The Joint Condensed Matter Seminar (JCMS) series is organised by KTH Royal Insitute of Technology, Nordita, and Stockholm University.
On Monday, October 13th, 2025 from 11.00 am to 12.00 we will host a seminar by Jonas Larson from Stockholm University.
Open quantum systems, chaos, and numerics
In this talk, we will bring together three research directions: (1) open quantum systems, (2) spectral statistics and quantum chaos, and (3) non-Hermitian physics. After developing the necessary background, I will demonstrate how numerical results applied to open quantum systems can be misleading and may even falsely predict physical phenomena.
In particular, numerical studies often rely on truncations of the state space, under the assumption that this is justified when focusing on low-energy physics. For open quantum systems, however, the dynamics is generated not by a Hamiltonian but by a Liouvillian. This distinction has far-reaching implications: truncation effectively introduces an artificial boundary in state space, which can induce a non-Hermitian skin effect and thereby cause spectral instabilities.
As a consequence, Liouvillian spectral statistics can appear to indicate chaos, when in fact this is merely the result of exponential amplification of numerical round-off errors. Two key lessons emerge: (1) numerical diagonalization of Liouvillians must be approached with great caution, and (2) level statistics in non-Hermitian systems, by themselves, cannot serve as reliable diagnostics of chaos.