Atomic Physics 2025

Ultracold polar molecular gases stabilized by microwave shielding provide a powerful platform for exploring quantum many-body physics with strong, long-range, and anisotropic interactions. We develop an extended Gross-Pitaevskii framework for bosonic dipolar molecules under single-microwave shielding, incorporating effective interactions and quantum fluctuation corrections. Benchmarking against path-integral Quantum Monte Carlo simulations shows excellent agreement across superfluid, supersolid, and droplet phases. Elliptic microwave polarization induces fully anisotropic superfluidity with direction-dependent sound velocities—an effect absent in atomic dipolar gases. A quasi-1D theory reveals roton softening and shows that instabilities can be tuned purely via polarization ellipticity. The ellipticity also controls the character of the superfluid–supersolid transition, from sharp to continuous, suggesting routes for low-entropy preparation of molecular supersolids. Our results demonstrate that even single-microwave shielded molecular gases exhibit rich, tunable quantum behavior, providing a solid foundation for future studies of more complex shielding schemes.

We investigate the role of quantum fluctuations in the dynamics of a bosonic Josephson junction across different spatial dimensions. Using beyond-mean-field Gaussian corrections, we systematically derive key dynamical properties, including the Josephson frequency in the regime of small population imbalance and the critical strength for macroscopic quantum self-trapping. Our results show that quantum fluctuations increase the Josephson frequency in two and three dimensions, but decrease it in one dimension. Similarly, the critical self-trapping strength is reduced by fluctuations in two and three dimensions, while enhanced in one dimension. This dimensional dependence can be traced back to the distinct scaling of the interaction strength with the scattering length in different geometries.

We investigate plasmon-assisted photoelectron emission using a one-dimensional time-dependent density-functional theory (TDDFT) model. The plasmons are excited nonlinearly by three laser photons. Photoelectron spectra are computed with the time-dependent surface-flux (t-SURFF) method. In addition to the expected above-threshold ionization (ATI) comb, we observe peaks that arise from long-lived plasmon oscillations and the associated electron emission occurring after the laser pulse. We further analyze the positions of these peaks and their scaling behavior with the laser intensity.

The study of the different interaction mechanisms between charged particle and atoms or molecules reveals several features of collision dynamics. In this study, the double differential cross section (DDCS) of electrons emitted from methane in collisions with protons having energies between 20-300 keV has been measured. The protons of different energies are obtained from an ECR-based low energy ion accelerator. A hemispherical electron analyzer with a CEM-mounted on a rotatable turn table, has been used for electron detection in the range of 1-400 eV. The energy and angular distributions of the e-DDCS have been studied along with the different theoretical models based on the CDW-EIS approximation. This investigation with such lower velocity projectile gives a stringent test to the perturbative models applied for a multi electronic target. The CNDO and molecular orbital (MO) approach, with two different scaling parameters (d) [1], are used for the target description. Agreement with the experimental data is found much better in case of the CDW-EIS (MO) model. The choice of a suitable theoretical scaling parameter also plays a crucial role. Fig: 1 and 2 show a typical energy distribution and the angular distribution spectrum of e-DDCS at 100 keV proton beam, respectively. The angular distribution reveals a large disagreement with the MO-based model in backward angles for lower energy (e.g. 100 keV) and better agreement at higher energies (e.g. 300 keV). The single differential cross section and the total cross section (TCS) have been deduced from the measured DDCS data. Projectile energy dependence of the TCS data [Fig: 3] shows a maximum at around 50 keV. Our data at lower energy falls well below the existing TCS data of Rudd and coworkers [2]. The detailed comparison with MO-based model will be presented along with our previously published data of HCI collisions with methane. Besides continuum, the KLL Auger line is also observed in Fig:1. Intensity of the C-KLL Auger line is found to be increasing with beam energy which reveals the typical behavior of the K-vacancy production cross section [3-5]. References: [1]. L. Gulyás, I. Tóth, L. Nagy, J. Phys. B: At. Mol. Opt. Phys. 46 (7) (2013) 075201. [2]. M. E. Rudd, Y. -K. Kim, D. H. Madison, and J. W. Gallagher, Rev. Mod. Phys. 57, 965 (1985). [3]. D. Chakraborty et al. J. Elec. Spec. and Related Phenomena 269, 147405 (2023). [4]. C. Bagdia, A. Bhogale, L. Guly ́as, and L. C. Tribedi, Phys. Rev. A 104, L060802 (2021). [5]. Anuvab Mandal et al., Phys. Rev. A 101, 062708 (2020).

In the last decades, the development of sophisticated cooling techniques has enabled the trapping of atomic gases in optical lattices, thus providing a suitable platform for the study of collective effects in many-body quantum systems. To a very good approximation, the behavior of bosonic atoms can be described by an extended Bose–Hubbard (EBH) Hamiltonian. In this contribution we focus on the finite-size properties of the EBH model. By employing exact diagonalization, we investigate the model in the hard-core approximation on both square and triangular grids of different sizes, with the purpose of reconstructing the zero-temperature "phase" diagram and the superfluid fraction through the whole range of parameters. Somewhat surprisingly, even with a small number of grid sites we observe a nontrivial phase behavior that is quite different from that found in the thermodynamic limit. In the square-grid case (9 sites) the addition of 4 further sites, makes the EBH phase diagram more in line with its infinite-size limit.

Dipolar Fermi gases are expected to show exotic phases of matter, such as the supersolid and the Wigner crystal phase. Their anisotropic long-range interactions make them also highly relevant for the study of unconventional superconductivity. In this work, we focus on the case of highly population-imbalanced dipolar Fermi gases to explore their few- and many-body physics. We analyze the quantum scattering of a single impurity in a dipolar Fermi sea, highlighting key differences from conventional short-range interaction models. Additionally, we discuss implications for the polaron-to-molecule transition and Anderson’s orthogonality catastrophe, introducing a new theoretical framework to address this problem. Our results provide insights into the interplay between s- and p-wave pairing, and the emergence of supersolid phases, in the dipolar BCS-BEC crossover. We propose an experimental protocol to test the predictions in this work using ultracold molecules.

In a Bose-Einstein condensate, the long-range nature of Rydberg interactions are characterised by a scattering length that may rival or even surpass the average spacing between the surrounding bosons. The significance of these interactions depends on the density; when the average distance between Bosons is smaller then the scattering length, the system exhibits a rich absorption spectrum which extends typical polaron physics, with large shifts and complex molaron structures. However, within a dense bath, the absorption spectrum consists only of a broad single Gaussian, indicating an almost classical behaviour. Extending the scope of interactions even further, and changing the electronic angular momentum of the Rydberg atom to l>0 can introduce anisotropic and non-additive interactions, breaking spherical symmetry and leading to l(l+1)-degenerate electronic potential energy surfaces. This degeneracy leads to a non-additive interaction potential, where the full interaction between impurity and bath depends explicitly on the positions of each bosons. To capture these effects, we employ a multichannel version of the functional determinant approach for bosons and scattering theory, revealing how anisotropy and non-additivity shape the absorption spectrum of a Rydberg impurity in an ideal BEC.

We study the incoherent transport of bosonic particles on a lattice in one and two dimensions in the case of asymmetric hopping rates between the lattice sites. In one dimension, it has recently been predicted that, in the case of sufficiently asymmetric hopping rates, at the steady state, the bosons spontaneously exhibit a real-space zig-zag pattern in their populations in the vicinity of the lattice boundary (SciPost Phys. 16, 029 (2024)). Here, we extend the previous work by considering a loss and hopping impurity at one of the sites. We demonstrate that both types of impurity act as a boundary, giving rise to a skin state in its vicinity. Moreover, we study the case of periodically alternating hopping rates on a one-dimensional lattice, similar to an SSH-type hopping configuration. Finally, we present some novel results regarding the asymmetric transport on a 2D lattice with a circular metastable hopping configuration.

We provide a geometric description of coherent states of quantum light in a time-periodically varying medium in terms of the Poincaré disk model of the hyperbolic plane. This model provides a geometric interpreteation of the distance between, which can be closely related to the degree of bipartite entanglement between counter-propagating bosonic modes, while evolution operators acting on the coherent states take the form of Möbius transformations of the unit disk. We link the nature and topology of Poincaré disk trajectories to stability and the photonic band structure. In addition to this, we utilise the underlying Lie group symmetry of the system to obtain the geometric phase accumulated by Floquet states in the traversal of a closed loop in the projectve Hilbert space.

We investigate finite, self-bound ensembles of microwave-shielded polar molecules that naturally arrange themselves in quasi-two-dimensional geometries, focusing on the crossover from a membrane-like droplet phase to a crystalline monolayer. The competition between long-range antidipolar interactions and quantum fluctuations gives rise to rich structural and coherence properties, including signatures consistent with a transitional supersolid regime. Our simulations are based on the Variational Monte Carlo method with a neural-network quantum state as the trial wave function. The parameters are optimized using stochastic reconfiguration, with the covariance matrix efficiently estimated through the KFAC (Kronecker-Factored Approximate Curvature) approximation. This framework enables stable energy minimization and accurate treatment of strongly correlated, finite systems with open boundary conditions. We report results for up to N=40 particles and interaction strengths ranging from C=10 to C=100, including pair-correlation functions and radial and two-dimensional density profiles. As the interaction increases, the system evolves from a delocalized droplet membrane to a structured crystalline monolayer. Remarkably, our approach does not exhibit the fragmentation into smaller subsystems reported in previous studies, indicating that coherence and self-binding are maintained throughout the transition. These findings demonstrate the ability of neural-network quantum states to capture supersolid-like behavior and emergent order in finite, strongly interacting molecular systems.

We reveal the emergence of polarizing phases for the enantiomers of cold, interacting chiral molecules in a helicity lattice. These recently proposed lattices have sites with alternating helicity which exert a discriminatory force on chiral molecules. We find that a strong dipolar repulsion between molecules results in the separation of left and right enantiomers.

The dipole approximation, a cornerstone of light-matter theory, breaks down in regimes of high laser intensity and short wavelength. In this beyond-dipole regime, phenomena such as quadrupole transitions, radiation pressure, and magnetic field interactions become significant. In this work, the interplay between dipole and quadrupole interaction terms is investigated to uncover new insights into the dynamics of quantum systems. Time-dependent density functional theory (TDDFT), as implemented in the OCTOPUS code, is employed to model these effects in atoms and small molecules with representative symmetries. The accuracy and reliability of this approach for capturing the full electromagnetic interaction are validated by benchmarking TDDFT results against exact numerical solutions of the time-dependent Schrödinger equation , paving the way to study absorption spectra of realistic quantum systems, like extended carbon-conjugated molecules, where the dipole approximation is no longer sufficient.

We study ultracold bosons confined in two-dimensional hexagonal optical lattices where a tunable geometric phase $\phi_g$ continuously transforms the potential from a symmetric honeycomb to a triangular geometry passing trough an asymmetric hBN-like geometry. Using continuous-space worm-algorithm path-integral Monte Carlo (WORM-PIMC) simulations, we compute low-temperature phase diagrams as functions of the chemical potential and interaction strength. For $\phi_g=0$, we recove standard superfluid–Mott-insulator transitions, although with important disagreement with the single-band Bose–Hubbard predictions on the honeycomb lattice from the literature. For $\phi_g=\pi/30$, the sublattice energy offset $\Delta_{AB}$ introduces a rich hierarchy of Mott plateaus with occupations $(n_A,n_B)$, governed by the interplay between tunneling, onsite interactions, and the offset energy. Our results demonstrate the crossover from an effectively triangular to an hBN-like lattice and benchmark the limits of lattice-model descriptions in optical-lattice experiments.

We investigate the role of quantum fluctuations in dipolar Bose-Einstein condensates (BECs), going beyond the usual Lee-Huang-Yang (LHY) description. By building a set of self-consistent equations for the BEC and its excitations, we obtain a new Bogoliubov spectrum with contributions that stabilize the BEC, consistent with experimental observations. In contrast to the established LHY theory, the corrections we obtain are purely real. We apply the theory to a homogeneous BEC with dipole-dipole interactions and obtain new corrections to the ground state energy and chemical potential, without the need to neglect imaginary contributions. Furthermore, we show that the system can form droplets in a harmonic trap, and we predict the critical atom number. Finally, we apply the theory to a system of single-dressed molecules with an effective anti-dipole interaction, and find that also for this interaction, our obtained corrections give rise to a significant stabilization of the system.

Optical lattices have been used to structure ultracold gases into periodic atomic arrays, typically with large micrometer-scale periods. On an independent research line, high-harmonic generation and X-ray free-electron lasers produce intense, high-frequency standing waves whose ponderomotive potentials generate lattices with orders-of- magnitude smaller periods. A third advance routinely prepares giant Rydberg atoms with $n \gtrsim 100$, whose orbital sizes $\sim n^{2}a_{0}$ are comparable to optical-lattice periods. Here we bridge these ultrafast and ultracold fronts by systematically characterizing the spectral response of a single Rydberg electron to a standing-wave field across lattice wavelengths from effectively infinite to extremely short. We find rich spectral features that change qualitatively across five regimes, with behavior that is nearly universal when the lattice wavelength is scaled by the adequate Rydberg spatial scale.

The interaction of intense laser fields with atoms provides a powerful means to probe the nonlinear response and dynamics of atomic systems. Among the theoretical models describing laser–atom interactions, the strong-field approximation (SFA) offers an intuitive and widely used framework for understanding strong-field ionization of an atom. SFA is often formulated with the plane-wave representation of the continuum electrons and primarily limited to describing ionization from an $s$-orbital where the orbital’s binding energy is adjusted to the ionization energy of the atom. Here, we present an SFA formulation with the continuum electron wavefunction expanded in terms of partial waves. This partial-wave expansion allows a consistent description of ionization from the outer orbitals with higher orbital angular momentum. This consideration of a more realistic atomic-bound electron wavefunction ensures a better representation of the bound–continuum transition matrix elements. We performed a comparative analysis of our partial-wave expansion with the widely used saddle-point method (SPM). More significantly, a strong quantitative agreement is established between the \textit{s}-wave ionization probability and the total probability calculated via the SPM. This suggests that the SPM, in the studied regime, primarily captures the dynamics of the $l=0$ ionization channel. Further, this present approach establishes a systematic foundation for future extensions to multi-electron systems, where electron–electron correlations can be incorporated through configuration-interaction schemes.

We employ few-level model systems to investigate the polarization and phase characteristics of below-threshold harmonics emitted from aligned molecules. In a two-level system, we observe a distinct change in the phase of the harmonic response: for photon energies below the transition energy of the dominant field-dressed states, the harmonic phase alternates by $\pi$ between consecutive odd orders, whereas it remains constant above this threshold. Building on this observation, we introduce a four-level model consisting of two independent two-level systems oriented along perpendicular axes. For suitable transition energies, the polarization of the lower-order harmonics align with the polarization of the driving field, while higher orders display a mirrored polarization. The model suggests that aligned molecules with orthogonal transition dipoles can exhibit a comparable phase and polarization behavior in the below-threshold regime.

We theoretically investigate the properties of ultracold dipolar atoms in radially coupled, concentric annular traps created by a potential barrier. The nonrotating ground-state phases are investigated across the superfluid-supersolid phase transition, revealing a particle imbalance between the two rings and a preferential density modulation in the outer ring in the absence of rotation. Near the phase transition on the superfluid side, applying rotation can induce density modulations in either ring, depending on the angular momentum and barrier strength. For low angular momentum, such rotation-induced density modulation forms in the outer ring, while for high angular momentum and weak barriers, it emerges in the inner ring. Rotation can lead to persistent currents and the nucleation of a vortex residing either at the center (central vortex) or at the ring junction (Josephson vortex). Josephson vortices can also form at the junctions of the localized density sites induced by rotation in the inner ring, a behavior that is unique to our system. By switching off the trap and allowing the system to expand, distinct interference patterns emerge, which can be analyzed to identify and distinguish between various vortex configurations, and thus can be observed in current state-of-the-art experiments.

The Kibble-Zurek mechanism (KZM) predicts the spontaneous formation of topological defects in a continuous phase transition driven at a finite rate. We propose the generation of spontaneous quantum turbulence (SQT) via the KZM during Bose-Einstein condensation induced by a thermal quench. Using numerical simulations of the stochastic projected Gross-Pitaevskii equation in two spatial dimensions, we describe the formation of a newborn Bose-Einstein condensate proliferated by quantum vortices. We establish the nonequilibrium universality of SQT through the Kibble-Zurek and Kolmogorov scaling of the incompressible kinetic energy. Additionally, we study the circulation statistics within an area A enclosed by a loop C, and show that the Migdal area rule (probability density function of circulation around a small closed contour is independent of the contour's shape in classical turbulence) holds for small loops, independent of their shape.

The $3^+\ ^1P^o$ resonance in He with 63.66 eV excitation energy autoionizes within 80 fs but may also decay by spontaneously emitting a 40.75 eV photon to populate the $1s3s\ ^1S^e$ atomic state with $3\cdot 10^{-4}$ probability \cite{Penent2001,Soder2008}. Despite such a small fluorescence branching ratio, our recent calculations in the paraxial approximation predicted strong ASE in the forward direction if dense and long enough column of He gas is traversed by an intense resonant XUV pump pulse \cite{Krusic2023}. Indeed, we have observed amplification of the weak $3^+\ ^1P^o\rightarrow 1s3s\ ^1S^e$ fluorescence decay at the EIS-TIMEX beamline using light pulses from the free electron laser (FEL-1) facility FERMI in Trieste, Italy. A He gas column in the open-end glass microcapillary was a few mm long with nearly constant He pressure that went up to 100 mbar. The 50 fs long FEL pulses with few tens of $\mu J$ energy were focused to the $15\times 26\ \mu$m cross section in the center of the gas target. A maximum observed average conversion factor from the number of probe photons to the number of ASE photons was 4.1\%. The theory reproduces well the ASE yield with regard to the pump energy and wavelength, He gas pressure, ASE spectral shape and photon-number distribution, especially at high pump energies where up to a few percent conversion efficiency was detected. Significant discrepancies are found at high gas pressure where the observed shift of the ASE spectrum is not well reproduced by the model. The effect may be attributed to the presence of transient molecular configurations of He atoms that effectively broaden/shift energies of the upper and lower ASE state. Simultaneously with ASE, we have measured position-resolved VIS emission spectra collecting the light emitted by gas from different positions along the microcapillary. The VIS signal was due to late transitions between singlet and triplet singly excited states in the atomic He, as well between the excited states in He$^+$ ions. The intensity of VIS lines was found to diminish with increasing distance from the cell entrance as dictated by absorption of the FEL light along the cell. A weak correlation of the single-shot VIS spectra with the ASE emission was detected. It was strongest at high pressures and also showed positional dependance.

We investigate the finite-temperature properties of a bosonic Josephson junction composed of N interacting atoms confined by a quasi-one-dimensional asymmetric double-well potential, modeled by the two-site Bose-Hubbard Hamiltonian. We numerically compute the spectral decomposition of the statistical ensemble of states, the thermodynamic and entanglement entropies, the population imbalance, the quantum Fisher information, and the coherence visibility. We analyze their dependence on the system parameters, showing, in particular, how finite temperature and on-site energy asymmetry affect the entanglement and coherence properties of the system. Moreover, starting from a quantum phase model which accurately describes the system over a wide range of interactions, we develop a reliable description of the strong tunneling regime, where thermal averages may be computed analytically using a modified Boltzmann weight involving an effective temperature. We discuss the possibility of applying this effective description to other models in suitable regimes.

Entanglement in position and momentum, despite the close connection of the Einstein–Podolsky–Rosen gedankenexperiment, remains largely unexplored in particle scatterings. Here we address this question with fully coherent calculations of bipartite scattering in three-dimensional space. We show that the standard plane-wave description of scattering fails to capture the entanglement properties, due to the essential role of quantum uncertainty in the initial state. The entanglement generated in scattering, quantified by the Schmidt number $K$, saturates with the time of flight. For sufficiently narrow initial momentum dispersion, $K$ scales linearly with the scattering cross section. These results provide a benchmark for future experiments and establish a framework for exploring resonance-enhanced entanglement in realistic quantum collisions.

In this work, we exploit the Magnus expansion and SU(2) algebra to study quasienergy of periodically driven two-level systems, paying attention to its convergence condition. We emphasize that elementary unitary transforms significantly extend the validity of Magnus expansion and higher order terms admit particular explanation of Bloch-Sigert shift and/or Stokes phase in Landau-Zener transition.

The immersion of an impurity in a bosonic medium has enabled systematic exploration of the Bose polaron problem across the entire range of impurity–bath coupling strengths. Both attractive and repulsive polarons—arising from inherently attractive impurity–medium interactions, such as those involving Rydberg or ionic impurities in neutral ultracold gases—have been extensively investigated. While the attractive polaron represents the ground state of the many-body impurity–bath system, the nature of the metastable repulsive polaron remains less understood. Here, we present a unified framework for describing both attractive and repulsive polarons in one- and two-dimensional (1D and 2D) Bose gases. By obtaining ground- and excited-state solutions of the Gross–Pitaevskii equation for a finite-range impurity potential in a weakly interacting Bose medium, we demonstrate that repulsive polarons are adiabatically connected to topological defects supported by the condensate. In 2D, these defects correspond to vortices and dark ring solitons, while in 1D they manifest as distinct solitonic configurations. Furthermore, we uncover a crossover between the repulsive and attractive polaron branches as the impurity–bath coupling strength increases. The analysis identifies universal regimes characterized by the zero-energy impurity–bath scattering length and the condensate coherence length.

We propose an experimental scheme to realize non-Abelian dynamical gauge field for ultracold fermions, which induces a novel pairing mechanism of topological superfluidity. The dynamical gauge fields arise from nontrivial interplay effect between the strong Zeeman splitting and Hubbard interaction in a two-dimensional (2D) optical Raman lattice. The spin-flip transitions are forbidden by the large Zeeman detuning, but are restored when the Zeeman splitting is compensated by Hubbard interaction. This scheme allows to generate a dynamical non-Abelian gauge field that leads to a Dirac type correlated 2D spin-orbit interaction depending on local state configurations. The topological superfluid from a novel pairing driven by 2D dynamical gauge fields is reached, with analytic and numerical results being obtained. Our work may open up a door to emulate non-Abelian dynamical gauge fields and correlated topological phases with experimental feasibility.