At a Glance
- Researchers have established a second law of entanglement manipulation, proving that transformations between quantum states can be made perfectly reversible under certain conditions.
- Previously, converting entangled states was considered irreversible when parties were restricted to local operations and classical communication, resulting in a net loss of entanglement.
- The key to achieving reversibility is a theoretical “entanglement battery,” an auxiliary quantum system that stores and supplies entanglement during the transformation process.
- This new framework allows for any mixed-state entanglement transformation to be reversed by ensuring the total entanglement, including the battery’s, is never depleted.
- The battery concept can be generalized to other quantum resource theories, offering a unified method for proving reversibility and designing efficient quantum technologies.
An international team of researchers has unveiled a quantum mechanical version of the second law of thermodynamics, a principle first formulated over 200 years ago. Their discovery, published in Physical Review Letters, proves that the quantum phenomenon of entanglement can be manipulated reversibly, much like energy in an idealized engine. This finding resolves a long-standing debate in quantum information science and deepens the fundamental understanding of one of the most central features of quantum theory.
Entanglement describes a unique connection between two or more quantum particles, where measuring a property of one instantly influences the others, regardless of the distance separating them. While this property is a key resource for technologies such as quantum computing and cryptography, scientists have long struggled with a significant limitation. When two distant parties—often referred to as Alice and Bob—attempt to transform one entangled state into another using only local actions and classical communication (LOCC), the process has been proven to be irreversible, meaning entanglement is always lost. “So the question is, can we somehow go beyond LOCC in a meaningful way, and recover reversibility?” explained Alexander Streltsov, the study’s senior author, in a University of Warsaw press release.
The team’s answer is yes, by introducing an auxiliary tool they call an “entanglement battery.” This hypothetical quantum system functions like a regular battery, but instead of storing energy, it stores and supplies entanglement. During a transformation, Alice and Bob can draw from or add to the battery, with the single rule that the battery’s overall entanglement level cannot decrease. Using this battery-assisted method, the researchers demonstrated that any entanglement transformation between states can be made perfectly reversible, with no net loss of the precious quantum resource.
This breakthrough provides a conclusive answer to the reversibility question and offers a powerful new framework for studying other quantum resources. The battery concept can be generalized to reversibly manipulate other properties, such as quantum coherence or thermodynamic free energy, creating a unified approach to establish second-law-like principles across quantum physics. According to the researchers, these findings could pave the way for understanding complex quantum networks and developing highly efficient future quantum technologies.
References
- Ganardi, R., Kondra, T. V., Ng, N. H. Y., & Streltsov, A. (2025). Second law of entanglement manipulation with an entanglement battery. Physical Review Letters, 135(1), 010202. https://doi.org/10.1103/kl56-p2vb
- University of Warsaw. (2025, July 3). Quantum equivalent of thermodynamics’ second law discovered for entanglement manipulation. Phys.Org; University of Warsaw. https://phys.org/news/2025-07-quantum-equivalent-thermodynamics-law-entanglement.html
