# Observation of Roton Mode Population in a Dipolar Quantum Gas

## TL;DR

This paper reports the first experimental observation of a roton mode in a dipolar quantum gas of erbium atoms, confirming theoretical predictions and advancing understanding of excitations in quantum liquids.

## Contribution

It provides the first direct evidence of roton modes in dipolar quantum gases through experimental momentum distribution measurements.

## Key findings

- Roton peaks appear at predicted momenta after interaction quench.
- Roton momentum scales inversely with confinement length.
- Roton softening and imaginary gap are observed over time.

## Abstract

The concept of a roton, a special kind of elementary excitation, forming a minimum of energy at finite momentum, has been essential to understand the properties of superfluid $^4$He. In quantum liquids, rotons arise from the strong interparticle interactions, whose microscopic description remains debated. In the realm of highly-controllable quantum gases, a roton mode has been predicted to emerge due to magnetic dipole-dipole interactions despite of their weakly-interacting character. This prospect has raised considerable interest; yet roton modes in dipolar quantum gases have remained elusive to observations. Here we report experimental and theoretical studies of the momentum distribution in Bose-Einstein condensates of highly-magnetic erbium atoms, revealing the existence of the long-sought roton mode. By quenching the interactions, we observe the roton appearance of peaks at well-defined momentum. The roton momentum follows the predicted geometrical scaling with the inverse of the confinement length along the magnetisation axis. From the growth of the roton population, we probe the roton softening of the excitation spectrum in time and extract the corresponding imaginary roton gap. Our results provide a further step in the quest towards supersolidity in dipolar quantum gases.