University researchers have developed a new quantum heat engine that produces work (i.e., mechanical energy created by converting heat into motion or power) and refrigeration at the same time.
Researchers at Qufu Normal University, the University of Hong Kong and the University of Palermo observed an anomalous thermal effect that allows a quantum system to absorb heat from colder thermal reservoirs.
As the team described in Physical Review Letters, this unusual effect informed the creation of a new quantum Otto engine.
“Indefinite causal order (ICO) allows two events to occur in a superposition of orders,” Zhong-Xiao Man, co-senior author of the paper, commented.
“In quantum thermodynamics, these events are modelled as thermalisation 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?”
Quantum system thermodynamics can behave differently to standard conditions
Heat naturally flows from warmer places to colder places until a state of thermal equilibrium is met. However, quantum mechanics and quantum systems can act differently, in ways that are not easily explained by our current understanding of physics. These unusual behaviours could be utilised to create new, more efficient thermal technologies.
The researchers wanted to explore what happens to heat when a quantum system encounters two thermal resevoirs. Their theory led to demonstrating an anomalous reversed flow, where the quantum system absorbs heat from colder thermal resevoirs.
The team were able to use this demonstration to design a new quantum Otto engine, a mechanism that converts heat into useful work.
“We experimentally realised 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 new engine performs the roles of two devices
This new quantum heat engine is able to use the unusual heat flow to work a quantum Otto cycle that simultaneously generates useful work and provides refrigeration, functions which would normally require two 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 realisation. 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.”
