PRL study shows quantum Otto engine that works and refrigerates via anomalous heat flow

Researchers at Qufu Normal University, the University of Hong Kong, and the University of Palermo report a quantum heat engine that can produce work and refrigeration at the same time by exploiting anomalous heat flow, Phys.org reported, citing a Physical Review Letters paper. The team theoretically predicted and experimentally demonstrated reversed heat flow in which a quantum system absorbs heat from colder thermal reservoirs. Co-senior author Zhong-Xiao Man told Phys.org that indefinite causal order lets thermalization processes act in a superposition of orders via a control qubit, so the system need not equilibrate as classical thermodynamics would require. Building on that effect, the group designed a quantum Otto engine that converts heat into useful work while also cooling another system, unlike conventional Otto engines. Man said both the anomalous flow and the engine were realized on a photonic platform with results that match the theory. Phys.org said the unusual heat flow lets the engine draw energy from a colder environment in a way that would not be possible classically. The writeup also notes the phenomena can arise under definite causal structures as well. The journal paper is credited to Qing-Feng Xue and colleagues in Physical Review Letters with DOI 10.1103/sx1m-pdhz, and an arXiv version under DOI 10.48550/arxiv.2511.04028.