arXiv subjects

Chris H. Greene

Publications and source records attributed to Chris H. Greene.

At least 19 recordsLinked to original sources

arXiv2026

Observation of $Π$-symmetry ultralong-range Rydberg molecules

We observe weakly-bound $Π$-symmetry electronic states in the spectroscopy of $^{87}$Rb$(nP_{3/2})$+$^{87}$Rb($5S_{1/2}$) ultralong-range Rydberg molecules. We detect these molecules in Rydberg states having principal quantum number $13\le n \le 16$. Their $Π$-state character is unambiguously identified via their observed multiplet structure: the $2F+1$ magnetic sublevels of the ground-state rubidium atom separate, as in the Zeeman effect, because of the spin-spin coupling between the Rydberg and valence electrons. We find a rapid decrease in the molecular binding energy $\propto (n-μ_{P_{3/2}})^{-11}$, where $μ_{P_{3/2}}$ is the quantum defect, indicating that the low-$n$ regime of Rydberg states is ideally suited for studies of $Π$-symmetry molecules. Our observations are in good agreement with Green's function-based calculations for $14\le n\le 16$, with poorer agreement for $n=13$ hinting at the beginning of a breakdown of the Fermi pseudopotential approach at low $n$.

physics.atom-ph↗

arXiv2026

Quantum science with arrays of metastable helium-3 atoms

The motion of atoms in programmable optical tweezer arrays offers many new opportunities for neutral atom quantum science. These include inter- and intra-site atom motion for resource-efficient implementations of fermionic and bosonic modes, respectively, as well as tweezer transport for efficient compilation of arbitrary circuits. However, the exploitation of atomic motion for all three purposes and others is limited by the inertia of the atoms. We present a comprehensive architectural blueprint for the use of fermionic metastable helium-3 ($^3$He$^*$) atoms -- the lightest trappable atomic species -- in programmable optical tweezer arrays. This includes a concrete analysis of atomic structure considerations as well as Rydberg-mediated interactions. We show that inter-tweezer hopping of $^3$He$^*$ atoms can be $\gtrsim3\times$ faster than previous demonstrations with lithium-6. We also demonstrate a new toolbox for encoding and manipulating qubits directly in the tweezer trap potential, uniquely enabled by the light mass of $^3$He$^*$. Finally, we provide several examples of new opportunities for fermionic quantum simulation and computation that leverage the transport and inter-tweezer hopping of $^3$He$^*$ atom arrays. These tools present new methods to improve the resource efficiency of neutral atom quantum science that may also enable quantum simulations of lattice gauge theories and quantum chemistry outside the Born-Oppenheimer approximation

quant-ph↗

arXiv2026

An alternative representation of multichannel Rydberg spectra: a modified K-matrix and Lu-Fano plot, applied to manganese spectroscopy

The well-known graphical representation called the Lu-Fano plot was originally developed for multi-channel Rydberg spectroscopy, especially in quantum defect theory. The present study shows some of the limitations of this traditional Lu-Fano plot that are desirable to improve on, when there are closely split ionization thresholds as in many current generation quantum information applications involving hyperfine-split thresholds, or when there are more than two ionization threshold energies. The modified representation introduced here is especially simplifying in the situation where one is exploring the bound states lying very far below those closely split thresholds. Moreover, it overcomes one limitation, namely that in contrast to the traditional Lu-Fano plot, the modified representation developed here can be utilized for problems where there more than two ionization thresholds. An example application to Rydberg series of the manganese atom illuminates its use in a practical problem.

physics.atom-ph↗

arXiv2026

Efimov Effect in Ultracold Microwave-Shielded Polar Molecules

A quantum-mechanical description is presented for the three-body physics of shielded dipolar molecules, including a prediction of observable Efimov physics. Despite the anisotropic and long-range nature of the interaction, shielding enables a regime in which universality emerges already at the two-body level and extends to the three-body sector, where Efimov physics emerges. On the negative side of the scattering-length resonance, computed trimer binding energies display the characteristic scaling expected for Efimov resonances. Finally, the sudden approximation can be used to create trimer bound states, starting from positive energy trap states as a way to create or detect these molecular trimers. Moreover, the three-body parameter expressed in dipolar units is found to be universal.

physics.atom-ph↗

arXiv2026

Mechanisms of anomalous three-body loss in a population-imbalanced three-component Fermi gas

Achieving precise control of ultracold atomic gases requires a detailed understanding of atom loss mechanisms. Motivated by the anomalous three-body decay in a three-component Fermi gas reported in Ref. [1], this work investigates mechanisms that possibly contribute to the observed loss. The three-body Schrödinger equation is solved in the hyperspherical adiabatic representation with pairwise van der Waals interactions, and the $S$-matrix is obtained via the eigenchannel $R$-matrix method to compute recombination rate coefficients $K_3$ and two-body cross sections. At the magnetic field strength where the anomalous decay occurs, $K_3$ is unitary limited, exhibiting the threshold energy scaling $K_3(E)\propto E^{-1}$. Consequently, the thermally averaged $\langle K_3 \rangle$ acquires a temperature dependence. Because the experiment is performed in the degenerate regime, $\langle K_3 \rangle$ also explicitly depends on the per-spin densities through the per-spin Fermi energies $E_{F}^{(i)}\propto n_i^{2/3}$. As the gas is diluted and degeneracy is reduced, $\langle K_3 \rangle$ approaches the non-degenerate value and becomes a function of temperature only. Channel-resolved branching ratios and cross sections are folded into a Monte Carlo cascade simulation of secondary collisions and trap escape. The analysis indicates that typical three-body recombination events remove fewer than three atoms on average, and that the atom losses are primarily due to the ejection of secondary collision products, rather than the initial three-body recombination products. Therefore, a significant fraction of the released binding energy remains in the trapped ensemble as kinetic energy. Retained energy drives evaporative loss, offering a plausible, partial explanation for the anomalous decay.

cond-mat.quant-gas↗

arXiv2025

Barium Autoionization for Efficient Ion Trap Loading

We report a theoretical and experimental investigation of autoionizing resonances from the $5d6p\,{}^3\mathrm{D}_1^o$ manifold in neutral barium for efficient loading of ion traps. Our calculations predict large resonant cross sections for many narrow autoionizing resonances, but we find experimentally that for most of these, Doppler broadening during trap loading depresses the effective cross sections that can be achieved in practice. We identify and demonstrate a strong, broad transition at $531\,\mathrm{nm}$, and show that it furnishes an order-of-magnitude increase in trap loading efficiency compared to other demonstrated resonances.

physics.atom-ph↗

arXiv2025

Connecting Collisional and Photofragmentation Resonances in the H$_2$ Ungerade Symmetry

A recently developed energy-dependent frame transformation theory that incorporates both ionization and dissociation channels of the H$_2$ molecule, is extended to treat the ungerade states that occur both in dissociative recombination and as the final state in ground state photoabsorption. The theoretical treatment includes the rotational degrees of freedom and is benchmarked against a two-dimensional model that can be solved with high accuracy and also compared with photoabsorption experiments. Analysis of the resulting spectra shows how the same resonances appear in very different observables, often with quite different line shapes.

physics.chem-ph↗

arXiv2025

Observable Resonances in Efimov-unfavored Systems

Three-body loss resonances associated with heavy-heavy-light Efimov states have been observed for over a decade in ultracold mixtures tuned near interspecies Feshbach resonances. For light-light-heavy systems, observing such resonances has been far more challenging due to the substantially large Efimov spacing. In these Efimov-unfavored systems, the intraspecies scattering length $a_\text{BB}$ has been shown to significantly affect the overall Efimov scenario, namely, the positions of the Efimov resonances $a_{-}^{(n)}$ and the three-body parameter (3BP) $a_{-}^{(0)}$. The present article explains the origin behind this influence by highlighting two primary mechanisms via which both the magnitude and sign of $a_\text{BB}$ govern the Efimov spectrum and set the resulting 3BP $a_{-}^{(0)}$. By employing van der Waals interactions for $^{23}$Na$_2{}^{40}$K, we attribute the vital role of $a_\text{BB}$ in Efimov-unfavored systems to the large difference between the Efimov scaling parameters for two and three resonant interactions, $s_0$ and $s_0^*$. In particular, we account for the unusually large $a_{-}^{(0)}$ obtained in light-light-heavy systems with $a_\text{BB}>0$ (e.g., $^{41}$K$_2{}^{87}$Rb), and show that the first Efimov resonance can still occur at an experimentally accessible value when $a_\text{BB}<0$.

cond-mat.quant-gas↗

arXiv2025

Broadband femtosecond lasers enable efficient two-photon excitation of the ultranarrow linewidth singlet 1s2s state in helium

We propose a broadband, femtosecond two-photon excitation scheme for efficient population transfer to the ultra-narrow linewidth $1s2s\ ^1S_0$ metastable state in helium. Using $120$ nm vacuum ultraviolet (VUV) femtosecond laser pulses, we theoretically demonstrate that a direct two-photon excitation process can achieve a population transfer efficiency of $25-30\%$, even when photoionization losses are included. The use of broadband pulses enables multiple excitation pathways to populate the excited state, in addition to compensating for significant AC Stark shifts occurring within the pulse duration. Furthermore, we introduce a two-color two-photon extreme ultraviolet-near infrared (XUV-IR) excitation scheme that will further reduce ionization losses and can achieve significantly higher transfer efficiencies of $\sim 70\%$. These results demonstrate that high excitation probability of ultra-narrow linewidth ($\sim 50$ Hz) excited states can be achieved with experimentally accessible femtosecond laser sources with a few THz bandwidth.

physics.atom-ph↗

arXiv2025

Observability of modified threshold behavior near unitarity

A number of recent references have pointed out that an N-particle system having short-range interactions at S-wave and/or P-wave unitarity can exhibit modified threshold behavior for various reactive processes. But the question of how close to unitarity one must get in order to observe such modifications has not been addressed. The present study quantities this question by treating cases involving 3- or 4-neutrons, at the physical value of the neutron-neutron singlet scattering length a and at artificially altered values. One major conclusion is that the neutron-neutron scattering length is not yet sufficiently large for the 3n or 4n systems to demonstrate the unitarity threshold exponent.

hep-ph↗

arXiv2024

Application of the Variational R-matrix Method for the Dirac Equation to the Be Atom

This paper presents an implementation of the non-iterative eigenchannel R-matrix method for the Dirac equation. It includes a brief introduction, implementation details, and results for the photoionization cross-section of the beryllium atom. Beryllium is a convenient test due to small but significant relativistic effects. The current calculation aligns with other R-matrix calculations and experiments. It observes the change in the Fano line shape of the (2pnd)1P series and, reveals a previously unnoticed coupling between triplet and singlet series observable in the ground state photoionization cross-section.

physics.atom-ph↗

arXiv2024

Resonances and collisional properties of neutron-rich helium isotopes in the adiabatic hyperspherical representation

This work treats few-body systems consisting of neutrons interacting with a $^{4}{\mathrm{He}}$ nucleus. The adiabatic hyperspherical representation is utilized to solve the $N$-body Schr$\ddot{\mathrm{o}}$dinger equation for the three- and four-body systems, treating both $^{6}{\mathrm{He}}$ and $^{7}{\mathrm{He}}$ nuclei. A simplified central potential model for the $^{4}{\mathrm{He}}-n$ interaction is used in conjunction with a spin-dependent three-body interaction to reproduce $^{6}{\mathrm{He}}$ bound-state and resonance properties as well as properties for the $^{8}{\mathrm{He}}$ nucleus in its ground-state. With this Hamiltonian, the adiabatic hyperspherical representation is used to compute bound and scattering states for both $^{6}{\mathrm{He}}$ and $^{7}{\mathrm{He}}$ nuclei. For the $^{6}{\mathrm{He}}$ system, the electric quadrupole transition between the $0^{+}$ and $2^{+}$ state is investigated. For the $^{7}{\mathrm{He}}$ system, $^{6}{\mathrm{He}}+n$ elastic scattering is investigated along with the four-body recombination process $^{4}{\mathrm{He}}+n+n+n\rightarrow$$^{6}{\mathrm{He}}+n$ and breakup process $^{6}{\mathrm{He}}+n\rightarrow$$^{4}{\mathrm{He}}+n+n+n$.

nucl-th↗

arXiv2024

Three-Body Recombination of Ultracold Microwave-Shielded Polar Molecules

A combined experimental and theoretical study is carried out on the three-body recombination process in a gas of microwave-shielded polar molecules. For ground-state polar molecules dressed with a strong microwave field, field-linked bound states can appear in the intermolecular potential. We model three-body recombination into such bound states using classical trajectory calculations. Our results show that recombination can explain the enhanced loss rates observed at small microwave detunings in trapped samples of bosonic NaCs [Bigagli, $\textit{et al.}$, Nat. Phys. $\textbf{19}$ 1579-1584 (2023)]. Specifically, our calculations reproduce the experimentally measured three-body loss rates across a wide range of microwave Rabi couplings, detunings, and temperatures. This work suggests that for bosonic shielded molecular systems in which the two-body loss is sufficiently suppressed and a field-linked bound state is present, the dominant loss process will be three-body recombination.

cond-mat.quant-gas↗

arXiv2024

Dissociative Recombination of CF$^+$

This work presents our theoretical study of the dissociative recombination (DR) of the closed-shell diatomic system CF$^+$ based on an approach recently applied to the CH$^+$ molecule. Our extended treatment uses the UK R-matrix theory and the multichannel quantum-defect theory procedure to uniformly resolve the direct and indirect mechanisms of DR while bypassing explicit dissociative state and electronic coupling calculations. The theoretical results exhibit good overall agreement with previous experimental measurements. At lower scattering energies, good agreement is found only if the rotational structure of the ion is included and the theoretical cross sections are averaged over initial rotational levels corresponding to the temperature at which the experimental measurements were made. At higher scattering energies, our rotationally resolved results are very similar to those obtained without including the ion's rotational structure.

physics.chem-ph↗

arXiv2024

Adiabatic and post-adiabatic hyperspherical treatment of the huge ungerade proton-hydrogen scattering length

While the hydrogen molecular ion is the simplest molecule in nature and very well studied in all of its properties, it remains an interesting system to use for explorations of fundamental questions. One such question treated in this study relates to finding an optimal adiabatic representation of the physics, i.e. the best adiabatic description that minimizes the role of nonadiabatic effects. As a test case explored here in detail, we consider the ungerade symmetry of H$_2^+$, which is known to have a huge scattering length of order 750 bohr radii, and an incredibly weakly bound excited state. We show that a hyperspherical adiabatic description does an excellent job of capturing the main physics. Our calculation yields a competitive scattering length and shows that nonadiabatic corrections are small and can even be adequately captured using the postadiabatic theory of Klar and Fano.

physics.atom-ph↗

arXiv2023

Kinetic rate coefficients for electron-driven collisions with CH$^+$: dissociative recombination and rovibronic excitation

Cross sections and rate coefficients for rovibronic excitation of the CH$^+$ ion by electron impact and dissociative recombination of CH$^+$ with electrons are evaluated using a theoretical approach combining an R-matrix method and molecular quantum defect theory. The method has been developed and tested, comparing the theoretical results with the data from the recent Cryogenic Storage Ring experiment. The obtained cross sections and rate coefficients evaluated for temperatures from 1~K to 10,000~K could be used for plasma modeling in interpretation of astrophysical observations and also in technological applications where molecular hydrocarbon plasma is present.

astro-ph.GA↗

arXiv2023

Green's function treatment of Rydberg molecules with spins

The determination of ultra-long-range molecular potential curves has been reformulated using the Coulomb Greens function to give a solution in terms of the roots of an analytical determinantal equation. For a system consisting of one Rydberg atom with fine structure and a neutral perturbing ground state atom with hyperfine structure, the solution yields potential energy curves and wavefunctions in terms of the quantum defects of the Rydberg atom and the electron-perturber scattering phaseshifts and hyperfine splittings. This method provides a promising alternative to the standard currently utilized method of diagonalization, which suffers from problematic convergence issues and nonuniqueness, and can potentially yield a more quantitative relationship between Rydberg molecule spectroscopy and electron-atom scattering phaseshifts.

physics.atom-ph↗

arXiv2023

Wigner Time Delay in Photoionization: A 1D Model Study

In scattering theory, the Wigner-Smith time delay, calculated through a phaseshift derivative or its multichannel generalization, has been demonstrated to measure the amount of delay or advance experienced by colliding particles during their interaction with the scattering potential. Fetic, Becker, and Milosevic argue that this concept cannot be extended to include photoionization, viewed as a half-scattering experiment. Their argument is based on the lack of information about scattering phaseshifts in the part of the wavefunction (satisfying the ingoing-wave boundary condition) going to the detector. This article aims to test this claim by examining a photodetachment process in a simple 1D model with a short-range symmetrical potential. Using time-dependent perturbation theory with a dipole interaction, the relevant wavepacket of the outgoing particle is analyzed and compared to the free wavepacket as a reference. Our findings confirm that, indeed, a time delay arises in the liberated fragmentation wavepacket, which is expressed as an energy derivative of the scattering phaseshift. Our study highlights that the source of the phaseshift content in the wavepacket arriving at the detector is the dipole matrix element, which is a direct consequence of imposing the ingoing-wave boundary condition. We illustrate our results through numerical simulations of both the non-free and free wavepackets. The amount of the observed time delay is found to be half of that appearing in a typical scattering experiment.

physics.atom-ph↗