Open quantum systems appear in quantum computers, quantum magnets and spintronics, but their behaviour is extremely difficult to model. The environment introduces memory effects (non‑Markovian dynamics) and strong system-bath interactions (non‑perturbative regimes), where most existing methods fail or require switching between entirely different techniques depending on the parameters. This research presents a single unified framework that can handle all these regimes for interacting quantum spins coupled to bosonic environments.

The approach combines Schwinger-Keldysh field theory with the two‑particle‑irreducible (2PI) effective action and crucially uses a 1/N expansion of Schwinger bosons rather than a perturbative expansion in the system-bath coupling. This allows the method to remain accurate even in strongly non‑perturbative regimes. The framework can compute advanced quantities such as multitime spin correlations, which are essential for understanding quantum phase transitions and nonequilibrium transport in quantum materials.

The authors benchmark their method against quasi‑exact tensor‑network simulations of the spin‑boson model, showing excellent agreement in the regimes where tensor‑network methods are applicable, and then apply it to more complex spin‑chain models with multiple baths where no other method currently works. Because it supports arbitrary spin value, geometry, dimensionality, and bath spectral function, the framework offers a general and computationally tractable route to simulating many‑body open quantum systems.

Overall, this work provides a powerful field‑theoretic tool for studying driven‑dissipative quantum systems, with applications ranging from quantum computing to quantum magnonics and spintronics.

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Keldysh field theory for driven open quantum systems by L M Sieberer, M Buchhold and S Diehl (2016)