Comparing contact and dipolar interaction in a Bose-Einstein condensate

TL;DR

This paper measures the relative strength of magnetic dipole-dipole interactions versus contact interactions in a chromium Bose-Einstein condensate, providing precise values that align with previous Feshbach resonance data.

Contribution

It introduces a method to accurately determine the dipole-dipole interaction strength and s-wave scattering length in a dipolar BEC through expansion velocity analysis.

Findings

Measured the dipole-dipole interaction strength as 0.159±0.034.

Determined the s-wave scattering length as 96±20 a0.

Confirmed consistency with previous Feshbach resonance measurements.

Abstract

We have measured the relative strength $\epsilon_dd$ of the magnetic dipole-dipole interaction compared to the contact interaction in a chromium Bose-Einstein condensate. We analyze the asymptotic velocities of expansion of a dipolar chromium BEC with different orientations of the atomic magnetic dipole moments. By comparing them with numerical solutions of the hydrodynamic equations for dipolar condensates, we are able to determine $\epsilon_dd = 0.159\pm0.034$ with high accuracy. Since the absolute strength of the dipole-dipole interaction is known exactly, the relative strength of the dipoledipole interaction can be used to determine the s-wave scattering length $a = 5.08\pm1.06\cdot10^-9 m = 96\pm20 a0$ of 52Cr. This is fully consistent with our previous measurements on the basis of Feshbach resonances.