Optimal Thermalization under Indefinite Causal Order Achieves Precise Temperature Control Using Two Thermal Baths

The fundamental process of thermalization, where a system reaches equilibrium with its environment, receives a surprising boost from a concept borrowed from quantum foundations, indefinite causal order. Neeraj Sharma and Parveen Kumar, both from the Department of Physics at the Indian Institute of Technology Jammu, demonstrate how manipulating the order in which a quantum system interacts with its surroundings can dramatically alter its final temperature. Their work reveals that by placing these interactions in a superposition, researchers gain control over heating and cooling processes, achieving outcomes impossible with traditional, fixed causal sequences. This advance establishes a systematic method for evaluating the potential of indefinite causal order and highlights coherence as a valuable, tunable resource for optimising thermal processes, potentially impacting fields from quantum thermodynamics to information processing.

Indefinite Causal Order and Thermalisation Efficiency

Optimal thermalisation, the process by which a system reaches thermal equilibrium, can be achieved using indefinite causal order, where the order of quantum operations exists in a coherent superposition. This research investigates how indefinite causal order influences the efficiency and speed at which a quantum system equilibrates with its environment, analysing scenarios with both identical and asymmetric thermal baths. The study establishes a theoretical framework for quantifying thermalisation performance under indefinite causal order, revealing that specific configurations can accelerate the rate at which a quantum system reaches equilibrium. Furthermore, the investigation demonstrates that greater asymmetry in the thermal baths generally leads to more pronounced improvements in thermalisation performance.

The research centres on a two-level system interacting with two thermal baths under a quantum switch, where an ancillary qubit controls the order of operations. Scientists derived precise mathematical expressions for the effective temperature of the system after this interaction, identifying control settings that maximise heating or cooling. The analysis reveals how the diagonal and coherent components of the control qubit’s state separately contribute to temperature shifts, and how their interplay enables thermal responses unattainable with fixed causal orders. Greater bath asymmetry enhances these effects.

Indefinite Causal Structures Discriminate Thermal States

This research explores how indefinite causal structures, which challenge the classical notion of a definite cause-effect relationship, can improve the ability to distinguish between two weakly interacting thermal states. Scientists employed quantum Fisher information, a metric quantifying the maximum information extractable from a quantum state, to optimise the discrimination process. The findings demonstrate that implementing an indefinite causal structure significantly enhances the accuracy of discriminating between these thermal states compared to traditional approaches. The performance of this scheme depends critically on the parameters defining the structure, and the research details how to optimise these parameters to achieve the best possible discrimination accuracy. The results are supported by both analytical calculations and numerical simulations, providing strong evidence for the effectiveness of the proposed scheme.

The research relies on theoretical modelling and mathematical analysis of quantum systems, utilising concepts from quantum information theory. Numerical simulations validate the analytical results and explore the system’s behaviour under different conditions. The findings have potential applications in quantum technologies such as quantum sensing, quantum imaging, and quantum communication, and contribute to advancements in quantum metrology. The work deepens our understanding of quantum information processing and the role of indefinite causal structures, with implications for quantum thermodynamics and the resource theory of quantum superposition.

Indefinite Causal Order Controls Thermalization Rates

This research demonstrates how indefinite causal order alters the process of thermalisation when a two-level system interacts with two thermal baths. Scientists derived precise mathematical expressions describing the effective temperature of the system after this interaction, identifying specific control settings that maximise either heating or cooling. The analysis reveals that both the diagonal components and the coherence of the control qubit contribute to temperature shifts, with their interplay enabling thermal responses unattainable under conventional, fixed causal orders. The team found that asymmetry between the temperatures of the two thermal baths amplifies these effects, while imperfections in the control qubit diminish them. This work extends previous studies by considering a broader range of control parameters and explicitly demonstrating how bath asymmetry reshapes the temperature shifts induced by indefinite causal order.

While the observed temperature changes occur only in the successful outcome of the quantum switch, quantum coherence in the control qubit serves as a tunable resource for modifying thermalisation. Future research will explore the potential for indefinite causal order to facilitate work extraction, design thermodynamic cycles that leverage this phenomenon, and assess its robustness to noise and imperfections in the system.