arXiv subjects

Michał Tomza

Publications and source records attributed to Michał Tomza.

At least 19 recordsLinked to original sources

arXiv2026

Diatomic molecular anions of alkali-metal and alkaline-earth-metal atoms

Studies of anions are inherently more challenging than investigations of neutrals and cations because of the diffuse and weakly bound character of an anionic electron. Here, we present a comprehensive computational examination of ground-state diatomic molecular anions composed of alkali-metal (Li, Na, K, Rb, Cs, Fr) and alkaline-earth-metal (Be, Mg, Ca, Sr, Ba, Ra) atoms. We study 21 alkali-metal diatomic anions in the X$^{2}Σ^{+}$ electronic state and 36 alkali-metal--alkaline-earth-metal diatomic anions in the X$^{1}Σ^{+}$ electronic state. The calculations employ a hierarchy of the coupled cluster methods, combined with large Gaussian basis sets and small-core relativistic energy-consistent pseudopotentials for heavier elements. We compute potential energy curves, permanent electric dipole moments, and static polarizabilities, and we assess convergence and uncertainties of our results. Additionally, using the multireference configuration interaction and equation-of-motion electron-attachment coupled cluster methods, we investigate excited electronic states of alkali-metal molecular anions, including valence-bound and dipole-bound states. We predict crossings between ground neutral and excited anionic states, which may enhance resonant electron attachment and subsequent anion dissociation. This finding may be relevant for experiments with mixtures of ultracold ground-state alkali-metal molecules and Rydberg atoms.

physics.atom-ph↗

arXiv2026

Tunable two-component ultracold molecular gases with vibrational shielding

We propose a method to realize stable, tunable two-component quantum mixtures of ultracold polar molecules. First, we show that a pair of polar molecules in two distinct rovibrational states exhibits a repulsive interaction, thereby leading to collisional shielding without requiring any external field. We refer to this as "vibrational shielding". This intercomponent interaction is tunable by an external static electric or a microwave field, with the latter stabilizing both inter- and intracomponent interactions in a two-component bulk mixture. Additionally, we show that two microwave fields can independently tune the interactions of the individual components. Our findings thus open the door to the experimental realization of tunable quantum mixtures using polar molecules, analogous to tunable magnetic Feshbach resonances in two-component atomic quantum gases.

cond-mat.quant-gas↗

arXiv2026

Universality in Ionic Three-body Systems Near an Ion-atom Feshbach Resonance

We calculate bound and scattering properties of a system of two neutral atoms and an ion near an atom-ion Feshbach resonance. Our results indicate that long-range atom-ion interactions lead to significant deviations from universal behavior derived from contact or van der Waals potentials. We find that ionic systems display an overall suppression of inelastic transitions leading to recombination rates and lifetimes of Efimov state orders of magnitude smaller with respect to those for neutral atoms. We further characterize the dense spectra of triatomic molecular ions with extended lifetimes. Our results provide a deeper insight on the universality and structure of three-body ionic systems and establishing them as a promising platform for exploring novel few- and many-body phenomena with long-range interactions.

physics.atom-ph↗

arXiv2026

Charge Exchange Dynamics in Cold Collisions of $^{40}$CaH$^+$ and $^{39}$K

We report the observation of charge-exchange collisions between trapped calcium monohydride molecular ions ($^{40}$CaH$^+$) and ultracold potassium atoms ($^{39}$K) in a hybrid ion-atom trap. The measured charge-exchange rate coefficient is significantly suppressed relative to the Langevin rate constant for the system. We use $\mathit{ab\ initio}$ quantum-chemical calculations to model the (CaH-K)$^+$ complex in the ground and excited electronic states and to identify possible charge-exchange mechanisms. Our calculations rule out a direct non-radiative charge-exchange reaction and instead point to a radiative mechanism, but do not quantitatively reproduce the measured rate, highlighting the need for a full-dimensional quantum dynamics treatment that includes vibrational motion and intermediate complex formation. Our work demonstrates that cold hybrid ion-atom platforms with molecular ions enable access to richer chemical complexity and collisional dynamics inaccessible in purely atomic systems.

physics.atom-ph↗

arXiv2026

Infrared absorption spectroscopy of a single polyatomic molecular ion

Absorption spectroscopy is a fundamental tool for probing molecular structure. However, performing absorption spectroscopy on individual molecules is challenging due to the low signal-to-noise ratio. Here, we report on a nondestructive absorption spectroscopy on a mid-infrared vibrational transition in a single molecular ion that is co-trapped with an atomic ion. The absorption of a single photon is detected via the momentum transfer from the absorbed photon onto the molecule. This recoil signal is amplified using a non-classical state of motion of the two-ion crystal and subsequently read out via the atomic ion. We characterize the recoil detection method and use it to investigate the interaction between femtosecond laser pulses and the O-H stretching vibration in individual CaOH+ molecular ions. Furthermore, we present the single-photon absorption spectrum obtained for the vibrational transition. This method represents a milestone towards quantum non-demolition measurements of complex polyatomic molecules, providing high-fidelity methods for preparation and measurement of the quantum state of a wide range of molecular species.

quant-ph↗

arXiv2026

Generalized Gross-Pitaevskii Equation for 2D Bosons with Attractive Interactions

We introduce a generalized Gross-Pitaevskii equation that provides a nonlinear framework for studying two-dimensional (2D) attractive Bose systems. Its defining feature is the logarithmic density dependence of the coupling constant, which breaks the scale invariance inherent in the standard mean-field equations. This framework allows straightforward calculations of the system properties arising from the quantum anomaly. As a first illustration, we study universal bound states in free space, commonly referred to as quantum droplets. Then, we analyze breathing modes and quench dynamics in trapped systems, paving the way for a systematic exploration of non-equilibrium phenomena in 2D attractive Bose systems. Finally, we predict the existence of universal excited states, including vortex configurations, which may be more accessible to experimental investigation than the ground state. Our results provide a robust theoretical foundation for studying both static and dynamical properties of finite systems, and offer guidance for the design of future experiments.

cond-mat.quant-gas↗

arXiv2026

Optical excitation and stabilization of ultracold field-linked tetratomic molecules

We propose a coherent optical population transfer of weakly bound field-linked (FL) tetratomic molecules (tetramers) to deeper FL bound states using stimulated Raman adiabatic passage. We consider static-electric-field shielded polar alkali-metal diatomic molecules and corresponding FL tetramers in their $\textrm{X}^1Σ^+$+$\textrm{X}^1Σ^+$ ground electronic state. We show that the excited metastable $\textrm{X}^1Σ^+$+$\textrm{b}^3Π$ electronic manifold supports FL tetramers in a broader range of electric fields with collisional shielding extended to zero field. We calculate the Franck-Condon factors between the ground and excited FL tetramers and show that they are highly tunable with the electric field. We also predict photoassociation of ground-state shielded molecules to the excited FL states in free-bound optical transitions. We propose proof-of-principle experiments to implement stimulated Raman adiabatic passage and photoassociation using FL tetramers, paving the way for the formation of deeply bound ultracold polyatomic molecules.

physics.atom-ph↗

arXiv2025

CaF+CaF interactions in the ground and excited electronic states: implications for collisional losses

Accurate \textit{ab initio} potential energy surfaces are essential to understand and predict collisional outcomes in ultracold molecular systems. In this study, we explore the intermolecular interactions between two laser-cooled CaF molecules, both in their ground and excited electronic states, aiming to understand the mechanisms behind the observed collisional losses on the non-reactive, spin-polarized surface of the CaF+CaF system. Using state-of-the-art \textit{ab initio} methods, we compute twelve electronic states of the Ca$_2$F$_2$ complex within the rigid rotor approximation applied to CaF. Calculating the potential energy surfaces for the excited electronic states of Ca$_2$F$_2$ is challenging and computationally expensive. Our approach employs the multireference configuration interaction method, restricted to single and double excitations, along with a reasonably large active space to ensure the convergence in the excited states. We also compute the spin-orbit coupling between the ground state and the lowest spin-polarized triplet state, as well as the spin-spin coupling within the lowest triplet state (1) $^3\mathrm{A}'$. Additionally, we determine the electric transition dipole moments for the (1) $^3\mathrm{A}'$-(2) $^3\mathrm{A}'$ and (1) $^3\mathrm{A}'$-(1) $^3\mathrm{A}''$ transitions. Notably, we find that the lowest spin-polarized state (1) $^3\mathrm{A}'$, shifted by 1064 nm of laser light from the optical dipole trap, intersects several electronically excited states. Finally, by analyzing the potential energy surfaces, we discuss two plausible pathways that may account for the observed collisional losses on the spin-polarized surface of the CaF+CaF system.

physics.atom-ph↗

arXiv2025

Ultracold high-spin $Σ$-state polar molecules for new physics searches

We propose high-spin $Σ$-state polar molecules assembled from ultracold atoms to probe charge-parity violating physics beyond the Standard Model. We identify YbCr as a prime candidate to search for the electric dipole moment of the electron. We show that the combination of relativistic ytterbium and high-spin chromium, amenable to magneto-association, leads to molecules with easy-to-polarize parity doublets and large intramolecular electric fields. Based on \textit{ab initio} results for molecular constants, we predict a sensitivity of $δd_{\textrm{e}}= ( 6 \times 10^{-31} / \sqrt{n_{\mathrm{day}}})\,e\,\mathrm{cm}$ via standard spin-precession measurements, we assess the experimental feasibility, and discuss potential extensions to more advanced quantum control as well as searches of the nuclear magnetic quadrupole moment. This work paves the way to next-generation searches for new physics with ultracold molecules in both the leptonic and hadronic sectors.

cond-mat.quant-gas↗

arXiv2025

Magnetic Feshbach resonances in Ba$^+$+Li collisions due to strong spin-orbit coupling

We report a pronounced dependence of magnetic Feshbach resonance spectra on the initial hyperfine-Zeeman state of Li in ultracold $^{138}$Ba$^+$+$^6$Li collisions. The measured number and distribution of resonances differ significantly between the two lowest states despite their similar electron spin character. We address this puzzle by developing a comprehensive yet generic computational model calibrated against key statistical features in the experimental spectrum. We confirm that strong spin-orbit coupling induces essential changes in the distribution of resonances, leading to an increase in the number of resolvable resonances. Our model reproduces the statistics of the spectrum with the lowest Li state but struggles with the second-lowest state, where a significantly smaller number of resonances is experimentally observed.

physics.atom-ph↗

arXiv2025

Modern applications of machine learning in quantum sciences

In this book, we provide a comprehensive introduction to the most recent advances in the application of machine learning methods in quantum sciences. We cover the use of deep learning and kernel methods in supervised, unsupervised, and reinforcement learning algorithms for phase classification, representation of many-body quantum states, quantum feedback control, and quantum circuits optimization. Moreover, we introduce and discuss more specialized topics such as differentiable programming, generative models, statistical approach to machine learning, and quantum machine learning.

quant-ph↗

arXiv2025

Tunable two-species spin models with Rydberg atoms in circular and elliptical states

We propose a scheme for constructing versatile quantum simulators using ultracold Rydberg atoms in long-lived circular and elliptical states. By exciting different subspaces of internal atomic states, the atoms can be used to simulate two effective spin species with different spin-spin interactions. The strengths of transverse and longitudinal spin-spin interactions, both intra- and inter-species, can be controlled within a wide range of values. This setup can be used to simulate two-species spin models or lattice models with two sublattices. We show examples of specific models which can be realized.

quant-ph↗

arXiv2025

Quantum rotor in a two-dimensional mesoscopic Bose gas

We investigate a molecular quantum rotor in a two-dimensional Bose-Einstein condensate. The focus is on studying the angulon quasiparticle concept in the crossover from few- to many-body physics. To this end, we formulate the problem in real space and solve it with a mean-field approach in the frame co-rotating with the impurity. We show that the system starts to feature angulon characteristics when the size of the bosonic cloud is large enough to screen the rotor. More importantly, we demonstrate the departure from the angulon picture for large system sizes or large angular momenta where the properties of the system are determined by collective excitations of the Bose gas.

cond-mat.quant-gas↗

arXiv2025

Quantum control of ion-atom collisions beyond the ultracold regime

Tunable scattering resonances are crucial for controlling atomic and molecular systems. However, their use has so far been limited to ultracold temperatures. These conditions remain hard to achieve for most hybrid trapped ion-atom systems -- a prospective platform for quantum technologies and fundamental research. Here we measure inelastic collision probabilities for ${\text{Sr}^++\text{Rb}}$ and use them to calibrate a comprehensive theoretical model of ion-atom collisions. Our theoretical results, compared with experimental observations, confirm that quantum interference effects persist to the multiple-partial-wave regime, leading to the pronounced state and mass dependence of the collision rates. Using our model, we go beyond interference and identify a rich spectrum of Feshbach resonances at moderate magnetic fields with the Rb atom in its lower ($f=1$) hyperfine state, which persist at temperatures as high as 1 mK. Future observation of these predicted resonances should allow precise control of the short-range dynamics in ${\text{Sr}^+}+{\text{Rb}}$ collisions under unprecedentedly warm conditions.

physics.atom-ph↗

arXiv2024

Ultralong-range Rydberg molecules of Hg atoms

Ultralong-range Rydberg molecules, composed of an excited Rydberg atom and a ground-state atom, are characterized by large bond lengths, dipole moments, sensitivity to external fields, and an unusual binding mechanism based on low-energy elastic electron scattering. Although Rydberg molecules formed between alkali atoms have received the most attention, the additional complexity found in atoms with more than a single valence electron poses new theoretical challenges as well as new possibilities for control and design of the molecular structure. In this paper, we extend the theory of Rydberg molecules to include the additional spin coupling of the Rydberg states of a multivalent atom. We employ this theory to describe the properties of Rydberg molecules composed of mercury atoms. We calculate the potential energy curves of both heteronuclear (Hg*Rb) and homonuclear (Hg*Hg) molecules. In the former case, we propose the realization of long-range spin entanglement and remote spin flip. In the latter, we show how long-lived metastable molecular states of Hg*Hg exist as resonances above the dissociation threshold.

physics.atom-ph↗

arXiv2024

$\mathcal{P,T}$-odd effects in YbCu, YbAg and YbAu

In this work, the molecular enhancement factors of the $\mathcal{P,T}$-odd interactions involving the electron electric dipole moment ($W_\mathrm{d}$) and the scalar-pseudoscalar nucleon-electron couplings ($W_\mathrm{s}$) are computed for the ground state of the bimetallic molecules YbCu, YbAg and YbAu. These systems offer a promising venue for creating cold molecules by associating laser cooled atoms. The relativistic coupled-cluster approach is used in the calculations and a thorough uncertainty analysis is performed to give accurate and reliable uncertainties to the obtained values. Furthermore, an in-depth investigation of the different electronic structure effects that determine the magnitude of the calculated enhancement factors is carried out, and two different schemes for computing $W_\mathrm{d}$ are compared. The recommended values for the enhancement factors are $(13.24\pm0.03)\times10^{24}\frac{h\,\text{Hz}}{e\,\text{cm}}$, $(12.15\pm0.08)\times10^{24}\frac{h\,\text{Hz}}{e\,\text{cm}}$ and $(2.13\pm0.28)\times10^{24}\frac{h\,\text{Hz}}{e\,\text{cm}}$ for $W_\mathrm{d}$, and $(-48.36\pm0.18)\;h\,\text{kHz}$, $(-45.51\pm0.43)\;h\,\text{kHz}$ and $(5.31\pm1.80)\;h\,\text{kHz}$ for $W_\mathrm{s}$, for YbCu, YbAg and YbAu, respectively.

physics.chem-ph↗

arXiv2024

Hyperfine-to-rotational energy transfer in ultracold atom-molecule collisions

Energy transfer between different mechanical degrees of freedom in atom-molecule collisions has been widely studied and largely understood. However, systems involving spins remain less explored, especially with a state-to-state precision. Here, we directly observed the energy transfer from atomic hyperfine to molecular rotation in the $^{87}$Rb ($|F_a,M_{F_a}\rangle = |2,2\rangle$) + $^{40}$K$^{87}$Rb (in the rovibronic ground state $N=0$) $\longrightarrow$ Rb ($ |1,1\rangle$) + KRb ($N=0,1,2$) exothermic collision. We probed the quantum states of the collision products using resonance-enhanced multi-photon ionization followed by time-of-flight mass spectrometry. We also carried out state-of-the-art quantum scattering calculations, which rigorously take into account the coupling between the spin and rotational degrees of freedom at short range, and assume that the KRb monomer can be treated as a rigid rotor moving on a single potential energy surface. The calculated product rotational state distribution deviates from the observations even after extensive tuning of the atom-molecule potential energy surface, suggesting that vibrational degrees of freedom and conical intersections play an important part in ultracold Rb + KRb collisions. Additionally, our ab initio calculations indicate that spin-rotation coupling is dramatically enhanced near a conical intersection, which is energetically accessible at short range. The observations confirm that spin is coupled to mechanical rotation at short range and establish a benchmark for future theoretical studies.

physics.atom-ph↗

arXiv2024

Characterizing out-of-distribution generalization of neural networks: application to the disordered Su-Schrieffer-Heeger model

Machine learning (ML) is a promising tool for the detection of phases of matter. However, ML models are also known for their black-box construction, which hinders understanding of what they learn from the data and makes their application to novel data risky. Moreover, the central challenge of ML is to ensure its good generalization abilities, i.e., good performance on data outside the training set. Here, we show how the informed use of an interpretability method called class activation mapping (CAM), and the analysis of the latent representation of the data with the principal component analysis (PCA) can increase trust in predictions of a neural network (NN) trained to classify quantum phases. In particular, we show that we can ensure better out-of-distribution generalization in the complex classification problem by choosing such an NN that, in the simplified version of the problem, learns a known characteristic of the phase. We show this on an example of the topological Su-Schrieffer-Heeger (SSH) model with and without disorder, which turned out to be surprisingly challenging for NNs trained in a supervised way. This work is an example of how the systematic use of interpretability methods can improve the performance of NNs in scientific problems.

quant-ph↗