Modelling non-Markovian and non-perturbative environments with unphysical modes*

Pseudo-modes are discrete effective modes used to model the effect of continuum environments on open quantum systems. Here we develop a new approach to pseudomodes to deal with a quantum system ultra-strongly-coupled to a bosonic continuum even at zero temperature [1]. This is made possible by defining pseudomodes which have a non-Hermitian interaction with the system, and which in isolation are unphysical. We show how they successfully describe the appearance of virtual excitations in the zero-temperature steady-state, which we analyze and bench-mark with the hierarchy-equations-of-motion and the reaction-coordinate approaches. We also discuss new applications of our approach to other types of environments and to error mitigation.

[1] N. Lambert, S. Ahmed, M. Cirio and Franco Nori, Nature Communications 10, 3721 (2019).