Observation of low-lying impurity states in Bose-Einstein condensates

TL;DR

This study observes low-energy impurity states in a Bose-Einstein Condensate, revealing states below the polaron energy and comparing experimental results with two theoretical models, suggesting possible bipolaron formation.

Contribution

First experimental observation of low-lying impurity states in BEC, providing insights into impurity-boson interactions and bipolaron formation.

Findings

Spectroscopic evidence of impurity states below the polaron energy.

Agreement of observed spectra with theoretical models, especially bipolaron.

Identification of strong interaction regimes where low-energy states emerge.

Abstract

Impurities embedded in a Bose-Einstein Condensate (BEC) of 39K atoms are investigated with a pump-probe ejection spectroscopy sequence. The spectroscopic signal exhibits a strong feature corresponding to a Bose polaron in agreement with prior injection spectroscopy and theory. In addition, significant spectral weight at energies well below the energy of the polaron is observed, which is absent in injection spectroscopy. The energy and spectral weight of this signal are measured as a function of interaction strength and evolution time between the pump and probe pulses. We tentatively compare these results to two different theoretical models: a low-energy impurity state dressed by many bosonic excitations and a bipolaron state formed by two polarons due to attractive interactions mediated by the BEC. Such states can exist due to the large compressibility of the weakly interacting BEC. Both theories predict ejection spectra consistent with the low-energy signal, but only the bipolaron model is compatible with its spectral weight. These results indicate that lowenergy states below the usual polaron exist for strong interactions, calling for further experimental investigations