Creation of Rydberg Polarons in a Bose Gas

Abstract

We report spectroscopic observation of Rydberg polarons in an atomic Bose gas. Polarons are created by excitation of Rydberg atoms as impurities in a strontium Bose-Einstein condensate. They are distinguished from previously studied polarons by macroscopic occupation of bound molecular states that arise from scattering of the weakly bound Rydberg electron from ground-state atoms. The absence of a p-wave resonance in the low-energy electron-atom scattering in Sr introduces a universal behavior in the Rydberg spectral line shape and in scaling of the spectral width (narrowing) with the Rydberg principal quantum number, n. Spectral features are described with a functional determinant approach (FDA) that solves an extended Fröhlich Hamiltonian for a mobile impurity in a Bose gas. Excited states of polyatomic Rydberg molecules (trimers, tetrameters, and pentamers) are experimentally resolved and accurately reproduced with a FDA.

Theory of excitation of Rydberg polarons in an atomic quantum gas

“…The system is described by the HamiltonianĤ 0 given by the first two terms in Eq. (9). In contrast, when the atom is in its Rydberg state |ns the potential V (q) is switched on.…”

“…We note that the depletion of a BEC by a single electron in a BEC has been studied in [44] including only the linear Fröhlich term in Eq. (9). While such an approach can approximately account for the rate of depletion of the condensate, it fails to describe the formation of molecules that is essential for the formation of Rydberg polarons.…”

“…In contrast, when the atom is in its Rydberg state |ns the potential V (q) is switched on. In experiments [9], transitions between both states are driven by a two-photon excitation.…”

“…In this paper, which accompanies the publication heralding the observation of Rydberg Bose polarons [9], we present details of calculations and the interpretation of this observation as a new class of Bose polarons, formed through excitation of Sr(5sns 3 S 1 ) Rydberg atoms in a strontium Bose-Einstein condensate (BEC). We begin with a general outline of different polaron Hamiltonians and construct the Rydberg polaron Hamiltonian used in this work.…”

“…In the companion paper [9], we provide experimental evidence for the observation of polarons created by excitation of Rydberg atoms in a strontium Bose-Einstein condensate, with an emphasis on the determination of the excitation spectrum in the absence of density inhomogeneity. Here we provide a detailed discussion of the theoretical methods and experimental analysis.…”