Observation of bound state self-interaction in a nano-eV atom collider

TL;DR

This paper reports the direct observation of bound state self-interaction in a nano-eV atom collider, revealing a large self-energy shift in a Feshbach resonance between potassium and rubidium atoms.

Contribution

It introduces a method to measure bound state self-interaction directly by tracking Feshbach resonances in a tunable atomic system, revealing large self-energy effects.

Findings

Observation of a strongly non-monotonic resonance trajectory.

Identification of an exceptionally large self-interaction energy.

Demonstration of a virtual bound state influencing the resonance.

Abstract

Quantum mechanical scattering resonances for colliding particles occur when a continuum scattering state couples to a discrete bound state between them. The coupling also causes the bound state to interact with itself via the continuum and leads to a shift in the bound state energy, but, lacking knowledge of the bare bound state energy, measuring this self-energy via the resonance position has remained elusive. Here, we report on the direct observation of self-interaction by using a nano-eV atom collider to track the position of a magnetically-tunable Feshbach resonance through a parameter space spanned by energy and magnetic field. Our system of potassium and rubidium atoms displays a strongly non-monotonic resonance trajectory with an exceptionally large self-interaction energy arising from an interplay between the Feshbach bound state and a different, virtual bound state at a fixed energy near threshold.