Fundamental limitations in Lindblad descriptions of systems weakly coupled to baths

TL;DR

This paper investigates the fundamental limitations of Lindblad-form quantum master equations used to describe weakly coupled open quantum systems, highlighting common violations of physical principles and conservation laws.

Contribution

It establishes fundamental conditions that approximations to Redfield equations must satisfy to be physically consistent and demonstrates that existing Lindblad descriptions often violate these conditions.

Findings

Violations of local conservation laws in existing Lindblad models

Inaccurate coherences despite correct populations in some Lindblad equations

Numerical validation in an interacting quantum spin system

Abstract

It is very common in the literature to write down a Markovian quantum master equation in Lindblad form to describe a system with multiple degrees of freedom and weakly connected to multiple thermal baths which can, in general, be at different temperatures and chemical potentials. However, the microscopically derived quantum master equation up to leading order in system-bath coupling is of the so-called Redfield form which is known to not preserve complete positivity in most cases. Additional approximations to the Redfield equation are required to obtain a Lindblad form. We lay down some fundamental requirements for any further approximations to Redfield equation, which, if violated, leads to physical inconsistencies like inaccuracies in the leading order populations and coherences in the energy eigenbasis, violation of thermalization, violation of local conservation laws at the non-equilibrium steady state (NESS). We argue that one or more of these conditions will generically be violated in all the weak system-bath-coupling Lindblad descriptions existing in literature to our knowledge. As an example, we study the recently derived Universal Lindblad Equation (ULE) and use these conditions to show violation of local conservation laws due to inaccurate coherences but accurate populations in energy eigenbasis. Finally, we exemplify our analytical results numerically in an interacting open quantum spin system.