Open harmonic chain without secular approximation

TL;DR

This paper compares exact and approximate Markovian dynamics in an open quantum harmonic chain, revealing how different approximations affect energy transport descriptions and emphasizing the importance of choosing appropriate models.

Contribution

It provides a detailed analysis of how local, global, and coarse-grained Markovian approximations influence energy transport in an open quantum harmonic chain, highlighting the impact of the rotating wave approximation.

Findings

Exact and local dynamics preserve divergence form of energy flow.

Global dynamics with RWA introduces non-divergence sink/source terms.

Sink and source contributions depend on the coarse-graining parameter t.

Abstract

We study particle and energy transport in an open quantum system consisting of a three harmonic oscillator chain coupled to thermal baths at different temperatures placed at the ends of the chain. We consider the exact dynamics of the open chain and its so-called local and global Markovian approximations. By comparing them, we show that, while all three yield a divergence-like continuity equation for the probability flow, the energy flow exhibits instead a distinct behavior. The exact dynamics and the local one preserve a standard divergence form for the energy transport, whereas the global open dynamics, due to the rotating wave approximation (RWA), introduces non-divergence sink/source terms. These terms also affect the continuity equation in the case of a master equation obtained through a time-coarse-graining method whereby RWA is avoided through a time-zoom parameter $\Delta t$. In such a scenario, sink and source contributions are always present for each $\Delta t>0$. While in the limit $\Delta t\to+\infty$ one recovers the global dissipative dynamics, sink and source terms instead vanish when $\Delta t\to 0$, restoring the divergence structure of the exact dynamics. Our results underscore how the choice of the dissipative Markovian approximation to an open system dynamics critically influences the energy transport descriptions, with implications for discriminating among them and thus, ultimately, for the correct modeling of the time-evolution of open quantum many-body systems.