Linear and nonlinear spectroscopy from quantum master equations

TL;DR

This paper assesses the accuracy of the TCL2 quantum master equation for linear and nonlinear spectroscopy in multichromophore systems, and introduces a hybrid quantum-classical scheme to improve third-order spectroscopic predictions.

Contribution

It demonstrates the broad accuracy of TCL2 for linear spectra and develops a hybrid approach combining TCL2 with classical sampling to enhance nonlinear spectroscopy results.

Findings

TCL2 accurately predicts linear absorption spectra over a wide parameter range.

The hybrid scheme improves third-order spectroscopy accuracy by combining quantum and classical methods.

The approach scales efficiently and handles non-adiabatic multilevel systems.

Abstract

We investigate the accuracy of the second-order time-convolutionless (TCL2) quantum master equation for the calculation of linear and nonlinear spectroscopies of multichromophore systems. We show that, even for systems with non-adiabatic coupling, the TCL2 master equation predicts linear absorption spectra that are accurate over an extremely broad range of parameters and well beyond what would be expected based on the perturbative nature of the approach; non-equilibrium population dynamics calculated with TCL2 for identical parameters are significantly less accurate. For third-order (two-dimensional) spectroscopy, the importance of population dynamics and the violation of the so-called quantum regression theorem degrade the accuracy of TCL2 dynamics. To correct these failures, we combine the TCL2 approach with a classical ensemble sampling of slow microscopic bath degrees of freedom, leading to an efficient hybrid quantum-classical scheme that displays excellent accuracy over a wide range of parameters. In the spectroscopic setting, the success of such a hybrid scheme can be understood through its separate treatment of homogeneous and inhomogeneous broadening. Importantly, the presented approach has the computational scaling of TCL2, with the modest addition of an embarrassingly parallel prefactor associated with ensemble sampling. The presented approach can be understood as a generalized inhomogeneous cumulant expansion technique, capable of treating multilevel systems with non-adiabatic dynamics.