Unusual quantum effect used to build a heat engine that produces work and cooling

A collaboration of researchers across international borders observed an anomalous thermal effect that allows quantum systems to absorb heat from cold thermal reservoirs. The team then leveraged this behavior to build a quantum heat engine that can produce mechanical work and refrigeration at the same time. Classical physics tells us that heat naturally flows from systems at higher temperature to those at lower temperature. This principle has been used to develop a range of technologies from power plants to household refrigerators, and we all know that it works until an equilibrium between the two systems has been reached. So, when researchers at Qufu Normal University, the University of Hong Kong and the University of Palermo observed an anomalous thermal behavior due to quantum effects, they immediately thought of possible applications it could drive. The result was a heat engine that works on two things simultaneously. Indefinite Causal Order Quantum systems often display behaviors that are difficult to explain with classical physics. “Indefinite Causal Order (ICO) allows two events to occur in a superposition of orders,” explained Zhong-Xiao Man, a researcher in quantum mechanics at Qufu Normal University. “Previous work has shown that even with identical channel temperatures, the system need not equilibrate to that temperature,” added Man. This is a major deviation from classical thermodynamics, and so the researchers questioned what would happen to the heat flow if the system and channel started at different temperatures or when there are two thermal reservoirs. In their theoretical work, the researchers predicted that the quantum system could absorb heat from colder thermal reservoirs. They then built a new quantum Otto engine that experimentally demonstrated their predictions. “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,” explained Man further. What happens next? By bringing a theoretical concept to physical reality, the researchers have demonstrated that the effects are not just signatures of indefinite causal order but can also be reproduced with defined causal structure. This is likely to help design other quantum technologies that can manipulate heat and energy in different ways. The quantum heat engine built by the researchers is only a proof of principle, opening up applications for a wide range of devices ranging from quantum processors to sensors and much more. “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,” said Rosario Lo Franco, a researcher in Open Quantum Systems at the University of Palermo, who was also involved in the work. “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.” The research findings were published in the journal Physical Review Letters.