Atomic Physics 2025

Dipolar Bose-Einstein condensates continue to intrigue as they provide access to exotic states such as supersolids and offer a platform to explore beyond mean-field physics due to their susceptibility to quantum and thermal fluctuations. We present recent results on the interplay between kinetic energy, long-range interactions and fluctuations which give rise to a rich diversity of emergent states.

Supersolids are a remarkable state of matter, exhibiting both superfluidity and crystalline properties. We predict a rich excitation spectrum for a binary dipolar supersolid in a linear crystal geometry, where the ground state consists of two partially immiscible components forming alternating, interlocking domains. We identify three Goldstone branches, each exhibiting first-sound, second-sound, or spin-sound character. By analogy with a diatomic crystal, the resulting lattice has a two-domain primitive basis, and we find that the crystal (first-sound-like) branch splits into optical and acoustic phonons. Additionally, we identify a spin-Higgs branch associated with the supersolid modulation amplitude [1]. [1] W. Kirkby, Au-Chen Lee, D. Baillie, T. Bland, F. Ferlaino, P. B. Blakie, R. N. Bisset, Phys. Rev. Lett. 133, 103401 (2024)

We discuss the physics of a doubly dipolar Bose gas at zero and finite temperatures. The gas transitions to droplets or mean-field Bose-Einstein condensation at low temperatures, depending on the relative strength of dipolar interactions and contact interactions. The droplet phases encompass self-confined droplets and droplet arrays in a harmonic potential. Additionally, we explore the nature of supersolid phases in doubly dipolar Bose gases. References: 1. R. Ghosh, M. Ciardi, R. Nath and F. Cinti, Phys. Rev. B 110, 014513 (20224) 2. R. Ghosh, C. Mishra, L. Santos and R. Nath, Phys. Rev. A 106, 063318 (2022) 3. C. Mishra, L. Santos and R. Nath, Phys. Rev. Lett. 124, 073402 (2020)

Dipolar molecules are emerging as a viable ultracold platform in strongly correlated regimes, opening new possibilities for quantum metrology, computing and simulations. Recent experiments have demonstrated Bose-Einstein condensation and droplet formations, reaching densities similar to ultracold atoms with much larger gas parameters. This talk will discuss current experimental results and explore theoretical predictions of strongly correlated phases, including strongly anisotropic droplets and monolayer crystals, predicted by our path integral Monte Carlo simulations to form in realistic experimental regimes.

We report a new framework to realize supersolid crystals in the strong-interaction regime, by confining dipolar bosons in a lattice with long-range hopping. We study dipolar excitons that genuinely realize this lattice model. At fractional lattice fillings – 1/4, 1/3 and 1/2 – we report dipolar quantum solids, with mesoscopic dimensions i.e. extending across over a few hundreds lattice sites, spontaneously breaking translational symmetry. At the same time, we show that off-diagonal long-range order is induced by long-range hopping, such that exciton solids are superfluids. Numerically exact calculations quantitatively confirm that supersolidity builds up in the ground-state of the lattice Hamiltonian.

The time-dependent dynamics of doped or pure $^4$He nanodroplets poses numerous challenges due to the highly quantum nature of this unusual ``solvent'' and to its eminent superfluid properties. Helium Time-dependent Density Functional Theory (4He-TDDFT), which describes the time evolution of the helium density rather than that of the many-body wave function, results from a compromise between accuracy and feasibility. It has emerged as a powerful tool to simulate and help understand many experimental results over the years [1]. This talk will focus on some of the most recent results obtained in our group. These include: - A direct view on time-dependent ion solvation using pump-probe techniques [2,3] - Coulomb explosion of dialkali molecules on the droplet surface - A stability study on multicharged helium droplets [4]. [1] F. Ancilotto, M. Barranco, F. Coppens, J. Eloranta, N. Halberstadt, A. Hernando, D. Mateo, and M. Pi. Density functional theory of doped superfluid liquid helium and nanodroplets, Int. Review in Phys. Chem. 36, 621{2017). [2] E. García-Alfonso, M. Barranco, N. Halberstadt, and M. Pi. Time-resolved solvation of alkali ions in superfluid helium nanodroplets, J. Chem. Phys., 160, 164308 (2024). [3] E. García-Alfonso, M. Barranco, M. Pi, and N. Halberstadt, Time-resolved solvation of alkali ions in superfluid helium nanodroplets: Theoretical simulation of a pump–probe study, J. Chem. Phys. 163, 144309 (2025) [4] E. García-Alfonso, F. Ancilotto, M. Barranco, F. Cargnoni, N. Halberstadt, and M. Pi., Revisiting Thomson’s model with multiply charged superfluid helium nanodroplets. J. Chem. Phys., 161, 224303 (2024).

We develop a self-consistent approach that provides an explicit potential for a Rydberg electron whose ionic core consists of a polarizable medium, typically realized with superfluid droplets. The electron's motion remains separable in spherical coordinates, but the radial force exerted by the droplet breaks degeneracy of the angular momentum states non-perturbatively. The ensuing electron spectrum reveals intriguing properties dependent on droplet size and electron excitation. Deviations of the polarizable medium from the continuous spherical distribution can be taken into account as a perturbation of this redefined Rydberg dynamics. We discuss specific but paradigmatic examples for superfluid helium and also propose a way to probe droplet properties including its possible crystallized fraction through stimulated transitions of the Rydberg electron.

Interactions between particles are one of the key factors leading to the rich phenomena in quantum many-body physics. By adding more species, it allows access to even more complexity and intricate behaviors. We explore phases of two-component Rydberg-dressed Bose-Einstein condensates in three spatial dimensions. By proposing a detailed scheme to independently tune the inter- and intra-component interacting ranges, we find that the competition between the effective ranges of inter- and intra-component soft-core interactions leads to a rich variety of ground states. These include states resembling ionic compounds with face-centered cubic or simple cubic lattice structure. Upon increasing the scattering length, the dimensionality of the symmetry-breaking is lower due to the suppression of large densities, leading to segregated planar or tubular density profiles. Owing to the superfluidity property of the BEC, it offers a promising route to investigate 3D supersolids with a rich variety of spatial symmetry breakings. We also show that these states are not only stable ground states, but can also emerge dynamically upon time evolution.

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.

We study tetratomic and hexatomic bound states of ultracold molecules dressed by an elliptic microwave field. We show that these bound states can be accurately predicted by effective one-dimensional (1D) models that incorporate angular quantum fluctuations, despite the physical system is in 3D free space. Such dimension reduction is purely due to the anisotropy of long-range interactions, in contrast to the conventional case due to external confinements. The hard-core feature of 1D models ensures Bose-Fermi duality of these bound states, i.e., they share identical eigen-energies and spatial densities in bosonic and fermionic systems. Moreover, we find the hexatomic bound state is much more favored than tetratomic one, with binding energy even exceeding twice of the latter for a wide range of microwave couplings. Extending to large ensemble of molecules, our results suggest the formation of elongated self-bound droplets with crystalline patterns in both bosonic and fermionic ultracold molecules. Tingting Shi, Haitian Wang, Xiaoling Cui, arxiv: 2504.21535.

Recently, the first BEC of polar molecules was realized at Columbia. By eliminating two- and three-body collisional losses via double microwave shielding, gases of sodium-cesium molecules are evaporatively cooled to quantum degeneracy. Dipolar interactions can be induced by unbalancing the two microwave fields or by inducing ellipticity in one of the fields. As the dipolar interactions increase, the gas transitions to either self-bound droplets or droplet arrays. For very strong dipolar interactions we estimate that their characteristic length scale can exceed the interparticle separation. In totality, polar molecules offer a new paradigm for quantum gases with strong and flexible long-range interactions.

The introduction of disorder in the extended Bose-Hubbard model gives rise to new glassy quantum phases, namely the Bose-glass and disordered solid phases. We explore the interplay of disorder and long-range interactions, focusing on the quantum phase transition between these two glassy phases. Using Gutzwiller mean-field theory, we systematically probe the effect of disorder and long-range interactions truncated to the nearest neighbors and next-nearest neighbors on the zero-temperature phase diagram. At sufficiently high disorder strength, we observe a quantum phase transition between the disordered solid and Bose-glass phases. We investigate this transition in greater detail using cluster Gutzwiller mean-field theory to study the effect of intersite correlations and found it to be robust. We also investigate this phase transition from the perspective of percolation theory.

A Supersolid, as realized for example in dipolar quantum gases, is a curious state of matter which combines superfluid flow with a self-induced crystalline lattice structure. This state of matter simultaneously breaks two continuous symmetries: The U(1) gauge symmetry as well as the continuous translational symmetry. In current experimental realizations in finite sized dipolar quantum gases the translational symmetry is, however, already explicitly broken by the confining potential. This has profound consequences, in particular on the mode structure close to the phase transition between superfluid and supersolid. In this talk, I will present our experimental efforts towards the realization of toroidal supersolids in a new generation dipolar quantum gas experiment under construction in Stuttgart. In addition, I will present recent numerical studies on the expected mode structure in annular geometries in collaboration with the group of Stephanie Reimann at Lund university. In particular we identify effectively decoupled first and second sound branches along with well isolated Higgs excitations. The latter enables us to numerically construct Higgs quasi particle wave packets, which disperse akin to a free particle. Self-interference as well as interference with the underlying self-induced lattice potential then leads to the emergence of a quantum carpet interference structure, reflecting the quadratic dispersion relation of the Higgs particle.

The talk will summarize recent works on dipolar and anti-dipolar quantum gases under toroidal confinement, discussing the effects of a weak link on the generation of persistent currents and vortices. In a multiply-connected confinement, the strong in-plane attraction of an anti-dipolar condensate can form stacks of ring-shaped droplets, which may coherently overlap to form a supersolid along the azimuthal symmetry axis of the system. Intriguingly, the functional behavior of the energy-angular momentum dispersion in this case differs from that of a usual repulsive superfluid. The periodic maxima between persistent flow and the non-rotating ground state flatten significantly, and the typical pronounced cusps in the energy dispersion also occur in the rotationally symmetric supersolid state. With an asymmetric weak link, one can selectively induce superfluid and rigid-body rotation in different layers within the same system, offering new perspectives for the controllability of persistent flow. [1] M. Nilsson Tengstrand, P. Sturmer, J. Ribbing, and S.M. Reimann, Phys. Rev. A 107, 063316 (2023). [2] J. Hertkorn, P. Sturmer, K. Mukherjee, K.S.H. Ng, P. Uerlings, F. Hellstern, L. Kavoine, S.M. Reimann, T. Pfau, and R. Klemt, Phys. Rev. Research 6, L042056 (2024). [3] K. Mukherjee, M. Schubert, R. Klemt, T. Bland, T. Pfau, and S.M. Reimann, arXiv:2507.00989 (2025) (accepted in PRL). [4] M. Schubert, K. Mukherjee, T. Pfau and S.M. Reimann, Phys. Rev. Research 7, 033110 (2024). [5] K. Mukherjee, T. Arnone Cardinale, and S.M. Reimann, Phys. Rev. A 111, 033304 (2025).

Twisted materials are attracting a broad attention in condensed-matter physics. These materials show a wide range of intriguing phenomena, including unconventional superconductivity and correlated insulator behaviors. While fermionic matter is widely studied in the context of electronic systems, its bosonic counterpart is much less explored and finds new applications in ultracold atoms and other quantum simulation platforms based on bosonic systems, such as cavity polaritons for instance. Here we discuss bosonic quantum matter is twisted potentials. We use continuous-space quantum Monte Carlo simulations to determine the unique phase diagrams of correlated Bose gases in such potentials. The latter encompass a series of superfluid, Mott insulating, and density-wave phases. Experimental perspectives are discussed.

Driven-dissipative quantum systems can undergo transitions from stationary to dynamical phases, reflecting the emergence of collective non-equilibrium behavior. We study such a transition in a Bose-Einstein condensate coupled to an optical cavity and develop a cavity-assisted Bragg spectroscopy technique to resolve its collective modes. We observe dissipation-induced synchronization at the quasiparticle level, where two roton-like modes coalesce at an exceptional point. This reveals how dissipation microscopically drives collective dynamics and signals a precursor to a dynamical phase transition. In a second set of experiments, we make use of the cavity-mediated interaction to induce the formation of pairs of correlated atoms. We demonstrate that this process is based on the amplification of vacuum fluctuations.

Understanding quantum phase transitions in low-dimensional systems with long-range interactions lies at the forefront of many-body physics. This work focuses on the ground-state properties of strongly correlated one-dimensional (1D) bosonic systems with power-law hopping, where quantum coherence and collective effects give rise to behaviors fundamentally distinct from those in short-range models. Using exact numerical methods, including Path Integral Monte Carlo and the Worm Algorithm, we investigate both disorder- and interaction-driven localization transitions from superfluid to insulating phases, in systems where hopping decays with distance as $1/r^\alpha$. A key result of this study is the discovery of a family of continuous, scale-invariant quantum phase transitions that deviate from the conventional Berezinskii-Kosterlitz-Thouless (BKT) scenario typically expected in 1D. This indicates that long-range hopping leads to a fundamentally different localization mechanism, and we propose a new phase diagram capturing this behavior. Our findings pinpoint the regime $\alpha\leq\alpha_* = 3$ as one where long-range effects dominate—challenging earlier predictions from bosonization and finite-size numerical studies. The results serve as a benchmark for future theoretical efforts and offer quantitative guidance to experimental platforms capable of realizing XY models with long-range couplings, such as dipolar atoms and molecules, Rydberg arrays, and trapped ion chains.

Since the first dipolar Bose-Einstein condensate (BEC) made of strongly magnetic atoms, novel fascinating phenomena genuinely arising from the long-range and anisotropic dipole-dipole interaction have been observed. This includes the discovery of the long-sought after supersolid phase—a state that simultaneously manifests a crystalline order and superfluid properties. Moreover, the recent achievement of two-dimensional supersolidity has opened a new avenue for exploring the system’s rotational response, which is intimately connected to the contrasting yet coexisting nature of superfluidity and solidity. In this talk, I will present the theoretical and experimental advances achieved during my PhD in Innsbruck on vortex nucleation in two-dimensional supersolids, driven by magnetostirring, along with the rich and intricate vortex–crystal dynamics that follow. Remarkably, we find that vortex nucleation locks the revolution and orbital motions of the crystal’s droplets into a synchronous behaviour. This mechanism may offer insights into vortex unpinning processes thought to underlie “glitches”— instantaneous jumps of the rotation frequency, akin to observations in neutron stars.

Experiments on dipolar mixtures open interesting possibilities in the context of quantum droplets and dipolar supersolids. In this talk I will review some of our works in this topic. After briefly commenting on single quantum droplets of miscible and immiscible components, I will discuss how even a slight admixture of a second component may result in droplet nucleation in unmodulated condensates due to the effective local modification of the dipolar strength. I will then comment on double supersolids formed by miscible components, and their particularly intriguing excitation spectrum. I will then move to admixture of particles with dipole moments oriented in opposite directions, and how this admixture may result in crystals with actual cohesion, contrary to usual droplet crystals which lack cohesion and are only maintained due to an external trap. Finally, I will make some remarks concerning dipolar relaxation, a major handicap for the realization of spinor dipolar gases.

We discuss the effects of quantum fluctuations on a two-component Bose gas of interacting alkali atoms, considering also the effect of a Rabi coupling. The divergent zero-point energy of gapless and gapped elementary excitations of the uniform system is properly regularized obtaining a meaningful analytical expression for the beyond-mean-field equation of state. In the case of attractive inter-particle interaction we show that the quantum pressure arising from Gaussian fluctuations can prevent the collapse of the mixture with the creation of a self-bound droplet [1]. We also discuss the collective modes of a binary Bose mixture across the soliton to droplet crossover in a quasi one dimensional waveguide with a beyond-mean-field equation of state and a variational Gaussian ansatz for the scalar bosonic field of the corresponding effective action functional [2]. Finally, we consider the breathing mode and the stability of a quantum droplet in a tightly trapped one-dimensional dipolar gas of bosonic atoms. Quite remarkably, when the droplet with a flat-top density profile is formed, the breathing mode frequency scales as the inverse of the number of atoms in the cloud [3]. [1] A. Cappellaro, T. Macrì, G.F. Bertacco, and L. Salasnich, Equation of state and self-bound droplet in Rabi-coupled Bose mixtures, Sci. Rep. 7, 13358 (2017). [2] A. Cappellaro, T. Macrì, and Luca Salasnich, Collective modes across the soliton-droplet crossover in binary Bose mixtures, Phys. Rev. A 97, 053623 (2018). [3] E. Orignac, S. De Palo, L. Salasnich, and R. Citro, Breathing mode of a quantum droplet in a quasi-one-dimensional dipolar Bose gas, Phys. Rev. A 109, 043316 (2024).

Quantum cluster quasicrystals represent a fascinating class of structures in which long-range orientational order emerges without translational symmetry. In this talk, I will present recent theoretical advances in identifying the key ingredients, particularly in terms of pair interactions, required to stabilize self-organized two-dimensional aperiodic structures with various rotational symmetries as the ground states of interacting bosonic gases. Using a spectral variational method alongside Gross–Pitaevskii simulations, we demonstrate that, in contrast to the classical case, the stabilization of quantum quasicrystals with different rotational symmetries requires a distinct number of unstable wave vectors in the pair interaction potential. To deepen our understanding of these systems, we investigate both global and local superfluid properties. Our results reveal that not only can these systems support global superfluidity in a so-called super quasicrystal phase, but they can also exhibit self-organized Bose glass phases characterized by localized superfluid domains in the absence of global coherence. Finally, I will comment on the nature of low-energy excitations in these structures and show how their properties can be analytically addressed from first principles.

Transition-metal dichalcogenide materials can support intervalley excitons characterized by long life time, which implies a possibility of their condensation into various excitonic phases including BEC. The crystal point group symmetry dictates that such a condensate is multi-component, with each valley characterized by a finite quasi-momentum. The excitons from different valleys can be coupled by internal Josephson effect. However, the first order coupling is forbidden due to a large quasi-momentum difference between valleys. At the same time the second order coupling between pairs of time-reversed valleys is allowed. A simplistic analytical argument based on Mean Field predicts that this coupling should induce pairing effect for sufficiently large number of the components. In contrast, Monte Carlo simulations [1] at finite temperature show that such pairing occurs only in 2D, while in 3D the transition between the normal and condensed phases is of strongly Ist order. In other words, in 3D the second order Josephson effect induces the first-order transition into the multi-component condensate rather than the paired condensate. The nature of the transition in 2D is found to be of the BKT type at finite temperature. Using the classical-to-quantum mapping (which makes a 2D system effectively d=2+1-dimensional) one can argue that the T=0 transition in 2D should also be of first order into the multi-component condensate. Thus, the pairing affect can occur only at finite temperatures, with the phase diagram featuring a tricritical point at some temperature.

I will present the results of recent experiments where we have investigated the Hall response of 173Yb ultracold fermions in synthetic flux ladders in the regime of strong atom-atom interactions. I will discuss the results of transport measurements evidencing a strong dependence of the Hall response upon changing interactions and the emergence of a universal regime in the strongly repulsive limit [1]. I will then discuss recent developments with the electric-like measurement of Hall voltages and Hall resistances, as those performed in Hall-bar devices, which provides a direct connection between quantum simulations and transport measurements performed in solid-state physics [2]. Other research directions in the group, involving the investigation of multi-component Hubbard models [3], will be briefly presented. [1] T. Zhou et al., Science 381, 427 (2023). [2] T. Zhou et al., arXiv:2411.09744 (2024). [3] D. Tusi et al., Nature Physics 18, 1201 (2022).

A supersolid is a counter-intuitive phase of matter in which its constituent particles are arranged into a crystalline structure, yet they are free to flow without friction. This requires the particles to share a global macroscopic phase while being able to reduce their total energy by spontaneous, spatial self-organization. The existence of the supersolid phase of matter was speculated more than 50 years ago. However, only recently has there been convincing experimental evidence, mainly using ultracold atomic Bose–Einstein condensates (BECs) coupled to electromagnetic fields. There, various guises of the supersolid were created using atoms coupled to high-finesse cavities, with large magnetic dipole moments, and spin–orbit-coupled, two-component systems showing stripe phases. Here we provide experimental evidence of a new implementation of the supersolid phase in a driven-dissipative, non-equilibrium context based on exciton–polaritons condensed in a topologically non-trivial, bound state in the continuum (BiC) with exceptionally low losses, realized in a photonic-crystal waveguide. We measure the density modulation of the polaritonic state indicating the breaking of translational symmetry with a precision of several parts in a thousand. Direct access to the phase of the wavefunction allows us to also measure the local coherence of the supersolid. We demonstrate the potential of our synthetic photonic material to host phonon dynamics and a multimode excitation spectrum.

Computer simulations yield evidence of superfluid behavior of nanoscale size clusters of parahydrogen adsorbed on a graphite substrate at low temperature (T $\lesssim$ 0.25 K). Clusters with a number of molecules between 7 and 12 display concurrent superfluidity and crystalline order, reflecting the corrugation of the substrate. Remarkably, it is found that specific clusters with a number of molecules ranging between 7 and 12 self-diffuse on the surface like free particles, despite the strong pinning effect of the substrate. This effect is underlain by coordinated quantum-mechanical exchanges of groups of identical molecules; that is, it has no classical counterpart.

Mixtures of ultracold gases with long-range interactions are expected to open new avenues in the study of quantum matter. Natural candidates for this research are spin mixtures of atomic species with large magnetic moments. However, the lifetime of such assemblies can be strongly affected by the dipolar relaxation that occurs in spin-flip collisions. Here we present experimental results for a mixture composed of the two lowest Zeeman states of $^{162}$Dy atoms, that act as dark states with respect to a spin-dependent light-shift. We show that, due to an interference phenomenon, the rate for such inelastic processes is dramatically reduced with respect to the Wigner threshold law. Additionally, we determine the scattering lengths characterizing the $s$-wave interaction between these states, providing all necessary data to predict the miscibility range of the mixture, depending on its dimensionality [1]. Building on these results, we demonstrate that the spin-dependent light-shift can be harnessed to displace the energy of the closed-channel molecular states of magnetic Fano–Feshbach resonances in the lowest Zeeman state [2]. This enables tuning of the interactions on spatial and temporal scales inaccessible to usual magnetic field control. Such local tunability opens a promising route towards engineering spatially varying interaction landscapes, providing new tools to explore and stabilize exotic quantum phases. [1] M. Lecomte, Phys. Rev. Lett. \textbf{134}, 013402 (2024). [2] A. Journeaux, arXiv:2509.03567 (2025).

Dipolar quantum gases demonstrated the possibility to realize supersolids, a new class of materials connecting superfluids to crystals. Although a dipolar supersolid has a reduced superfluid response due to its partial crystallization, it can still be described as quantum fluid with a single wavefunction. In this talk I will discuss two phenomena, which directly stem from this twofold nature: the presence of self-induced Josephson oscillations in a linear supersolid [1,2], and the emergence of “partially quantized” supercurrents in annular systems [3]. [1] Biagioni, G., Antolini, N., Donelli, B., Pezzè, L., Smerzi, A., Fattori, M., ... & Modugno, G. (2024). Measurement of the superfluid fraction of a supersolid by Josephson effect. Nature, 629(8013), 773-777. [2] Donelli, B., Antolini, N., Biagioni, G., Fattori, M., Fioretti, A., Gabbanini, C., ... & Pezzè, L. (2025). Self-induced Josephson oscillations and self-trapping in a supersolid dipolar quantum gas. arXiv preprint arXiv:2501.17142. [3] Preti, N., Antolini, N., Drevon, C., Lombardi, P., Fioretti, A., Gabbanini, C., ... & Biagioni, G. (2025). Single-fluid model for rotating annular supersolids and its experimental implications. arXiv preprint arXiv:2510.26753.

Helicity lattices are a novel form of optical lattice featuring regions of alternating optical helicity—a measure of the handedness of light. I will present the properties of these lattices, their connection to superchirality, and how they exert discriminatory forces on the enantiomers of chiral molecules. When polar chiral molecules are trapped in such lattices, their dipole-dipole interactions lead to the formation of polarising quantum phases, including phases where different enantiomers separate spatially within the lattice. I will demonstrate how these intermolecular interactions modify the rotational spectroscopy of the confined molecules.

Attosecond photoelectron interferometry provides access to the photoionization dynamics of a quantum system by measuring the photoelectron spectra generated in a two-color field composed of extreme ultraviolet (XUV) harmonics and a synchronized infrared (IR) field. A widely used implementation of this approach is the reconstruction of attosecond beating by interference of two-photon transitions (RABBIT) technique [1], which has provided valuable insights into electron correlation effects and coupled electronic-nuclear dynamics [2,3]. Additionally, by combining two-color interferometric technique with photoelectron-photoion coincidence spectroscopy, angle-resolved studies in the recoil frame can reveal information about the anisotropy of the molecular potential [4,5]. While such measurements typically access only the energy or momentum of the emitted photoelectron, they often leave the internal dynamics of the parent ion unexplored. In atoms, this is usually justified, as the ion remains in a single well-defined state or in states that are not efficiently coupled by the IR field, due to their energy difference. However, in molecules the IR field can induce polarization of the ion, which for $CO_2$ would be a coupling between $B ^2\Sigma^+_u$ and $C ^2\Sigma^+_g$ cationic states [6]. This would lead to a more complex scheme than the usual two-path interference picture for the RABBIT technique, with an additional pathway arising [7]. The interference of these three pathways introduces a measurable additional time delay in photoelectron emission. The observed effect, consistent with theoretical predictions, is not mediated by the Coulomb interaction between the ion and the photoelectron but depends on the degree of entanglement of the reduced density matrix of the ion subsystem conditioned on the energy absorbed from the harmonic $H_{2q+1}$ [8]. References [1] P. M. Paul et al., Science, 292, 1689 (2001). [2] M. Nisoli et al., Chem Reviews 117, 10760 (2017). [3] D. Ertel et al., Sci. Adv. 9, eadh7747 (2023). [4] J. Ullrich et al., Rep. Prog. Phys. 66, 1463 (2003). [5] J. Vos et al., Science 360 1326 (2018). [6] J. Benda et al., Phys. Rev. A, 105, 053101 (2022). [7] J. Delgado et al., Phys. Rev. A, 111, 063107 (2025). [8] I. Makos et al., Nature Commun 16, 8584 (2025).

High-harmonic generation (HHG) serves as a powerful tool for investigating the electronic structure, crystal order, and ultrafast dynamics of inorganic solids. In this work, we extend HHG studies to a new class of materials: thin organic molecular crystals (OMCs), using pentacene as a model system. Unlike covalent or ionic crystals, OMCs are composed of weakly bound molecules, resulting in relatively flat electronic bands. Yet, their inherent, perfect molecular alignment offers a promising platform for efficient high-harmonic spectroscopy, without relying on complex alignment procedures. We find that pentacene crystals sustain high laser intensities and support efficient HHG up to the 17th order. Both experimental data and time-dependent density functional theory simulations reveal a pronounced sensitivity of the harmonic yield to the polarization of the driving field. The higher harmonic orders, in particular, exhibit strong dependence on intermolecular couplings, including nearest and next-nearest neighbors. Single-molecule models fail to capture this behavior. Tight-binding calculations demonstrate that weaker intermolecular couplings require higher harmonic orders to probe the crystal structure. These results establish HHG as a viable spectroscopic tool for probing the intermolecular coupling in OMCs, advancing the development of all-optical methods for characterizing electronic structure and ultrafast processes in complex organic materials.

Helium nanodroplets offer a unique platform for studying electron dynamics and anion formation under intense laser radiation. In this work, we explore the use of a Timepix3 detector to capture electrons, anions, and ions emitted from helium nanoplasma generated via strong-field near-infrared (NIR) excitation. The Timepix technique successfully reproduces previous findings on electron emission from both pure and dopant-induced nanoplasmas, offering a direct comparison with earlier CCD-based VMI detection schemes. In addition, the Timepix camera provides both spatial and temporal resolution, enabling momentum-resolved detection of ions under free-field expansion conditions. This temporal resolution also allows us to simultaneously detect and clearly distinguish the electron signal from the nanoplasma and the emerging anion signal. These capabilities mark an important step toward momentum-resolved imaging of ions and anions in plasma environments.

We theoretically analyze quantum dynamics of Rydberg-dressed bosonic or fermionic cold atoms in structured light. We propose a scheme to produce spin-orbital-angular momentum coupled Rydberg-dressed atoms in a harmonic trap using a Laguerre-Gaussian (LG) beam and a pair of Gaussian beams. We show that the optical orbital angular momentum (OAM) of the LG beam can be coherently transferred to the center-of-mass (COM) motion of single trapped atoms through dark state resonances. Rydberg-dressing along with OAM transfer enables one to manipulate the interaction and rotational motion between a pair of atoms.

Tungsten-doped VO₂ undergoes a sharp insulator–metal transition that strongly reshapes its nonlinear optical response. We drive solid-state high-harmonic generation across the phase transition in this material with a femtosecond near-infrared laser and interrogate individual visible harmonics using photon-correlation measurements in a single-mode Hanbury Brown–Twiss setup. As the drive intensity increases, the harmonics shift between strongly bunched to nearly coherent light, with the insulating phase retaining bunching over a broader range of intensities than the metallic phase. These phase-dependent trends are robust to detector artifacts and analysis details, suggesting that they reflect genuine changes in emission pathways and modal structure through the transition. With our results, photon-correlation high-harmonic spectroscopy emerges as a new tool to probe correlated electron–lattice dynamics and motivate inter-harmonic and phase-sensitive extensions to test for squeezing and entanglement in solids.

Attosecond spectroscopic techniques employed in the last to decades have used HHG-generated (high-harmonic generation) attosecond pulses. Despite the success and the large number of applications, e.g. to retrieve photoionization time delays in atomic and molecular targets or to track ultrafast charge migration in large systems, the low repetition rates of these sources limited the observation of non-linear phenomena in matter. Alternatively, free electron laser (FEL) facilities generate high brilliance and high intense pulses with a high frequency tunability, and have been recently shown to enable the production of coherent sub-femtosecond pulses. The combination of these intense XUV/X-ray sources with advanced detection devices that enable coincident measurements of all charged fragments enables a complete dynamical characterization of non-linear phenomena that remained experimentally inaccessible until now [1]. This technological progress thus calls for accurate theoretical methods able to predict and interpret these XUV-induced non-linear phenomena and unravel the role of electron correlation in the excitation and ionization process [2]. Recent works on XUV-IR schemes on biomolecules and small molecules (e.g. N2O, C2H2) will be discussed [3]. This talk will also present our new implementation for a full dimensional solution for the two-photon double ionization of the H2 molecule. Very few theoretical works addressed this problem due to the difficulty and computational cost of achieving an accurate evaluation of the strong correlation between all fragments in the four-body Coulomb breakup, and only frozen-nuclei approaches were employed until now [4]. In contrast with our previous methods, which were very successful for H2 single ionization problems including autoionization [5], but unable to accurately describe double ionization, in this work, we have developed from scratch a new tool to describe the multiphoton double ionization of H2 including electronic and nuclear degrees of freedom on an equal footing, i.e., working beyond the Born-Oppenheimer approximation. We employ a numerical representation of the molecular wave function in a basis set of FEM-DVR (finite elements combined with a discrete variable representation) and apply an exterior complex scaling procedure to impose the appropriate many-body Coulomb boundary conditions [6,7]. Accurate angle and energy differential two-photon double ionization yields show significant energy displacements in the photoelectron spectra with respect to frozen nuclei approaches. More interestingly, counterintuitive angularly resolved double ionization yields with respect to its atomic analog are found, due to novel interferences that arise from sequential two-photon absorption paths through different cationic states [7]. References [1] X Li et al., PRR 4, 013029 (2022) [2] F Vismarra et al., Nat. Chem. 16, 2017 (2024) [3] C González-Collado et al., PRR 6, 033 342 (2024) [4] X Guan et al., PRA 90, 043416 (2014) [5] AJ Suñer Rubio et al., PRR 6, L022066 (2024) [6] D Jelovina et al., New J. Phys. 20, 123004 (2018) [7] K Arteaga, J Feist, D Jelovina, F Martín and A Palacios, Phys. Rev. Lett. 133, 123201 (2024)

Exploiting an electric dipole effect in ionization [1], photoelectron circular dichroism (PECD) is a highly sensitive enantioselective spectroscopy for studying chiral molecules in the gas phase using either single-photon ionization [2] or multiphoton ionization [3]. In the latter case, resonance-enhanced multiphoton ionization (REMPI) gives access to neutral electronic excited states. The PECD sensitivity opens the door to study control of enantiomers' coupled electron and nuclear motion. A prerequisite is a detailed understanding of PECD in REMPI schemes. In this contribution, I will report on our investigations on PECD with coherent light sources whose pulse durations span from femtoseconds to nanoseconds [4]. By this, we address impulsive excitation on the femtosecond time scale to highly vibrational state selective excitation in mixtures with the help of high-resolution nanosecond laser techniques [5]. Subcycle control of PECD in bichromatic fields will be discussed [6], as well as coherent control of the ion yield [7] and handedness. [01] Ritchie, B. Theory of the angular distribution of photoelectrons ejected from optically active molecules and molecular negative ions. Phys. Rev. A 13, 1411–1415 (1976). [02] Böwering, N., Lischke, T., Schmidtke, B., Müller, N., Khalil, T. & Heinzmann, U. Asymmetry in Photoelectron Emission from Chiral Molecules Induced by Circularly Polarized Light. Phys. Rev. Lett. 86, 1187 (2001). [03] Lux, C., Wollenhaupt, M., Bolze, T., Liang, Q., Köhler, J., Sarpe, C. & Baumert, T. Circular dichroism in the photoelectron angular distributions of camphor and fenchone from multiphoton ionization with femtosecond laser pulses. Angew. Chem. Int. Ed. 51, 5001–5005 (2012). [04] Lee, H., Ranecky, S. T., Vasudevan, S., Ladda, N., Rosen, T., Das, S., Ghosh, J., Braun, H., Reich, D. M., Senftleben, A. & Baumert, T. Pulse length dependence of photoelectron circular dichroism. Physical chemistry chemical physics : PCCP 24, 27483–27494 (2022). [05] Ranecky, S. T., Park, G. B., Samartzis, P. C., Giannakidis, I. C., Schwarzer, D., Senftleben, A., Baumert, T. & Schäfer, T. Detecting chirality in mixtures using nanosecond photoelectron circular dichroism. Phys. Chem. Chem. Phys. 24, 2758–2761 (2022). [06] Demekhin, P. V., Artemyev, A. N., Kastner, A. & Baumert, T. Photoelectron Circular Dichroism with Two Overlapping Laser Pulses of Carrier Frequencies ω and 2ω Linearly Polarized in Two Mutually Orthogonal Directions. Phys. Rev. Lett. 121, 253201 (2018). [07] Das, S., Ghosh, J., Sudheendran, V., Ranecky, S., Rosen, T., Ladda, N., Lee, H., Stehling, T.-J., Westmeier, F., Mikosch, J., Senftleben, A., Baumert, T. & Braun, H. Control of circular dichroism in ion yield of 3-methyl cyclopentanone with femtosecond laser pulses. Physical chemistry chemical physics : PCCP, (2025).

I will discuss our recent research on electron spin dynamics in atoms and small molecules. First, I will discuss various ways of efficiently generating spin-polarized photoelectrons from rare gas atoms, both in the multiphoton and single-photon regimes, and how we can use the spin- and angle-resolved photoelectron spectra to image the core hole motion. I will also present our recent discovery of a strong connection between the spatial orientation of the ion and the spin polarization of the photoelectron. This connection explains surprisingly strong chirality-induced spin selectivity (CISS) -- the ability of chiral matter to orient the electron spin, locking it to the orientation of the chiral system. Last but not least, I will introduce the new exciting field of spin migration: the new effect where the coupling of the nuclear and electronic motions in an argon dimer drives spin currents across the dimer.

Amplified Spontaneous Emission (ASE) is a phenomenon observed in lasers and optical devices. It starts with a spontaneous emission in an active gain medium, such as a crystal or a gas. Emitted photons stimulate radiative decay of other atoms in the medium that are still in the excited state. The process results in emission of additional photons with the same energy, phase, and direction, and thus lead to amplification of the spontaneous emission signal. Since the invention of optical lasers in the 1960s, laser-based technologies have emerged as cornerstones of numerous research methods and indispensable tools for various applications in everyday life. However, widespread lasers have a fundamental limit: their wavelength is limited to specific ranges: the discrete energy levels of the gain medium, whose excitation leads to ASE, as well as the characteristics of the optical cavity, dictate the frequency of light that can be emitted. The laser wavelengths are thus confined to the infrared and visible spectral ranges, with an addition of a few plasma-based stripped ionic systems that may emit amplified light in the extreme ultraviolet (XUV) range. This limitation impedes the flexibility needed for comprehensive exploration across various spectroscopic and imaging domains, underscoring the significance of alternative technologies, like free-electron lasers (FELs), in overcoming these constraints. FELs are capable of producing ultra-fast light pulses with tunable wavelength, and have empowered researchers to investigate matter with unparalleled sensitivity and resolution in both spatial and temporal dimensions. One of the intriguing activities in this new research field deals with generalization of nonlinear spectroscopies from an optical to the short wavelength region. ASE involving inner-shell transitions in a solid or diluted medium emerges as a powerful approach to nonlinear x-ray spectroscopy because it can enable acquisition of significantly amplified signals from weak overlapping valence-to-core transitions that hold a wealth of chemical information. However, XUV ASE from short-lived quantum states above the first ionization threshold was yet to be witnessed. This is primarily due to the swift nonradiative decay processes, such as autoionization or Auger electron emission, which take place on the femtosecond temporal scale. We have overcome the problem thanks to an open-end gas cell produced at Istituto di Fotonica e Nanotecnologie (IFN-CNR, Milano), by which a millimeters long column of He gas at tens of mbar pressure was submitted to the focused XUV light pulse (Fig. 1). At the EIS-TIMEX beamline of FERMI FEL facility in Trieste, Italy, we were able to trigger, observe and quantify XUV ASE from an autoionizing resonance in helium that emits at 30.4 nm. Despite 1600-times more probable autoionization with respect to the XUV emission, the tuned pump pulse was intense enough to create sufficiently large population of helium atoms in short enough time to initiate the ASE process from a short-lived doubly excited state. Such an ASE based approach to the nonlinear XUV spectroscopy resulted in eight orders of magnitude amplification of a weak spontaneous emission signal in the forward direction on account of high directionality and radiative to non-radiative branching ratio redistribution (Fig. 2). The measurements were performed under well-controlled experimental conditions and enable an absolute comparison with the state-of-art ASE theory for the simplest non-trivial atomic system. The new results are important in showing the necessity to consider the effects of inelastic electron scattering and transient molecular configurations in the modelling. This work represents a link between the two existing research lines: the advanced ASE studies in the x-ray domain and the impulsive XUV superfuorescence studies below the first ionization threshold. The nonlinear XUV spectroscopy of gases is expected to have a significant impact on XUV spectroscopy in general because it promises orders of magnitude higher efficiency, higher chemical sensitivity and upper-state control. Our results may motivate machine physicists to provide, simultaneously with the pump, an XUV light pulse with incommensurate wavelength to selectively seed and amplify secondary weak transitions.

In this talk, I will present our recent works on measuring and modifying the ultrafast electronic response with ultrashort intense laser fields (VIS or XUV FEL). We have theoretically proposed and experimentally revealed interesting phenomena in helium: weak transitions brought to light, and spatial redirection and spectral reshaping of intense XUV radiation. By performing XUV-VIS attosecond transient absorption measurements, we have identified a strong laser-coupled pathway and enhanced the spectroscopic signal of the quasi-forbidden transitions from the ground state 1s2 to the weakly coupled doubly excited 2p3d and sp2,4- states in helium. The presented method breaks the unfavorable scaling of the cross section with the transition matrix elements, and can be generally used for enhancing single-photon-suppressed transitions [1]. In addition, we have revealed the self-induced spatial redirection and spectral reshaping, a general phenomenon when an intense laser beam passes through a resonant medium. As an experimental demonstration, the intense-XUV-induced double-peak off-axis structure in the far-field spectrum obtained at FLASH shows indications of the underlying XUV-driven Rabi dynamics and resonant pulse propagation effects [2]. [1] Y. He et al., Nat. Commun. 16, 5322 (2025) [2] Y. He et al., arXiv:2503.16344 (2025)

Synchronization is one of the most striking manifestations of collective behavior out of equilibrium: a simple yet powerful form of self-organization in which order emerges spontaneously without any leader or external cue. With recent developments in quantum technology that allow one to exquisitely tailor both system and environmental properties, synchronization has also emerged in the quantum domain, attracting considerable interest from both fundamental and applied perspectives. In this talk, I will discuss how condensed matter physics and quantum synchronization can be combined to realize novel forms of collective behavior, including topological synchronization, which connects topological notions with collective dynamics, and synchronized Aharonov–Bohm cages, where quantum interference and engineered dissipation cooperate. These examples demonstrate that the strategic use of dissipation can serve as a powerful tool to generate nontrivial nonequilibrium many-body dynamics, here viewed through the lens of synchronization.

We present a scheme for the systematic design of quantum control protocols based on shortcuts to adiabaticity. To fight decoherence, the adiabatic dynamics is accelerated by introducing high-frequency modulations in the control Hamiltonian, which mimic a time-dependent counterdiabatic correction. We present several applications for the high-fidelity realization of quantum state transfers and quantum gates based on effective counterdiabatic driving, in platforms ranging from superconducting circuits to Rydberg atoms [1]. We briefly sketch as well related ideas to control many-body interactions [2] and evolution errors by compensating terms in the Hamiltonian [3]. [1] F. Petiziol, F. Mintert, S. Wimberger, EPL 145, 15001 (2024) [2] S. Dengis, S. Wimberger, P. Schlagheck, PRA 111, L031301 (2025) [3] M. Delvecchio, F. Petiziol, E. Arimondo, S. Wimberger, PRA 105, 042431 (2022)

This talk has three parts. The first part is an introduction to Hamilton’s two monumental papers from 1834-1835, which introduced the Hamilton-Jacobi equation, Hamilton’s equations of motion and the principle of least action. These three formulations of classical mechanics became the three forerunners of quantum mechanics; but ironically none of them is what Hamilton was looking for -- he was looking for a “magical” function, the principal function S(q1,q2,t) from which the entire trajectory history can be obtained just by differentiation (no integration). In the second part of the talk I argue that Hamilton’s principal function is almost certainly more magical than even Hamilton realized. Astonishingly, all of the above formulations of classical mechanics can be derived just from assuming that S(q1,q2,t) is additive, with no input of physics. The third part of the talk will present a new formulation of quantum mechanics in which the Hamilton-Jacobi equation is extended to complex-valued trajectories, allowing the treatment of classically allowed processes, classically forbidden process and arbitrary time-dependent external fields within a single, coherent framework. The approach is illustrated for barrier tunneling, wavepacket revivals, nonadiabatic dynamics, optical excitation using shaped laser pulses and high harmonic generation with strong field attosecond pulses.

Continuous efforts at CERN are devoted to the production and investigation of antihydrogen in order to understand the source for the obvious matter-antimatter asymmetry that is not reproduced by the Standard Model of physics. While most of these experiments aim at high-precision investigations on, mostly trapped, antihydrogen atoms, the question of its reactions with ordinary matter, especially its matter counterpart, the hydrogen atom, has been of continuous interest, especially when ultracold hydrogen atoms had been suggested about 30 years ago as a possible coolant for antihydrogen. In fact, the antihydrogen-hydrogen quasimolecule has turned out to be an interesting object by itself. After an overview over the previous theoretical results on antihydrogen-hydrogen collisions, very recent results will be presented.

In my talk I will advocate a parameter free ab-initio approach to treating ultrafast light-matter interactions uncovering novel and hitherto unsuspected early time spin dynamics phenomena [1,2]: (a) I will show that OISTR is one of the fastest ways to control spins by light and (b) I will make a case for femto- phono- magnetism by demonstrating that selective excitation of optical phonon modes exert a strong influence on femtosecond demagnetisation, generating an additional loss of moment [3] in laser pumped materials. In the second part of the talk I address the question of valley control in 2d transition metal dichalcogenides (TMDC), with current understanding that it couples exclusively via circularly polarized light. In our work we show that on femtosecond time scales valley coupling is a much more general effect. We find that two time separated linearly polarized pulses allow almost complete control over valley excitation. [1] Dewhust et al., Nano Lett. 18, 1842 (2018). [2] Siegrist et al. Nature 571, 240 (2019) [3] Sharma et al. Sci. Advs. 8, eabq2021 (2022) [4] Sharma et al. Optica 9 (8), 947-952 (2022)