Matthew T. Eiles

Divalent atoms have emerged as powerful alternatives to alkalis in ultracold atom platforms, offering unique advantages arising from their two-electron structure. Among these species, ytterbium (Yb) is especially promising, yet its anionic properties and its Rydberg spectrum remain comparatively unexplored. In this work, we perform a comprehensive experimental and theoretical investigation of ultralong-range Rydberg molecules (ULRMs) of Yb 174 in 6 s n s S 1 0 Rydberg states across nearly two decades in principal quantum number n and 3 orders of magnitude in molecular binding energy. Using the Coulomb Green’s function formalism, we compute Born-Oppenheimer molecular potentials describing the Rydberg atom in the presence of a ground-state perturber and achieve quantitative agreement with high-resolution molecular spectra. This enables the extraction of low-energy electron-Yb scattering phase shifts, including the zero-energy s -wave scattering length and the positions of two spin-orbit split p -wave shape resonances. Our results provide strong evidence that the Yb − anion exists only as a metastable resonance. We additionally show the sensitivity of ULRM spectra to the atomic quantum defects, using this to determine the quantum defect of the 6 s 23 f F 1 3 state. Together, these findings establish Yb ULRMs as a powerful probe of electron-Yb interactions and lay essential groundwork for future Rydberg experiments with divalent atoms.

Laser cooling and trapping of atomic matter waves in optical potentials has enabled rapid progress in quantum science, particularly when combined with Rydberg excitation of the atoms to induce long-range interactions. Here, we propose the local manipulation and spatiotemporal sculpting of the electronic matter wave of a Rydberg atom by a laser field focused so that its beam width is smaller than the Rydberg electron orbit. We compute the electronic eigenstates in the presence of a sharply focused Gaussian laser beam, and find strong Rydberg state mixing leading to large kilo-Debye dipole moments. These can be modulated with high bandwidth controlled by the local tweezer intensity. Oscillations in the position-dependent level shifts, analogous to the potential wells allowing ultralong-range Rydberg molecules to form, provide opportunities for eccentric radial trapping of the Rydberg electron via ponderomotive forces acting on suborbital length scales.

We report the observation of spin-singlet ultralong-range Rydberg butterfly molecules consisting of a ground-state atom bound to a Rydberg atom by P -wave scattering of Rb 87 Rydberg electrons from Rb 87 ( 5 s ) atoms. A three-photon excitation scheme enables the photoassociation of these molecules by weakly admixing Rb ( 18 f 7 / 2 ) states. The measured binding energies, kilo-Debye permanent electric dipole moments, and lifetimes are in excellent agreement with theory. Two long-lived vibrational levels, red detuned from the Rb ( 18 f 7 / 2 ) threshold, are observed. This experiment is a foundational step in the production of ultracold anions and heavy Rydberg ion-pair systems.

We present a framework for treating mesoscopic anisotropic and nonadditive impurity–bath interactions, ubiquitous in realistic quantum impurity problems, which are often neglected in conventional approaches relying on additive, spherically symmetric pseudopotentials. To illustrate this general approach, we focus on the specific impurity problem of a Rydberg atom immersed in a Bose-Einstein condensate, where the internal-state degeneracy of the Rydberg impurity gives rise to configuration-dependent nonadditive potentials. With increasing interaction strength, anisotropy-induced partial-wave mixing generates distinct polaron and molaron resonances, allowing for radially and angularly excited bound states to become accessible. This approach captures the anisotropy and nonadditivity characteristic of a Rydberg impurity immersed in a quantum bath, and is broadly applicable to a host of quantum impurity problems beyond the Fröhlich paradigm.

We present a unified description of attractive and repulsive polarons, formed in a one-dimensional Bose gas hosting an impurity particle, by obtaining all ground- and excited-state solutions to the Gross-Pitaevskii equation. Modeling the impurity with an attractive square-well potential, we characterize the excited-state energy branches as a function of interaction strength. As the impurity-bath coupling increases, the excited states change from distinct soliton configurations to hybridized soliton-polaron states, eventually crossing over from repulsive to attractive polarons at unitarity. We identify a universal regime near this crossover where the polaron properties are accurately characterized by the zero-energy scattering length.

An impurity immersed in a Bose condensate can form a quasiparticle known as a Bose polaron. When the impurity-boson interaction is short ranged and three-body Efimov correlations are neglected, the quasiparticle properties can be characterized in terms of the impurity-boson scattering length a IB and the condensate coherence length ξ . This description remains valid irrespective of the bath density n 0 . Long-ranged interactions—such as provided by Rydberg or ionic impurities—introduce an effective interaction range r eff as the third length scale. The interplay among these competing length scales raises the question of whether a robust description across different bath densities can still be maintained. In this study, we discuss the quasiparticle nature of long-range impurities and its dependence on the length scales n 0 − 1 / 3 , r eff , and ξ . We employ two complementary theories—the coherent-state ansatz and the perturbative Gross-Pitaevskii theory—which incorporate beyond-Fröhlich interactions. We derive an analytical expression for the beyond-Fröhlich effective mass for a contact interaction and numerically compute the effective mass for long-range impurities. We argue that the coupling parameter | a IB | n 0 1 / 3 remains the principal parameter governing the properties of the polaron. For weak ( | a IB | n 0 1 / 3 ≪ 1 ) and intermediate ( | a IB | n 0 1 / 3 ≃ 1 ) values of the coupling parameter, long-range impurities in a Bose condensate are well described as quasiparticles with a finite quasiparticle weight and a well-defined effective mass. However, the quasiparticle weight becomes significantly suppressed as the effective impurity volume is occupied by an increasing number of bath particles ( r eff n 0 1 / 3 ≫ 1 ).

We report on the observation of highly excited ( ν ∼ 100 ) vibrational states of a trilobite ultralong-range Rydberg molecule in Rb 87 . These states manifest spectroscopically in a regularly spaced series of peaks red-detuned from the 25 f 7 / 2 dissociation threshold. The existence and observed stability of these states require the almost complete suppression of the adiabatic decay pathway induced by the P -wave shape resonance of Rb. This stabilization is predicted to occur only for certain Rydberg levels where the avoided crossing between trilobite and P -wave-dominated butterfly potential energy curves nearly vanishes, allowing the vibrational states to diabatically traverse the crossing with almost unit probability. This is the first direct measurement of beyond-Born-Oppenheimer physics in long-range Rydberg molecules, and paves the way for future experiments to access and manipulate wave packets formed from high-lying vibrational states.

We report the observation of spin-singlet ultra-long range Rydberg butterfly molecules consisting of a ground-state atom bound to a Rydberg atom by $P$-wave scattering of $^{87}$Rb Rydberg electrons from $^{87}$Rb(5s) atoms. A three-photon excitation scheme enables the photoassociation of these molecules by weakly admixing Rb($18f_{7/2}$) states. The measured binding energies, kilo-Debye permanent electric dipole moments, and lifetimes are in excellent agreement with theory. Two long-lived vibrational levels, red detuned from the Rb($18f_{7/2}$) threshold, are observed. This experiment is a foundational step in the production of ultra-cold anions and heavy Rydberg ion-pair systems.

We perform three-photon photoassociation to obtain high resolution spectra of Rb 87 trilobite dimers for the principal quantum numbers n = 22 , 24, 25, 26, and 27. The large binding energy of the molecules in combination with a relative spectroscopic resolution of 10 − 4 provides a rigorous benchmark for existing theoretical models. A recently developed Green’s function framework, which circumvents the convergence issues that afflicted previous studies, is employed to theoretically reproduce the vibrational spectrum of the molecule with high accuracy. The relatively large molecular binding energy is primarily determined by the low-energy S -wave electron-atom scattering length, thereby allowing us to extract the S 1 3 scattering phase shift with unprecedented accuracy, at low-energy regimes inaccessible to free electrons. Published by the American Physical Society 2025