Hybrid Quantum-Classical Stochastic Approach to Spin-Boson Models

Abstract: Interacting spin-boson models encompass a large class of physical systems, spanning models with a single spin interacting with a bosonic bath -- a paradigm of quantum impurity problems -- to models with many spins interacting with a cavity mode -- a paradigm of quantum optics. Such models have emerged in various quantum simulation platforms which are further subject to noise and lossy dynamics. As generic many-body systems, dynamics of spin-boson models constitutes a challenging problem. In this paper, we present an exact hybrid quantum-classical stochastic approach to different spin-boson models which are typically treated using distinct techniques. In this approach, the solution of a classical stochastic equation (mimicking the bosonic modes) is input into a quantum stochastic equation for the spins. Furthermore, the spins are effectively decoupled for each stochastic realization, but this comes at the expense of sampling over unphysical states. Remarkably, the dynamics remains Markovian in our approach even in the strong coupling regime. Moreover, we utilize Markovian dissipation to make \textit{causality} manifest, thus ensuring hermiticity (though not positivity) of the density matrix for each realization. Finally, in contrast with many existing methods, we place no restriction on the initial state, and further argue that an intrinsic nonlinearity of the bosonic modes can be tackled within this framework. We benchmark and showcase the utility of our approach in several examples, specifically in cases where an exact numerical calculation is far from reach.