Quantum heat engine refrigeration shown in photonic experiment

Researchers have demonstrated quantum heat engine refrigeration in a proof-of-principle device that can produce useful work while also cooling another system. The result matters because heat engines and refrigerators normally perform separate jobs, while the reported quantum setup combines both functions under conditions that would not occur in a classical device.

The work, by researchers at Qufu Normal University, the University of Hong Kong and the University of Palermo, appears in Physical Review Letters. According to the paper, the team predicted and observed an anomalous heat flow in which a quantum system absorbs heat from colder thermal reservoirs.

In ordinary thermodynamics, heat moves from hotter bodies or regions toward colder ones until temperatures balance out. The reported effect does not replace that rule for everyday systems, but it shows how quantum behavior can change the way heat is exchanged in specially prepared systems.

How can a quantum heat engine cool while producing work?

The researchers used a quantum version of an Otto engine, a cycle that converts heat into work. In this case, the device draws on an unusual heat flow enabled by quantum processes and uses it to run a cycle that yields work output and refrigeration at the same time, according to the study.

A quantum Otto engine is the quantum-scale counterpart of a heat engine cycle used to describe how energy can be converted into useful work. Refrigeration means removing heat from a target system, so combining it with work production would usually require different machines or operating modes.

Zhong-Xiao Man, a co-senior author, told Phys.org that the work grew out of earlier studies of indefinite causal order. In that framework, two events can occur in a quantum superposition of different orders rather than in one fixed sequence.

In the team's thermodynamic model, those events are thermalization channels acting on a system through a control qubit, Man told Phys.org. Earlier work had shown that a system exposed to channels with the same temperature does not have to settle at that temperature, prompting the researchers to examine what happens when the system and channels begin at different temperatures.

What the experiment showed

The team reported both a theoretical prediction and an experimental realization of the anomalous heat flow on a photonic platform. Man told Phys.org that the measurements matched the team's theoretical expectations for the heat-flow effect and for the engine cycle.

Giulio Chiribella, a co-author, told Phys.org that the study identifies a new form of anomalous heat flow in which quantum coherence changes heat exchange between systems. He also said the work gives a theoretical and experimental demonstration of the thermodynamic effect in a photonic setup.

The paper also addresses a foundational point about the mechanism. Chiribella told Phys.org that the effects are not exclusive evidence of indefinite causal order, because the researchers showed they can also be reproduced within a definite causal structure.

What could it be used for?

The engine remains a proof of principle, according to Phys.org's report on the study. Possible future uses include thermal control for quantum processors and heat management in quantum sensors, imaging systems and nanoscale devices, though the researchers did not report a deployable technology.

Rosario Lo Franco, a co-senior author, told Phys.org that future work should move beyond idealized thermodynamic cycles and study versions in which operations take finite time. He said practical devices must account for speed because it affects both efficiency and power output, and because measurement and control can introduce trade-offs among energy cost, information gained and performance.

The paper is titled “Anomalous Heat Flows and Quantum Otto Engine with (In)definite Causal Order.” It lists Qing-Feng Xue and colleagues as authors and is also available through arXiv.

This story draws on original reporting from Phys.org.