Many-body exceptional points in colliding condensates

TL;DR

This paper investigates exceptional points in the Bogoliubov spectrum of colliding Bose-Einstein condensates, linking them to dynamical instabilities, superfluidity criteria, and proposing experimental detection methods.

Contribution

It identifies the role of exceptional points in the counterflow instability of Bose-Einstein condensates and connects this to broader concepts like superfluidity and classical scattering.

Findings

The instability arises from an exceptional point in the Bogoliubov spectrum.

Proposed an experimental setup to observe the exceptional point.

Numerical calculations support the feasibility of detection.

Abstract

Exceptional points describe the coalescence of the eigenmodes of a non-Hermitian matrix. When an exceptional point occurs in the unitary evolution of a many-body system, it generically leads to a dynamical instability with a finite wavevector [N. Bernier \etal, Phys. Rev. Lett. 113, 065303 (2014)]. Here, we study exceptional points in the context of the counterflow instability of colliding Bose-Einstein condensates. We show that the instability of this system is due to an exceptional point in the Bogoliubov spectrum. We further clarify the connection of this effect to the Landau criterion of superfluidity and to the scattering of classical particles. We propose an experimental set-up to directly probe this exceptional point, and demonstrate its feasibility with the aid of numerical calculations. Our work fosters the observation of exceptional points in nonequilibrium many-body quantum systems.