Correspondence: Reply to ‘The experimental requirements for a photon thermal diode’

Budaev 1 correctly identifies a fundamental symmetry error in several crucial experiments in our recent study 2 , specifically the results presented in Fig. 3c (the three filled and four striped bars, labeled ‘Col. 1’ and ‘Col. 2’, respectively) and Fig. 4. A suitable configuration for those measurements should have included identical (mirror-imaged) collimators at both hot and cold sides, as in Fig. 1a here. However, the actual experiments omitted the cold-side collimator, a choice made for experimental simplicity and which we believed was acceptable at the time based on a simple thermal model 3 and the fact that T BBC 4 ≫ T ∞ 4 , where T BBC and T ∞ are the temperatures of the blackbody (BB) cavity and cold-side plate, respectively. However, upon careful reconsideration we now find that thermal model to be flawed, and we believe that omitting the cold-side collimator invalidated several key measurements. We provide a more detailed discussion of that thermal estimate, and the reasons for its failure, elsewhere 3 . Figure 1: Alternative analyses reveal a fundamental problem with the thermalizing graphite plate concept for inelastic thermal collimation. The alternative text for this image may have been generated using AI. Full size image ( a ) Schematic of a configuration with proper symmetries 1 , using two mirror-imaged thermal collimators. The heat transfer response function Q ( T 1 , T 2 ) can be analyzed using either of the control volumes depicted in b , c . ( b ) In the original work (for example, Supplementary Fig. 5c of ref. 2 ), each graphite plate was considered part of its adjacent BB, corresponding to non-equilibrium reservoir boundary conditions f NE,1 ( T 1 , T 2 ) and f NE,2 ( T 2 , T 1 ). ( c ) Alternatively, the graphite plates can be considered with the pyramids as Σ B . In this case, the reservoir boundary conditions are perfect blackbodies with equilibrium Bose-Einstein statistics f BE ( T 1 ) and f BE ( T 2 ), analyzed further here in Fig. 2 . Clearly the Q ( T 1 , T 2 ) function must be the same whether calculated using b or c .