Stellenbosch University finds heat speeds up quantum decoherence

Physicists at Stellenbosch University, working with colleagues at the National Institute for Theoretical and Computational Sciences and KTH Royal Institute of Technology, have resolved a decades-old inconsistency within a key model describing how quantum systems lose coherence. The team discovered that heat does not preserve a particle’s ability to exist in multiple states, but instead accelerates decoherence through a previously unrecognised mechanism affecting high-frequency quantum fluctuations.

“The high-temperature limit of quantum Brownian motion has always left a mathematical loose end,” said Professor Francesco Petruccione of Stellenbosch University; by refining the Caldeira-Leggett master equation, the researchers restored physical consistency to the model and identified how heat influences the transition from quantum to classical behavior.

Restored Positivity Suppresses High-Frequency Quantum Fluctuations

The Caldeira-Leggett master equation, a model for quantum Brownian motion established in the 1980s, has long predicted physically impossible states characterized by negative probabilities. The Stellenbosch University team addressed this inconsistency by identifying a previously overlooked mechanism influencing decoherence. This finding resolves a mathematical ambiguity present in the standard model and offers a more accurate depiction of how quantum systems interact with thermal environments. Researchers discovered an additional contribution to the Caldeira-Leggett equation that restores a fundamental requirement for physically realistic quantum descriptions.

This correction specifically suppresses high-frequency quantum fluctuations, effectively damping the most rapidly oscillating quantum components and hastening the system’s move toward classicality. The team achieved this by carefully re-evaluating the decoherence kernel, which captures the environment’s influence on particle motion. “By retaining the next-to-leading contribution to the decoherence kernel, we obtain a description that is both physically consistent and independent of artificial cutoffs,” said Professor Francesco Petruccione, co-author and Pro Vice-Chancellor for Artificial Intelligence and Quantum Technologies at Stellenbosch University.

“High-frequency quantum noise is actively suppressed, providing a natural route from the quantum to the classical world.” The implications of this work extend beyond theoretical consistency, offering a more robust foundation for understanding decoherence at the quantum scale. Dr. Graeme Pleasance, lead author of the study, explained that master equations are often introduced without a firm grounding in first principles, potentially obscuring their validity; this research identifies the correct structure for describing Markovian quantum Brownian motion in the semiclassical limit.