Anisotropic excitation spectrum of a dipolar quantum Bose gas

TL;DR

This study experimentally measures the anisotropic excitation spectrum of a dipolar Bose-Einstein condensate, revealing how dipole orientation affects energy dispersion and confirming theoretical predictions about anisotropic sound speed.

Contribution

First experimental observation of anisotropic excitation spectrum in a dipolar BEC, validating Bogoliubov theory and numerical simulations of the Gross-Pitaevskii equation.

Findings

Excitation spectrum depends on dipole orientation.

Anisotropy observed in the speed of sound.

Results agree with theoretical models.

Abstract

We measure the excitation spectrum of a dipolar Chromium Bose Einstein Condensate with Raman-Bragg spectroscopy. The energy spectrum depends on the orientation of the dipoles with respect to the excitation momentum, demonstrating an anisotropy which originates from the dipole-dipole interactions between the atoms. We compare our results with the Bogoliubov theory based on the local density approximation, and, at large excitation wavelengths, with numerical simulations of the time dependent Gross-Pitaevskii equation. Our results show an anisotropy of the speed of sound