Seminar — Driven-dissipative spin systems in spintronics, magnonics and quantum computing

The driven-dissipative many-body systems remain one of the most challenging unsolvedproblems of quantum physics. When “body” is spin, such as of electrons or engineered like qubits, these systemsunderlie spintronics, magnonics and quantum computing technologies. In this talk, I will first explain conditions [1]under which quantum spins interacting with a dissipative environment can transition toward classical dynamics governedby the celebrated Landau-Lifshitz-Gilbert (LLG) equation. The extended LLG equation for classical spin dynamics, whichincludes non-Markovian and spatially nonlocal damping of quantum origin, can be rigorously derived fromSchwinger-Keldysh quantum field theory (SKFT) [2] by integrating out fermionic or bosonic bath and by neglectingquantum fluctuations of spin fields. Its application [3] to magnons explains recent experiments [4] where quantumsensing 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 opticallyexcited 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 have derived [6] time evolution of spin in archetypical open quantum system--the spin-boson model also of greatimportance for understanding decoherence of idle superconducting qubits. Despite only a class of Feynman diagrams beingresummed to infinite order by 2PI, where those diagrams are generated by expansion in 1/N (where N in the number ofSchwinger bosons to which spin is mapped) instead of expansion in coupling constant, our SKFT can track numericallyexact non-Markovian simulations from tensor networks (TN) methods. It also provides access to longer times and higherspatial dimensions where TN fail due to the emergence of “entanglement barrier.”

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, arXiv:2504.17769 (2025).
[6] F. Reyes-Osorio, F. Garcia-Gaitan, D. J.Strachan, P. Plecháč, S. R. Clark, and B. K.Nikolić, arXiv:2405.00765 (2024)