THURSDAY COLLOQUIUM | Quantum correlations in time: Macrorealism and its extreme limits

Speaker: A. R. Usha Devi Senior Professor, Department of Physics, Bangalore University, Bangalore Abstract The Leggett–Garg framework probes the classical notion of macrorealism using temporal correlations obtained from sequential measurements on a single system. Traditionally, such tests rely on correlation-based inequalities, which are sensitive to measurement invasiveness. In this colloquium, I will first review the conceptual foundations of macrorealism and the motivation behind Leggett–Garg inequalities. I will then introduce entropic Leggett–Garg inequalities, which replace correlation functions with information-theoretic quantities and provide a complementary way of detecting non-classical temporal behavior.  I will then discuss recent advances in pushing temporal quantum correlations beyond conventional bounds, including scenarios where temporal correlations approach or achieve their algebraic maximum. I will explain the physical mechanisms that enable such extremal temporal correlations.  The talk will be self-contained and aimed at a broad audience, while highlighting how temporal correlations offer both a deeper understanding of quantum foundations and new opportunities for quantum information processing. About the speaker: Dr. A. R. Usha Devi (Senior Professor) has been teaching in the Department of Physics, Bangalore University, Bangalore for nearly 30 years. Her research interests are in quantum information processing, open quantum systems and mathematical physics (theoretical). Professor Usha Devi is a M.Sc gold medalist (Mysore University) and she had won the Indian Physical Society (IPS) Young Physicist Award during the year 1997. During 2008 She was awarded the Commonwealth Fellowship and worked with the research group of Professor Sandu Popescu at Bristol University, Bristol. She worked as a member of the Editorial Board of Resonance, journal of science education published monthly by Indian Academy of Sciences during 2015-21. Dr. Usha Devi has published around 100 research papers in reputed international journals; has guided 10 Ph.D students; she serves as a Referee of national and international journals.  She has worked as a resource person in Indian Academy sponsored lecture series/workshops and delivered series of lectures; she has worked as program committee member in National/International  Conferences/Symposia in the area of Quantum Information Processing. More recently, her profile was featured as one of the leading women physicists contributing to the advancement of quantum science in India  in an article titled “The rise of women in quantum science in India and the legacy of Satyendra Nath Bose”  -- published by  "Physics World" during July 2025  (IOP publication  https://share.google/5QYmRKDwf5YWfQHDs ).