Quantum Decoherence, Energy Exchange and Thermodynamics of Open Systems.

The behaviour of quantum systems inevitably deviates from ideal isolation, interacting with surrounding environments and undergoing a process known as decoherence, where quantum superposition and entanglement erode. Understanding the thermodynamic consequences of this decoherence, particularly when considering energy exchange with the environment, presents a significant challenge. Researchers now meticulously compare ‘local’ and ‘global’ thermodynamic descriptions of these open quantum systems, differing in how they account for the thermal reservoir’s influence. Irene Ada Picatoste, from the Institute of Physics at the University of Freiburg, Alessandra Colla, of the Dipartimento di Fisica Aldo Pontremoli at the Università degli Studi di Milano, and Heinz-Peter Breuer, also at the University of Freiburg, present a detailed analysis in their article, “Local and global approaches to the thermodynamics of pure decoherence processes in open quantum systems”. Their work employs a solvable model to reveal substantial discrepancies arising from differing definitions of key thermodynamic quantities, such as internal energy and entropy production, and their implications for fundamental laws governing energy transfer.

Open quantum systems present considerable challenges to conventional thermodynamic descriptions, necessitating careful delineation of system boundaries and accounting for energy exchange with the surrounding environment. Researchers currently investigate the interplay between decoherence, the loss of quantum coherence due to environmental interaction, and established thermodynamic quantities, revealing discrepancies arising from differing perspectives on energy flow between an open system and its thermal reservoir. This study systematically compares local and global approaches to defining key thermodynamic concepts – internal energy, work, heat, and entropy production – providing a nuanced understanding of how theoretical frameworks shape interpretations of thermodynamic behaviour.

The investigation centres on pure decoherence processes, where the system loses quantum coherence through interaction with the environment, but its populations remain constant. This allows for a focused examination of how different definitions of thermodynamic quantities emerge. Researchers demonstrate that global approaches, explicitly considering the reservoir’s degrees of freedom, account for substantial energy exchange, while local approaches, focusing solely on the system, register a constant average energy. This difference arises because local approaches inherently omit the energy transferred to the reservoir as the system decoheres.

Scientists demonstrate that these global perspectives reveal a significant energy transfer between the system and the environment, a factor absent in local calculations. This stems from the nature of pure decoherence where the system’s populations remain constant, masking the energy flow at the interface. The researchers perform a detailed comparison of the two approaches, highlighting how the formulation of both the first and second laws of thermodynamics differs depending on whether the reservoir’s influence is considered. Specifically, the first law, concerning energy conservation, appears differently when reservoir energy changes are included, and the second law, relating to entropy increase, requires careful consideration of the system-reservoir correlation.

The research provides a nuanced understanding of how different theoretical frameworks can lead to divergent interpretations of thermodynamic quantities in open quantum systems. It demonstrates that a complete thermodynamic description necessitates considering the reservoir’s role in energy exchange, even when the system’s internal energy remains constant. Scientists contribute to a more comprehensive understanding of nonequilibrium dynamics and provide valuable insights for the development of accurate models for quantum technologies and complex physical systems, offering a crucial framework for interpreting experimental results and designing efficient quantum devices. This is particularly relevant as quantum technologies increasingly rely on systems interacting with their environment, where accurate thermodynamic modelling is essential for optimisation and control.