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Anomalous Heat Flows and Quantum Otto Engine with (In)definite Causal Order
Qing-Feng Xue1, Qi Zhang1, Xu-Cai Zhuang1
1Qufu Normal University, School of Physics and Physical Engineering, 273165, Qufu, China.
Physical Review Letters
|July 31, 2026
Summary
Quantum systems can absorb heat against the usual temperature gradient when undergoing thermalization in an indefinite causal order. This discovery enables a quantum Otto cycle that generates work and refrigeration simultaneously.
Area of Science:
- Quantum thermodynamics
- Quantum information science
- Thermodynamics
Background:
- Classical thermodynamics dictates heat flows from hot to cold reservoirs.
- This principle is well-established for macroscopic systems at thermal equilibrium.
Purpose of the Study:
- To investigate heat flow in quantum systems with indefinite causal order.
- To explore the potential for anomalous heat absorption and novel thermodynamic cycles.
Main Methods:
- Utilizing a quantum switch to implement two thermalization processes in an indefinite causal order.
- Analyzing the quantum Otto cycle under these unconventional conditions.
- Developing an equivalent causally ordered quantum circuit for comparison.
- Experimentally realizing photonic setups to demonstrate the phenomena.
Main Results:
- Demonstrated anomalous heat absorption in a quantum system, even when its initial temperature exceeds reservoir temperatures.
- Designed and analyzed a quantum Otto cycle exhibiting both work generation and refrigeration.
- Showcased that the quantum switch's effects can be replicated by parallel thermalizations in a causally ordered circuit.
- Confirmed the experimental feasibility using free-space photonic implementations.
Conclusions:
- Indefinite causal order enables heat absorption against the classical thermodynamic gradient.
- The quantum Otto cycle with indefinite causal order offers a novel approach to work extraction and refrigeration.
- Photonic implementations provide a practical platform for realizing these quantum thermodynamic effects.
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