Creation of Rydberg Polarons in a Bose Gas

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.

Figure 1

Schematic of the excitation of a Rydberg polaron in a uniform-density BEC. Laser excitation projects the system into final configurations involving atoms in bound and scattering states $|{\beta }_i⟩$. Bound states are confined in the Rydberg potential $V(\mathbf{r})$.

Figure 2

Spectrum for excitation of the $5s38s{{}^{3}S}_1$ Rydberg state in the few-body regime. Pure atomic excitation is at zero detuning, and lines for dimers ($X=D$) through pentamers ($X=P$) are visible. Subscripts ($X_{ij\dots }$) indicate the molecular levels of the Rydberg potential occupied by atoms after laser excitation. The FDA matches the data over several orders of magnitude in signal strength, which is normalized to the peak at zero detuning.

Figure 3

Rydberg excitation in the many-body regime for $n=49$, 60, 72. The mean-field prediction is for the condensate atoms only. Deviations between the mean-field result and data at small detuning ($\nu /{\mathrm{\Delta }}_{\max }<0.3$) represent contributions from the noncondensed thermal atoms. The shaded regions represent the predicted polaron response from the center of the atom cloud as obtained from the FDA. Theory for this high-density region matches the tail of the experimental data and represents the Rydberg-impurity spectral function for a nearly uniform-density region. For uniform density, the width of the spectrum scales as $n^{-3}$.