arXiv subjects

Tijs Karman

Publications and source records attributed to Tijs Karman.

At least 19 recordsLinked to original sources

arXiv2026

First-order Degenerate Symmetry-Adapted Perturbation Theory

Degenerate electronic states govern the photophysics of excimers and the molecular dynamics near conical intersections. In regions where potential energy surfaces are degenerate or nearly degenerate, intermolecular interactions mix the corresponding electronic configurations. We introduce a first-order degenerate formulation of symmetry-adapted perturbation theory (dSAPT) with weak symmetry forcing that quantifies the mixing in terms of electrostatic and exchange (Pauli repulsion) contributions. We compare two alternative ways of resolving the degeneracy, in which antisymmetry is enforced either after or before diagonalization of the first-order eigenproblem. Using CASSCF monomer wave functions, we apply dSAPT to two model spatially degenerate systems, H$_2$--F(${}^2P$) and H$_2$--NO(${}^2\Pi$), and show that exchange effects contribute significantly to configuration mixing already at intermediate intermolecular separations. For the H$_2$--NO(${}^2\Pi$) complex, we further demonstrate that second-order dispersion energy is essential for qualitatively reproducing the angular dependence of the diabatic mixing angle. Finally, using water and benzene excimers with monomers described either by CASSCF or CIS wave functions, we show that Pauli repulsion controls delocalization of the excitation at short intermolecular separations.

physics.chem-ph↗

arXiv2026

Electrostriction in a Bose-Einstein Condensate of Dipolar Molecules

The recent creation of a Bose-Einstein condensate (BEC) of dipolar molecules has opened a new frontier for many-body quantum systems in which dipolar interactions can drive novel self-organization phenomena. Here, we observe electrostriction in a molecular BEC, an elliptical deformation driven by anisotropic dipolar interactions. We use double microwave dressing, involving $\sigma$- and $\pi$-polarized fields, to control non-axially symmetric dipolar interactions. We compare the experimental observations of electrostriction to a model based on an extended Gross-Pitaevskii equation and find excellent agreement in the regime of weak to moderate interactions. Using electrostriction, we demonstrate that the molecular BEC can be torqued by dynamically changing the orientation of the elliptical $\sigma$ microwave field. This provides a route to setting molecular quantum gases into rotation, opening opportunities to probe vorticity, superfluidity, and supersolidity in strongly dipolar matter.

cond-mat.quant-gas↗

arXiv2026

Ro-vibrational van der Waals interaction between ultracold polar molecules

We describe the ro-vibrational van der Waals interaction between ultracold polar molecules. This interaction is strong, leading to fast elastic collisions and orders of magnitude suppression of collisional loss. This enables evaporative cooling of Fermi mixtures of molecules in different ro-vibrational states, without active shielding by applying external fields. The scheme is compatible with microwave shielding, where it enables controlled state dependent interactions, opening up new opportunities for quantum simulation and impurity physics. The interaction can also be used to stabilize fermionic molecules in optical lattices, to control interactions in synthetic dimensions, for enhanced tweezer loading, and direct infrared shielding.

cond-mat.quant-gas↗

arXiv2026

Tunable state-dependent interactions in collisionally stable mixtures of polar molecules

We propose encoding a pseudo-spin-$1/2$ system in the ground ($v=0$) and first excited ($v=1$) vibrational states of polar molecules. Double microwave shielding simultaneously shields molecules in both states, suppressing two-body losses by orders of magnitude while strictly avoiding three-body recombination. The microwave dressing is state-dependent and results in highly tunable, long-range dipolar Ising exchange ($J_z$), density-density ($V$), and spin-density ($W$) interactions. These interaction length scales readily exceed the typical interparticle spacing, pushing the molecules deep into the strongly interacting regime. In bulk gases, this enables the exploration of itinerant quantum magnetism and quantum droplets with novel anisotropic spin textures; in optical lattices, it naturally realizes extended Hubbard and $t$-$J_z$ models, opening new directions in quantum simulation.

cond-mat.quant-gas↗

arXiv2026

Deterministic loading of molecular arrays by microwave-assisted collisions

Molecular tweezer arrays offer great prospects for quantum simulation, sensing, and computing, and would benefit from methods that enhance loading efficiency. Whereas light-assisted collisions underpin enhanced loading methods for atomic tweezer arrays, this approach cannot be directly extended to molecular arrays due to collisional loss. We show how this collisional loss can be suppressed by shelving molecules in rotationally or vibrationally excited states, so that a shelved molecule interacts with a newly loaded molecule through a repulsive van der Waals interaction. By introducing microwave assisted collisions, we show how to control the final states and the energy released in a collision between a pair of molecules. Following this controlled collision, one of the two molecules can be ejected, and we explore several strategies for ensuring deterministic ejection. Our schemes rely on currently available techniques for laser-coolable molecules, and we predict achievable filling fractions up to 96%, paving the way for scalable molecular arrays.

physics.atom-ph↗

arXiv2026

Evolution of dipole-dipole dynamics in cold ammonia collisions

Cold polar molecules offer fascinating prospects for ultracold chemistry and quantum physics, including new platforms for quantum simulation or computation. However, their inherent collision properties remain largely unknown. It has proven extremely hard to experimentally probe collisions between two dipolar molecules at sufficiently low energies and high precision, as it appears fundamentally impossible to merge two beams of molecules with significant dipole moments. Here we report measurements of state-to-state cross sections for collisions between ammonia isotopologues at energies between 0.3 and 100 cm$^{-1}$ using a novel beam merger. We experimentally observed a local maximum in the cross sections that indicates the effective dipole moments can switch off at low collision energies. Scattering calculations reproduced this maximum in good agreement and explained the observed scaling with the parity splitting energies in the molecular energy level structures. Measurements of the correlated energy transfer in both collision partners yielded direct evidence of the suppression of the dipole-dipole interaction at low energies. Our results demonstrate how collisions between an important class of polar molecules evolve from the high temperature limit towards the ultracold regime in a counterintuitive way, have major consequences for the feasibility of future experiments and the interpretation of previous work, and offer distinctive opportunities to control cold molecular collisions with external fields.

physics.atom-ph↗

arXiv2026

Mapping the parameter space of double microwave shielding

Double microwave shielding employs $\sigma^{+}$- and $\pi$-polarized microwave fields, tuned close to the lowest rotational transition, to engineer a long-range repulsive barrier between polar molecules. By preventing molecules from reaching the short range, this technique suppresses detrimental two-body losses and recently enabled the realization of molecular Bose-Einstein condensates and self-bound droplets. Yet, the optimal operating regimes of the shielding mechanism remain largely unexplored. Here, by leveraging the underlying universality of the scattering problem, we systematically map the four-dimensional microwave parameter space-spanned by the detunings and intensities of the two fields-to identify configurations that maximize both shielding efficiency and interaction tunability. We define optimal operating regimes as configurations that are strictly free of field-linked bound states while sufficiently suppressing two-body losses to exceed typical lifetimes of ultracold samples. In these regimes, we evaluate the elastic-to-inelastic collision ratios required for efficient evaporative cooling and explore the accessible tuning range of the effective dipolar interactions. Finally, to identify the best platforms for future quantum simulation experiments, we conduct a global survey of candidate molecular species under realistic field constraints. We identify heavy, strongly dipolar molecules as the most promising candidates, demonstrating that they can achieve extreme loss suppression-reaching two-body loss rates as low as $10^{-18}$ cm$^{3}$/s-alongside robust interaction tunability using only moderate field strengths.

cond-mat.quant-gas↗

arXiv2026

Low-Energy Purification of Crystal Defects by Rydberg Excitons

Recent experiments show that optically generated Rydberg excitons in cuprous oxide can neutralize charged impurities, strongly reducing stray electric fields and effectively purifying the crystal. Here, we develop a multichannel theory of Rydberg exciton-impurity scattering that resolves the competing roles of capture, elastic scattering, and inelastic transitions between excitonic states. We find that at high collision energies, as effective under conventional single-photon excitation, purification is reduced relative to Langevin capture. These collisions are accompanied by inelastic redistribution and dominant elastic scattering, including pronounced glory scattering, which suppress purification efficiency. We identify a quantum regime at ultralow collision energies favorable for purification, where only the s-wave contributes: capture is enhanced while elastic and inelastic channels are strongly suppressed. This regime can be accessed via degenerate two-photon excitation of even-parity Rydberg excitons with tunable recoil, additionally enabling the systematic exploration of exciton-impurity scattering over a wide range of collision energies beyond what is readily achievable in atomic counterparts in atomic gas experiments.

cond-mat.mes-hall↗

arXiv2026

Low-entropy arrays of microwave-shielded molecules prepared by interaction blockade

Ultracold molecules are becoming an increasingly important technology for quantum simulation, computation, and sensing, but their state preparation in large, low-entropy arrays remains a key challenge. We propose to deterministically load single molecules into optical tweezer arrays or lattices from either thermal or degenerate gases, with a high probability of occupying the tweezer's motional ground state. Strong repulsion between microwave-shielded molecules prevents multiparticle occupancy. Our proposal represents a robust scheme for deterministic single molecule preparation directly in the motional ground state with expected fidelities exceeding 99 percent for small trap volumes and highly polar species. This method can be scaled to thousands of traps limited by the reservoir molecule number, opening the door to large, low-entropy polar molecule arrays for quantum computation, quantum simulation, and precision measurement.

quant-ph↗

arXiv2025

Strongly dipolar molecular Bose-Einstein condensates: From few- to many-body physics

Recent advances in molecular cooling have enabled the realization of strongly dipolar Bose--Einstein condensates (BECs) of molecules, and BECs of many different molecular species may become experimentally accessible in the near future. Here, we explore the unique properties of such BECs and the new insights they may offer into dipolar quantum fluids and many-body physics. We explore which parameter regimes can realistically be achieved using currently available experimental techniques, discuss how to implement these techniques, and outline which molecular species are particularly well suited to explore exotic new states of matter. We further determine how state-of-the-art beyond mean-field theories, originally developed for weakly dipolar magnetic gases, can be pushed to their limits and beyond, and what other long-standing questions in the field of dipolar physics may realistically come within reach using molecular systems.

cond-mat.quant-gas↗

arXiv2025

Data availability and requirements relevant for the Ariel space mission and other exoplanet atmosphere applications

The goal of this white paper is to provide a snapshot of the data availability and data needs primarily for the Ariel space mission, but also for related atmospheric studies of exoplanets and brown dwarfs. It covers the following data-related topics: molecular and atomic line lists, line profiles, computed cross-sections and opacities, collision-induced absorption and other continuum data, optical properties of aerosols and surfaces, atmospheric chemistry, UV photodissociation and photoabsorption cross-sections, and standards in the description and format of such data. These data aspects are discussed by addressing the following questions for each topic, based on the experience of the "data-provider" and "data-user" communities: (1) what are the types and sources of currently available data, (2) what work is currently in progress, and (3) what are the current and anticipated data needs. We present a GitHub platform for Ariel-related data, with the goal to provide a go-to place for both data-users and data-providers, for the users to make requests for their data needs and for the data-providers to link to their available data. Our aim throughout the paper is to provide practical information on existing sources of data whether in databases, theoretical, or literature sources.

astro-ph.IM↗

arXiv2025

Observation of Self-Bound Droplets of Ultracold Dipolar Molecules

Ultracold gases of dipolar molecules have long been envisioned as a platform for the realization of novel quantum phases. Recent advances in collisional shielding, protecting molecules from inelastic losses, have enabled the creation of degenerate Fermi gases and, more recently, Bose-Einstein condensation of dipolar molecules. However, the observation of quantum phases in ultracold molecular gases that are driven by dipole-dipole interactions has so far remained elusive. In this work, we report the formation of self-bound droplets and droplet arrays in an ultracold gas of strongly dipolar sodium-cesium molecules. Starting from a molecular Bose-Einstein condensate (BEC), microwave dressing fields are used to induce dipole-dipole interactions with controllable strength and anisotropy. By varying the speed at which interactions are induced, covering a dynamic range of four orders of magnitude, we prepare droplets under equilibrium and non-equilibrium conditions, observing a transition from robust one-dimensional (1D) arrays to fluctuating two-dimensional (2D) structures. The droplets exhibit densities up to 100 times higher than the initial BEC, reaching the strongly interacting regime, and suggesting the possibility of a quantum-liquid or crystalline state. This work establishes ultracold molecules as a system for the exploration of strongly dipolar quantum matter and opens the door to the realization of self-organized crystal phases and dipolar spin liquids in optical lattices.

cond-mat.quant-gas↗

arXiv2025

Hyperfine van der Waals repulsion between open-shell polar molecules

We describe a novel type of interaction between open-shell polar molecules at sub-millikelvin temperatures. This hyperfine van der Waals interaction occurs between two molecules in two rotational states that differ by one quantum. Normally, this induces resonant dipolar interactions that lead to rapid collisional loss. For specific hyperfine states, however, selection rules prevent this. One can effectively turn off the dipolar interaction by merely flipping a nuclear spin. The resulting van der Waals interaction can be repulsive and can suppress collisional loss. We focus on laser-coolable CaF, but show this effect occurs universally for open-shell molecules, including MgF, SrF, BaF and YO. We propose that this effect could be measured by merging molecules in optical tweezers, where flipping a spin in one of the tweezers enables tuning of collision rates by five orders of magnitude.

physics.atom-ph↗

arXiv2025

Complete insensitivity to ab initio data -- A new perspective on modeling collision-induced absorption of noble gas atoms

In this study, we systematically investigate how the accuracy of CIA spectra depends on the quality of the ab initio data used. We evaluate quantitatively the impact of different quantum chemical methods and basis sets on spectral features, finding that even the lowest-level calculations are accurate to approximately 10 % at room temperature, and better at higher temperatures. This study also reveals a previously unreported double-peak structure for the He-Ne complex, which cannot be described by simple but commonly used single-exponential models for the short-range dipole. Our analysis shows that the range of internuclear distances relevant for CIA spectra varies with temperature, with short-range interactions becoming increasingly important at high temperatures. The long-range van der Waals induced dipoles never contribute. These findings provide new insights into the temperature-dependent behavior of CIA spectra and emphasize the importance of accurate modeling of short-range interactions for reliable astronomical modeling.

physics.chem-ph↗

arXiv2025

Extreme Loss Suppression and Wide Tunability of Dipolar Interactions in an Ultracold Molecular Gas

Ultracold dipolar molecules hold great promise for the creation of novel quantum states of matter, but the realization of long-lived molecular bulk samples with strong dipole-dipole interactions has remained elusive. Here, we realize a collisionally stable gas of ultracold ground state molecules with a lifetime of several seconds. Utilizing double microwave dressing, we achieve an extreme suppression of inelastic two- and three-body losses by factors of more than 10,000 and 1,000, respectively. We find that losses remain suppressed across a wide range of dipole-dipole interactions, allowing the continuous tuning of the dipolar length from 0 to 1 um $\sim$ 20,000 $a_0$. Combined with the recent realization of Bose-Einstein condensation of dipolar molecules, our findings open the door to the exploration of strongly dipolar quantum liquids.

cond-mat.quant-gas↗

arXiv2025

Double Microwave Shielding

We develop double microwave shielding, which has recently enabled evaporative cooling to the first Bose-Einstein condensate of polar molecules [Bigagli et al., Nature 631, 289 (2024)]. Two microwave fields of different frequency and polarization are employed to effectively shield polar molecules from inelastic collisions and three-body recombination. Here, we describe in detail the theory of double microwave shielding. We demonstrate that double microwave shielding effectively suppresses two- and three-body losses. Simultaneously, dipolar interactions and the scattering length can be flexibly tuned, enabling comprehensive control over interactions in ultracold gases of polar molecules. We show that this approach works universally for a wide range of molecules. This opens the door to studying many-body physics with strongly interacting dipolar quantum matter.

cond-mat.quant-gas↗

arXiv2024

Dipolar droplets of strongly interacting molecules

We simulate a molecular Bose-Einstein condensate in the strongly dipolar regime, observing the existence of self-bound droplets, as well as their splitting into multiple droplets by confinement-induced frustration. Our quantum Monte Carlo approach goes beyond the limits of the established effective mean-field theories for dipolar quantum gases, revealing small droplets produced by strong dipolar interactions outside known stable regimes. The simulations include realistic molecular interactions and therefore have direct relevance for current and future experiments.

cond-mat.quant-gas↗

arXiv2024

Rotational-state dependence of interactions between polar molecules

The long-range electrostatic interactions between molecules depend strongly on their relative orientation, which manifests as a rotational state dependence. Interactions between molecules in the same rotational quantum state are well-known attractive rotational van der Waals interactions. Interactions in rotational states that differ by one quantum show resonant dipole-dipole interactions. We show that where molecules are in rotational states that differ by more than one quantum, they exhibit repulsive van der Waals interactions. At temperatures below a millikelvin, this effect can reduce collisional loss by multiple orders of magnitude. These repulsive interactions lead to applications in quantum simulation and impurity physics with ultracold polar molecules.

physics.atom-ph↗