists of two thermally coupled rings rotating in the opposite direction. At a particular rotation speed, known as the exceptional point, the APT symmetry of the system changes, resulting in the temperature profile switching between stationary and moving states. In fact, this seemingly unusual and exotic behavior can be elegantly captured and predicted using a conventional heat transfer framework with similarity and scaling analysis. In this work, we show the system behavior can be characterized into three zones by two widely-used dimensionless parameters on a regime map. The exceptional point, discovered using wave physics, is located precisely on the zone boundary on the regime map, indicating a balance between the contribution of thermal coupling and mechanical motion. Furthermore, the observed cessation of thermal diffusion is merely a result of the long diffusion time constant of the experimental setup. The unfamiliarity of concepts in another scientific field as well as the remarkable equivalence of the two points of view prompts this in-depth discussion of the analogy between wave physics and heat transfer. We believe that this work can help bridge the gap and promote new developments in the two distinctly different disciplines.