Researchers from the Normal University of Qufu, Hong Kong, and Palermo announced the development of a thermal quantum engine. This new engine has the capacity to produce both work and cooling at the same time. The study, which combines theoretical and experimental evidence of the phenomenon, was published in the journal Physical Review Letters.
The starting point for this discovery was the observation of an atypical thermal effect: a quantum system that absorbs heat from colder thermal reservoirs. This behavior contradicts the established classical principle, according to which heat always spontaneously moves from hotter to colder areas until thermal equilibrium is reached, a principle essential for technologies such as refrigerators and power plants.
This phenomenon is intrinsically linked to the concept of Indefinite Causal Order (ICO). Zhong-Xiao Man, a senior co-author, explained to Phys.org that ICO allows two events to exist in a superposition of orders. In quantum thermodynamics, these events are represented as thermalization channels acting on a system through a control qubit, generating this indefinite order.
Man added that previous research had already demonstrated that, even using channels at equal temperatures, the system did not need to stabilize at that temperature, representing a significant deviation from conventional thermodynamics. With this, the team investigated what happens to the heat flow when the system and the channels have different temperatures. Man stated: “The central idea of 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 extract energy from a colder environment in a way that would not be possible classically.”
Based on this abnormal heat flow, the scientists conceived a quantum Otto engine, a device designed to convert heat into usable energy. Unlike traditional Otto engines, this device operates a quantum cycle that can generate useful work and provide cooling simultaneously, something that would normally require two separate pieces of equipment. Both the anomalous flow and the engine were experimentally tested on a photonic platform, and the results fully confirmed the team's theoretical projections.
Giulio Chiribella, another co-author, highlighted three important advances of the work. Firstly, a new modality of anomalous heat flow was identified, proving that quantum coherence can drastically modify heat exchange between systems. Secondly, a complete demonstration, both theoretical and experimental, of these unconventional thermodynamic effects was carried out on a photonic platform. Finally, it was proven that such effects are not exclusive to indefinite causal order, but can be replicated within a defined causal structure.
Although the presented engine is still only a proof of concept, the authors suggest that the design and the anomalous heat flow can be applied in the development of quantum technologies. These include cooling quantum processors, managing heat in quantum sensors, as well as in imaging and nanoscale devices. Rosario Lo Franco, a senior co-author, indicated that future research should focus on going beyond ideal thermodynamic cycles, investigating more practical implementations where all operations occur within a limited time. Lo Franco observed that recent studies indicate that finite-time measurements and controls can introduce crucial trade-offs between energy cost, information gain, and performance, and the team plans to understand how these constraints impact the proposed engine.
Researchers from the University of Southampton and the National University of Singapore have presented a method for creating two-dimensional heterostructures without using polymers, which are traditionally employed in this process. The results of this study were published in the journal Nature Communications.
The proposed approach replaces adhesive synthetic materials with natural muscovite crystals, which is a type of mica. This allows for a reduction in microscopic contamination that typically degrades the quality of structures composed of layers only a few atoms thick.
This achievement has the potential to increase the accuracy of research using quantum materials and accelerate the development of more reliable electronic components. Surface purity is recognized as a critical factor for the performance of such devices.
The production of two-dimensional materials depends on stacking extremely thin atomic layers. Standard methods for this assembly use synthetic polymers, which leave microscopic residues that alter the electronic behavior of the structures. To overcome this problem, the team used muscovite, a mineral known as mica.
According to the researchers, because muscovite is an inorganic crystal, it provides a more uniform surface and significantly reduces contaminants during the assembly of heterostructures. The work also demonstrated that this substitution allows for the placement of atomic layers with greater precision. Such control is important because some two-dimensional materials acquire completely different properties when stacked at specific angles.
The study's lead author, Marcus Shishkin, a professor of experimental physics at the University of Southampton, emphasized that this level of precision is critical for research in quantum materials. He stated that when two-dimensional materials, such as graphene and hexagonal boron nitride, are stacked into structures with controlled angles between layers, they begin to exhibit entirely new properties.
The researcher explained that small amounts of contamination can distort experimental results, so a cleaner process expands the scope of scientific inquiry. He also noted that the new method simplifies the fabrication of structures and reduces process costs.
In Shishkin's assessment, improving contamination-free fabrication methods is an important step toward creating more efficient nanoelectronic components and, in the future, faster and more reliable microchips.
Alexey Berdyugin, a professor at the National University of Singapore and co-author of the study, noted that creating atom stacks without impurities remains one of the biggest challenges in nanotechnology. He added that since mica is an inorganic crystal rather than a soft polymer, it avoids many contamination issues inherent in traditional methods.
According to Berdyugin, this method provides ultra-pure surfaces, allowing electronic components to operate at maximum performance. The researcher believes this feature could pave the way for both fundamental research and future applications of quantum technologies.
