A quantum heat engine that simultaneously provides work and refrigeration - Physics - General Physics - Quantum Physics

The image visualizes a quantum Otto engine driven by indefinite causal order. Blue streams from the cold reservoirs intertwine in a quantum-switch geometry and flow toward the central two-level system, representing anomalous heat transfer from colder to hotter. The golden arrow shows useful work output, while the minimal interferometer at the bottom recalls the photonic experiment. Credit: Rosario Lo Franco.

The laws of thermodynamics state that heat naturally flows from hotter systems or regions to colder systems or regions until a state of thermal equilibrium is reached. This simple principle underpins the operation of numerous technologies, ranging from refrigerators to power plants.

Yet quantum systems, which are governed by quantum mechanics, can exhibit unusual behaviors that cannot be explained by classical physics. These behaviors could be used to create innovative thermal devices.

Researchers at Qufu Normal University, the University of Hong Kong and the University of Palermo recently observed an anomalous thermal effect that allows a quantum system to absorb heat from colder thermal reservoirs. This unusual effect, outlined in a paper in Physical Review Letters, was leveraged to develop a new quantum heat engine that simultaneously produces work (i.e., mechanical energy created by converting heat into motion or power) and refrigeration.

"Indefinite causal order (ICO) allows two events to occur in a superposition of orders," Zhong-Xiao Man, co-senior author of the paper, told Phys.org.

"In quantum thermodynamics, these events are modeled as thermalization channels acting on a system via a control qubit, creating an indefinite order. Previous work showed that even with identical channel temperatures, the system need not equilibrate to that temperature—a striking deviation from standard thermodynamics. Motivated by this, we asked: What happens to heat flow when the system and channels start at different temperatures?"

Designing a quantum engine with an unusual advantage

Building on earlier studies, Man and his colleagues set out to explore what happens to heat when a quantum system interacts with two thermal reservoirs. This led them to theoretically predict and experimentally demonstrate an anomalous reversed flow in which a quantum system absorbs heat from colder thermal reservoirs.

The researchers subsequently designed a new quantum Otto engine, a device that converts heat into useful work. In contrast with conventional Otto engines, however, the device they developed can simultaneously produce work and cool another system.

"We experimentally realized both the anomalous flow and the engine on a photonic platform, obtaining results that fully confirm our theoretical predictions," Man explained.

"The key idea behind our paper is that, in the quantum world, two thermal processes can occur in a superposition of different orders. This creates an unusual heat flow, allowing the engine to draw energy from a colder environment in a way that would not be possible classically."

The newly developed quantum heat engine uses the unusual heat flow to operate a quantum Otto cycle that concurrently generates useful work and provides refrigeration. This is a notable achievement, as enabling the two functionalities would typically require two separate and very different devices.

"First, we identified a new form of anomalous heat flow, showing that quantum coherence can fundamentally alter the way heat is exchanged between systems," said Giulio Chiribella, co-author of the paper.

"Second, we provided a complete theoretical and experimental demonstration of these unconventional thermodynamic effects in a photonic platform, bringing what was largely a theoretical concept closer to physical realization. Third, we clarified an important foundational issue by proving that these effects are not exclusive signatures of indefinite causal order but can also be reproduced within a definite causal structure."

Possible applications and future research avenues

In the future, the team's design and the anomalous heat flow they reported could be used to develop other promising quantum technologies that manipulate heat and energy in fundamentally new ways. While the engine introduced in the recent paper is still a proof of principle, it could potentially open new avenues for cooling quantum processors and managing heat in quantum sensors, imaging technologies or nanoscale devices.

"One area for future research is to move beyond idealized thermodynamic cycles and investigate more realistic implementations where all operations occur in a finite time," added Rosario Lo Franco, co-senior author of the paper.

"In practical devices, speed matters because it determines not only efficiency but also power output. Recent theoretical studies have shown that finite-time measurements and control operations can introduce fundamental trade-offs between energetic cost, information gain and performance, and we would like to understand how these limitations affect the type of engine proposed in our work." 

by Ingrid Fadelli, Phys.org

edited by Sadie Harley, reviewed by Robert Egan 

Source: A quantum heat engine that simultaneously provides work and refrigeration