Abstract: The driven-dissipative many-body systems remain one of the most challenging unsolved problems of quantum physics. When “body” is spin, such as of electrons or of engineered ones like qubits, these many-body systems underlie spintronics, magnonics and quantum computing technologies. In this talk, I first explain conditions [1] under which quantum spins interacting with a dissipative environment can transition toward classical dynamics governed by the celebrated Landau-Lifshitz-Gilbert (LLG) equation. Furthermore, after 90 years since the introduction of phenomenological LLG equation, we can now microscopically derive it from Schwinger-Keldysh quantum field theory (SKFT) [2], but with extra ingredients like non-Markovian and spatially nonlocal damping of quantum origin. The application [3] of such extended LLG equation application to magnons explains recent experiments [4] where quantum sensing has measured 100-fold increase of magnon damping in yttrium iron garnet (one of the key materials in magnonics) due to metallic overlayer. It also explains [5] how to properly bring light into the LLG equation and predict optically excited inertial magnons. In the case of fully quantum spin dynamics, by combining SKFT with two-particle irreducible effective (2PI) action formalism and 1/N expansion, both of which have been developed originally in elementary particle physics, we describe [6] time evolution of spin in archetypical open quantum system⸺the spin-boson model. Despite only a class of Feynman diagrams being considered, our SKFT approach can track numerically exact non-Markovian simulations from tensor networks (TN) methods [6], while providing access to longer times and higher spatial dimensions where TN can fail due to the emergence of “entanglement barrier.” These newly developed methods make it possible to understand spatial structure and fate of multipartite entanglement in open quantum spin liquids [7].
References
[1] F. Garcia-Gaitan and B. K. Nikolić, Phys. Rev. B 109, L180408 (2024).
[2] F. Reyes-Osorio and B. K. Nikolić, Phys. Rev. B 109, 024413 (2024).
[3] F. Reyes-Osorio and B. K. Nikolić, Phys. Rev. B 110, 214432 (2024).
[4] I. Bertelli et. al., Adv. Quantum Technol. 4, 2100094 (2021).
[5] F. Reyes-Osorio and B. K. Nikolić, Phys. Rev. Lett. 135, 246701 (2025).
[6] F. Reyes-Osorio, F. Garcia-Gaitan, D. J. Strachan, P. Plecháč, S. R. Clark, and B. K.Nikolić, Rep. Prog. Phys. 89, 018002 (2026).
[7] F. Garcia-Gaitan and B. K. Nikolić, New J. Phys. 28, 083503 (2026).
Biosketch
Branislav K. Nikolić is a Full Professor of Physics at the University of Delaware and a Senior Visiting Scientist at RIKEN in Japan. He received his Ph.D. in theoretical condensed matter physics from Stony Brook University, and a B.Sc. degree from the University of Belgrade, Serbia. He was visiting Professor at the University of Regensburg, National Taiwan University, Centre de Physique Théorique de Grenoble-Alpes and Beijing Normal University. His research focuses on nonequilibrium quantum many-body and field theory, computational quantum transport, multiscale quantum-classical approaches, and high-performance scientific computing, including applications to technologically relevant problems in spintronics, magnonics, quantum information, nanoelectronics and nano-bio interface.