Researchers at the Vienna University of Technology have made significant strides in understanding quantum entropy, challenging long-held beliefs in the field. Their study, published in PRX Quantum on February 3, 2025, reveals that entropy in a closed quantum system increases over time until it reaches a peak, aligning it with classical thermodynamic principles.
Traditionally, it was thought that quantum physics operated outside the laws of thermodynamics, particularly the second law, which states that entropy in an isolated system tends to increase. This misconception stemmed from the work of mathematician John von Neumann, who suggested that if complete information about a quantum system is known, its entropy remains constant.
However, the Vienna researchers argue that this perspective is flawed. They emphasize that complete knowledge of a quantum system is fundamentally unattainable due to inherent uncertainties in measurement. Instead, they propose using Shannon entropy, a concept developed by Claude Shannon in 1948, to gauge uncertainty in specific observable outcomes.
Florian Meier, the study's lead author, explained that when only one measurement outcome is possible with certainty, Shannon entropy equals zero, indicating no new information is gained. Conversely, a system with multiple possible outcomes results in high Shannon entropy, reflecting greater uncertainty and the potential for new insights.
The researchers illustrate that as time progresses, the entropy of a quantum system increases, mirroring the behavior observed in classical thermodynamics until it stabilizes at equilibrium. This finding implies that the second law of thermodynamics applies to isolated quantum systems, provided the correct questions are asked and appropriate definitions of entropy are utilized.
This research not only deepens our understanding of quantum mechanics but also has potential applications in quantum computing and information theory, where managing and manipulating entropy is crucial for developing efficient algorithms and systems.