The day in conversation for 2026-10-23

The discussion: Lev: I keep coming back to that definition in the caloritronics paper—defining the Onsager-Casimir reciprocal pair between heat and photon spin channels. That feels like it’s the necessary language if we are actually going to talk about these nonreciprocal photonic thermal devices. Mira: Exactly. Because if we can't cleanly separate those channels, then any claim about nonreciprocity in the system is just a poetic description; it’s not a fundamental thermodynamic constraint. It links directly back to the work we did last week on how energy flows through these coupled systems, and now it’s giving us the direction for engineering them. Kai: I agree with Mira on the importance of the framework, but I wonder if focusing so heavily on the theoretical pair overshadows what’s happening experimentally. The fact that we have to define this reciprocal relationship suggests that in practice, breaking that symmetry is incredibly difficult because it requires controlling things at a level where quantum coherence and thermal fluctuations are battling for dominance. Lev: That's fair, Kai. But I think the conceptual foundation is the bottleneck right now. Look at how this ties into the topological phases we touched on earlier this month in the Kitaev models—the idea that certain states have inherent robustness against perturbations. If we can map these thermal spin couplings onto a topological manifold, maybe that’s where the