The absolute speed limit for engines, taught in every physics class for 200 years, just got a new, even stricter set of rules.
April 14, 2026
Original Paper
An Information-Theoretic Bound on Thermodynamic Efficiency and the Generalized Carnot's Theorem
arXiv · 2604.10762
The Takeaway
Carnot's theorem has always been the theoretical ceiling for engine efficiency, but this new information-based bound is even tighter. It shows that the statistical correlation between a machine's internal state and its energy source can further limit how much work we can extract.
From the abstract
We derive a bound on the efficiency of thermal engines that can be sharper than Carnot's limit. It is a function of statistical correlations between the engine internal state and Hamiltonian, can be saturated even in finite-time cycles, and applies to both classical and quantum engines. Specifically, the bound establishes the exact maximal efficiency of engines operating with multiple baths, tightening the upper limit set by Carnot's theorem. Then, we show that an engine made of a quantum dot co