Microscopy of an ultranarrow Feshbach resonance using a laser-based atom collider: A quantum defect theory analysis

TL;DR

This paper uses quantum defect theory to analyze and control ultracold atom collisions, revealing how shape resonances can significantly enhance Feshbach resonance widths in experiments with rubidium-87.

Contribution

It introduces a quantum defect theory approach to describe and manipulate Feshbach and shape resonances in ultracold collisions, enabling precise control over resonance widths.

Findings

Resonance width can be tuned over several orders of magnitude.

Coincidence with a shape resonance significantly broadens the Feshbach resonance.

Experimental demonstration of a resonance width broadened to 8 G.

Abstract

We employ a quantum defect theory framework to provide a detailed analysis of the interplay between a magnetic Feshbach resonance and a shape resonance in cold collisions of ultracold $\rm ^{87}Rb$ atoms as captured in recent experiments using a laser-based collider [Phys. Rev. Research 3, 033209 (2021)]. By exerting control over a parameter space spanned by both collision energy and magnetic field, the width of a Feshbach resonance can be tuned over several orders of magnitude. We apply a quantum defect theory specialized for ultracold atomic collisions to fully describe of the experimental observations. While the width of a Feshbach resonance generally increases with collision energy, its coincidence with a shape resonance leads to a significant additional boost. By conducting experiments at a collision energy matching the shape resonance and using the shape resonance as a magnifying lens we demonstrate a feature broadening to a magnetic width of 8 G compared to a predicted Feshbach resonance width $\ll 0.1$~mG.