Felipe Reyes-Osorio, University of Delaware

Nonequilibrium field theory of open spin systems: from semiclassical to nonperturbative quantum dynamics

Open systems of many interacting spins—as realized by localized magnetic moments in a spintronic device, or qubits in a quantum computer—pose a formidable challenge for presently available theoretical methods, especially when the memory effects induced by the surrounding environment are relevant. Even archetypical examples like the spin-boson model, in which a single spin interacts with a continuum of bosonic modes requires switching between specialized methods for different choice of system-bath parameters. Here, we present a field theory (FT) of open quantum spin systems based on the Schwinger-Keldysh (SK) functional integral, which serves as the starting point for both semiclassical and fully quantum descriptions of the dynamics. In the semiclassical regime, we obtain corrections to the Landau-Lifshitz-Gilbert equations, conventionally employed in spintronics and magnonics, accounting for, e.g., nonlocal magnetic damping. On the other hand, the fully quantum regime is probed by combining SKFT with the two-particle irreducible (2PI) action resumming a class of Feynman diagrams to an infinite order. Remarkably, our SKFT+2PI closely tracks numerically exact benchmarks for the spin-boson and spin-chain-boson models, even in the nonperturbative and non-Markovian regime. The favorable numerical cost of solving integro-differential equations produced by SKFT+2PI framework with increasing number of spins, time steps or spatial dimensionality makes it a promising route for simulation of driven-dissipative systems in quantum computing or quantum spintronics and magnonics in the presence of a single or multiple dissipative environments.