Atomic Physics 2023

In our work we exploit femtosecond pulse shaping and interferometry in nonlinear spectroscopy and quantum control ranging from the visible to the extreme ultraviolet (XUV) spectral domain. With these tools, we developed a highly sensitive probe for inter-atomic interactions in thermal atomic ensembles [1]. This regime was so far difficult to access due to the strong inhomogeneous broadenings dominating the spectral signatures. Our method can resolve this problem and reveals interactions in ensembles with mean inter-atomic separation as large as 27 $\mu$m (for D-line excitations in alkali atoms) [2]. We assign these findings to the far-field contribution of the dipole interaction. In a more recent development, we extended pulse shaping to an XUV free-electron laser (FEL). Paired with the high peak intensities of FELs, this opens the door to the study of coherent strong-field phenomena in the XUV spectral domain. As an example, we demonstrated the strong-field quantum control in helium atoms. [1] L. Bruder et al., Phys. Rev. A 92, 053412 (2015) [2] L. Bruder et al., Phys. Chem. Chem. Phys. 21, 2276 (2019)

Scattering properties and time delays for isotropic and anisotropic potentials are discussed paradigmatically in one dimension in terms of the respective S-matrices for symmetric and non-symmetric potentials.

We report on the coherent control of photoelectron momentum distributions using polarization-tailed femtosecond laser pulses. We present recent results on chirped photoelectron vortices, strong field control of spin-orbit wave packets, and a novel scenario to manipulate the Autler-Townes doublet using bichromatic pulses. In our experiments, we combine supercontinuum polarization pulse shaping with high-resolution photoelectron tomography to map the atomic MPI dynamics. Recently, we have extended our previous scheme towards multichromatic polarization shaping. In addition, we introduce a holographic method to extract the phases of the photoelectron momentum distributions.

Manifestations of dipole–dipole interactions in dilute thermal atomic vapors are difficult to sense, because of strong Doppler broadening. Recent experiments with alkali atoms reported signatures of such interactions in fluorescence detection-based measurements of multiple quantum coherence (MQC) signals. We develop an original open quantum systems theory of MQC signals in dilute thermal gases, which allows us to obtain good quantitative agreement with the experimental observations. In the present talk, we outline the characteristic features of our theory which incorporates the vector character of the atomic dipoles, as well as driving laser pulses of arbitrary strength and polarization, includes the far-field coupling between the dipoles, which prevails in dilute ensembles, and effectively accounts for the atomic motion via a disorder average. We then discuss how the multilevel internal structure of alkali atoms affects the fundamental properties of MQC signals.

We experimentally demonstrate the use of a Bose-Einstein condensate (BEC) as a “tracking device” [1] for Rydberg chemistry, akin to a bubble chamber. A sin- gle ultra-long range Rydberg molecule embedded in the BEC can coherently ex- cite waves, carrying tell-tale signatures of its dynamics. Bond lengths exceeding a micrometer allow us to observe the fingerprint of these molecules on the BEC via optical microscopy. Simulations of expected in-situ condensate images evidence that the molecular electronic state rapidly transforms from an initially excited S- or D- orbital to a complex trilobite molecular state, marked by a strongly localized electron wave function [2]. The state change liberates energy, which should cause final state particles to move rapidly through the medium, which is however ruled out by com- paring experiment and simulations. The molecule thus must strongly decelerate in the medium, for which we propose a plausible mechanism. Our experiment features an ambient coherent medium that facilitates and records an electronic state change of exotic molecules in ultra-cold chemistry, with sufficient sensitivity to constrain velocities of final state products. The same platform can also provide controllable decoherence of superposition states and permit a glimpse of the environmental com- ponent of decohering entangled states [3]. [1] S. Tiwari and S. Wüster, Phys. Rev. A 99, 043616 (2019). [2] S. Tiwari, F. Engel, M. Wagner, R. Schmidt, F. Meinert and S. Wüster, New J. Phys. 24, 073005 (2022). F. Engel, S. Tiwari, T. Pfau, S. Wüster and F. Meinert, https://arxiv.org/abs/2308.13762 (2023). [3] S. Rammohan et al., Phys. Rev. A (Letters) 104, L060202 (2021), Phys. Rev. A 103, 063307 (2021).

We study the properties of intermediate few-atom complexes formed in nonreactive collisions, which is critical for understanding experiments with ultracold alkali-metal molecules. We analyze the three representative systems: KRb ($X^1\Sigma^+$)+Rb ($^2S$), NaLi ($a^3\Sigma^+$)+Na ($^2S$) and NaK ($X^1\Sigma^+$)+NaK ($X^1\Sigma^+$) with the \textit{ab initio} electronic structure quantum-chemical methods. We propose [1] that unexpectedly large loss rates of the closed-shell molecules, as in the case of NaK ($^1\Sigma^+$)+NaK ($^1\Sigma^+$) collision may be explained by the variation in the nuclear spin–spin and quadrupole couplings. Electronic structure calculations show that at small inter-monomer separations, those couplings may be strong enough to couple different rotational manifolds. Increased density of states leads to the extended lifetime of the collision complex. In the case of the system with an unpaired electron(s), the couplings that involve the electronic spin are far more critical. We have found that electron spin-rotation coupling is strongly enhanced by the conical intersection, which may lead to an increase in the lifetime of the collisional complex in KRb ($X^1\Sigma^+$)+Rb ($^2S$) system. The electron spin-spin or electron spin-rotation interaction has to join with the anisotropy of the interaction potential to increase the loss rate constant in the NaLi ($^3\Sigma^+$)+Na ($^2S$) [2]. Our results help interpret recent experimental [3,4] data obtained with nonreactive systems.\\ \textbf{References:} [1] K. Jachymski, M. Gronowski, M. Tomza, \textit{Collisional losses of ultracold molecules due to intermediate complex formation}; \textit{Phys. Rev. A} 106, L041301 (2022) [2] T. Karman, M. Gronowski, M. Tomza, J. J. Park, H. Son, Y.-K. Lu, A. O. Jamison, W. Ketterle \textit{Ab initio calculation of the spectrum of Feshbach resonances in NaLi + Na collisions}; \textit{Phys. Rev. A} 108, 023309 (2023) [3] J. J. Park, H. Son, Y.-K. Lu, T. Karman, M. Gronowski, M. Tomza, A. O. Jamison, W. Ketterle \textit{Spectrum of Feshbach resonances in NaLi + Na collisions}; \textit{Phys. Rev. X} 13, 031018 (2023) [4] M. A. Nichols, Y.-X. Liu, L. Zhu, M.-G. Hu, Y. Liu, K.-K. Ni \textit{Detection of Long-Lived Complexes in Ultracold Atom-Molecule Collisions}; \textit{Phys. Rev. X} 12, 011049 (2022)

Quantum impurities interacting with their environment via long-ranged forces can cause a significant perturbation in the host gas density, making their description challenging. I will discuss the construction of a simple density functional for the Fermi polaron problem based on the Thomas--Fermi approach. Within this approach the Fermi polaron is fully bosonized in two dimensions, as the model results in a suitable Landau--Pekar functional known from the Bose polaron problem. In turn, in other dimensions the impurity self--interacts with an infinite number of its own images, and no bosonization occurs. The theory allows for estimation of the effective mass and predicts a self--trapping transition with order depending on the dimensionality.

In my talk, I will start by giving an overview of this year’s Nobel Prize in Physics awarded to Anne L'Huillier, Ferenz Krausz and Pierre Agostini “for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter”. Following their pioneering works, we have witnessed a revolutionary development of attosecond light sources, and methods to conduct attosecond time-resolved measurements. These now allow researchers to explore electron dynamics at this timescale, including charge migration in molecules and ultrafast processes in semiconductors and dielectrics. While experimental techniques have progressed rapidly, theory has not developed at the same pace. Insights that can be derived from present-day theory formulated in terms of single-electron and classical modeling are reaching their barrier. This situation impedes a deeper understanding of the quantum motion of electrons in matter and the quantum properties of light. Therefore, in the second part of my talk, I will discuss the challenges faced by current theoretical models and discuss ongoing efforts to enhance them. Three key areas of focus include (i) Electron-electron correlations: Exploring ways to incorporate electron-electron correlation in theoretical frameworks and identify their effect, (ii) Relativistic corrections: accounting for relativistic corrections is needed to provide a more accurate description of electron motion (iii) Beyond classical treatment of light: Going beyond a classical description of the electromagnetic fields to better capture the quantum nature of light, and explore the merging of quantum optics and strong-field laser physics.

The Second Flavor of Hydrogen Atoms (SFHA) has been predicted theoretically and proven experimentally to exist for the 1st time – by analyzing atomic experiments related to the distribution of the linear momentum in the ground state of hydrogen atoms (J. Phys. B: At. Mol. Opt. Phys. 34 (2001), 2235). It was motivated by the extremely large discrepancy: the ratio of the experimental and previous theoretical results was up to tens of thousands. The essence of the theoretical discovery was that for the states of zero angular momentum (S-states), the so-called “singular” solution of the Dirac equation outside the atomic proton, which was usually disregarded, can be matched without any problem with the regular solution inside the proton with the allowance for the experimental fact that the charge density inside protons has the maximum at r = 0. So, for this second solution the wave function did not have a singularity at the origin and thus there was no reason to disregard this solution. This solution eliminated the above huge discrepancy between the theoretical and experimental results. No alternative explanation was ever provided. Later it was shown that the singular solution of the Dirac equation (with the allowance for the experimental charge distribution inside the proton) is legitimate for all discrete and continuum states of the zero angular momentum (Research in Astron. and Astrophys. 20 (2020) 109). Hydrogen atoms possessing only zero angular momentum states both in the discrete and continuous spectra constitute the second flavor of hydrogen atoms – called so by analogy with quarks, where, e.g., up and down quarks are called two flavors. The primary property of the second flavor of hydrogen atoms is that, since they have only the S-states, then according to the selection rules they cannot emit or absorb the electromagnetic radiation: they remain dark (except for the 21 cm spectral line). The 2nd experimental evidence of the existence of the SFHA was found by analyzing experiments on charge exchange of hydrogen atoms with incoming protons (Foundations 1 (2021) 265). The 3rd experimental proof of the existence of the SFHA was obtained by analyzing experiments on the excitation of n=2 states of atomic hydrogen by the electron impact (Foundations 2 (2022) 541). The 4th experimental proof of the existence of the SFHA was found by analyzing experiments on the excitation of the lowest triplet states of molecular hydrogen by the electron impact (Foundations 2 (2022) 697). In all of the latter three types of experiments, the allowance for the SFHA eliminated significant discrepancies with the previous theories: discrepancies up to the factor of two. Again, no alternative explanation was ever provided. Recently it was also shown (Symmetry 15 (2023) 1760) that the allowance for the SFHA eliminates the discrepancy between the proton charge radius rp ≈ 0.84 fm, deduced from the muonic hydrogen spectroscopy experiments and the value rp ≈ 0.88 fm extracted from the form factor experiments. There are also two kinds of the astrophysical evidence of the existence of the SFHA. The first one is related to the puzzling observation of the redshifted 21 cm spectral line from the early Universe where it was found that the absorption in this spectral line was about two times stronger than predicted by the standard cosmology (Nature 555 (2018) 67). The qualitative and quantitative explanation of this puzzle by using the SFHA made the latter a candidate for dark matter (Research in Astron. and Astrophys. 20 (2020) 109). The second astrophysical evidence of the existence of the SFHA related to recent perplexing observations that the distribution of dark matter in the Universe is smoother than predicted by Einstein’s gravitation (Monthly Not. Roy. Astron. Soc. 505 (2021) 4626). However, it turned out that this puzzle can be also explained qualitatively and quantitatively by using the SFHA (Research in Astron. and Astrophys. 21 (2021) 241). The theoretical discovery of the SFHA was based on the standard Dirac equation of quantum mechanics without any change of physical laws. This presentation should motivate further experiments of the above types – plus experiments on the formation of the molecular hydrogen ions by collision of protons with hydrogen atoms: the experiments that could yield yet another evidence of the existence of the SFHA.

In the past decades, time-resolved spectroscopy of photo-induced electron dynamics has provided unique tools for investigating a wide range of natural processes. This has opened up new avenues for understanding and manipulating matter on ultrafast timescales [1-4]. In the presentation, we will discuss new research directions aimed at expanding our current understanding of quantum control over photo-induced electron dynamics in matter. Specifically, the presentation will focus on two challenging aspects: the first involves exploring electron dynamics correlated with nuclear transitions, while the second centers around achieving strong-field photo-interactions at moderate, or even low, laser intensities. [1] Lambert, W. R., Felker, P. M. & Zewail, A. H. J. Chem. Phys. 75, 5958–5960 (1981). [2] Zewail, A. J. of Phys. Chem. A 104, 5660 (2000). [3] Hentschel, M. et al. Nature 414, 509–13 (2001). [4] Goulielmakis, E. et al. Nature 466, 739–743 (2010).

Although the neutrino is the most abundant known massive particle in the universe, many of its most basic properties are still unknown. This includes not only, e. g., the question whether neutrinos are Dirac or Majorana particles (in the latter case their own anti-particles), but even such a seemingly simple observable as its rest mass. In fact, while the neutrinos were supposed to be mass-less according to the standard model until the 1990s (and thus according to most text books at that time), the experimental confirmation of neutrino oscillations has proven that they possess a non-zero mass (and that mass eigenstates are not identical to the flavor states). The most accurate purely kinematical determination of an upper limit on the neutrino mass, more accurately the mass eigenstate corresponding to an electronic antineutrino, stems from tritium neutrino-mass experiments. In such experiments, the kinetic-energy distribution of the β electrons emitted in the radioactive decay of molecular tritium is analyzed. Very recently, the Karlsruhe neutrino experiment KATRIN has provided the so far lowest upper bound to the neutrino mass (mν < 0.9 eV c–2 at 90% confidence level), for the first time breaking the (“magic”) 1 eV c–2 threshold [1]. Since the energy released in the β decay is not only shared by the emitted (and measured) electron and the neutrino, but also by the remaining molecular ion 3HeT+, the analysis of an experiment like KATRIN requires the very precise knowledge with which probability which amount of energy is left in this ion, the so-called molecular final-state distribution. So far, this information is only available from theoretical calculations. After a brief introduction into the KATRIN experiment, this talk will discuss the challenges in precisely calculating the molecular final-states distribution and in quantifying the uncertainty of the finally extracted neutrino mass due to limitations in the calculation of this distribution. [1] Direct neutrino-mass measurement with sub-electronvolt sensitivity, Aker et al. (The KATRIN collaboration), Nature Physics 18, 160 (2022).

Tunnel ionization is a central phenomenon of strong-field physics and it is involved in essentially all intense laser-matter interactions. Strong-field ionization in the tunnelling regime takes place as discrete events which the strong-field approximation describes via saddle points that give rise to the well-established formalism of quantum orbits. In this work we consider the nonadiabatic above-threshold ionization of a 1D model atom by a bichromatic field. We pose the question of what happens to these ionization events (i.e., saddle points of the action) when we gradually replace a monochromatic beam with its second harmonic. Over this replacement, the number of ionization events per cycle of the fundamental changes from two to four. We therefore ask: Which ones are new? And, how did they get there? The transition comprises two interesting features. Firstly, we identify configurations in which the saddle points describing ionization events coalesce into a caustic and form a branch point. Here, continuous labelling of saddle points becomes impossible, the saddle point approximation breaks down, and novel uniform approximations have to be employed in its stead. More remarkably, we find that the new saddle points start contributing to the ionization yield long before the field changes sign. In other words, we present a tunnelling ionization event which occurs when the instantaneous electric field is zero, and hence at a time when there is no barrier. This results purely from a nonadiabatic picture of tunnelling, and presents a situation which cannot be modelled within the semi-classical and quasi-static pictures of optical tunnel ionization.

High harmonic generation (HHG) is a Nobel prize winning phenomenon in 2023 for physics, first observed in noble gases in 1987 [1], wherein a strongly driven electronic system may emit integer multiples of the driving laser field frequency in a nonlinear, nonperturbative up-conversion process. HHG offers an easily controllable source of ultra-short pulses of extreme Ultraviolet (XUV) photons to be used in subsequent attosecond techniques. These pulses, if infused with spin angular momentum via their state of polarisation, may permit the interrogation of chiral properties of matter. However, despite the versatile set of tools used for manipulating the emission, reliable control over the polarisation remains challenging. This is because, in the context of the three-step model [2], the ionised electron is driven away from the originating ion by an elliptically polarised driver – quenching that harmonic emission. One proposed technique to enable circularly polarised harmonic emission to incorporate the so-called “bicircular” field [3] in which two counter-rotating circularly polarised drivers of equal intensity and a 2:1 frequency mixing are combined to form a trefoil-shaped electric field. The critical initial attoseconds, the birth of a chemical reaction, may be observed via ultrafast spectroscopic techniques to monitor the electronic dynamics, excited-state populations, and photo-excitation. We explore the influence of the polarisation state and intensity of driving fields on single atoms, and simple (a)chiral molecules by means of time-dependent density functional theory (TDDFT). This is achieved by probing the electron response in the form of excited state populations, harmonic emission, and ionisation times, in the presence of linearly, elliptically, and bicircularly polarised driving laser fields of varying intensity. Through these investigations, we look through the attosecond window and observe the progression of chemical reactions with greater insight on how they may be manipulated and steered. References [1] A. McPherson, et al., J. Opt. Soc. Am. B 4, 595 (1987). [2] P. Corkum, Phys. Rev. Lett. 71 13, (1994) [3] E. Pisanty et al., Nat. Photon. 13, 569 (2019)

The ability to generate and manipulate photonic quantum states of light via effective photon-photon interactions holds numerous scientific and technological perspectives. In this talk, I will present different ideas to realize optical nonlinearities that are sufficiently strong to generate effective interactions between individual photons. To this end, we will consider emerging photon-photon interactions in mesoscopic regular structures of individual atoms and explore their effect on the dynamics of multi-photon quantum states. Potential applications and experimental prospects will also be discussed.

We investigate the ground state properties of N bosons with attractive zero-range interactions characterized by the scattering length a > 0 and confined to the surface of a sphere of radius R. We present the analytic solution of the problem for N = 2, mean-field analysis for N → ∞, and exact diffusion Monte-Carlo results for intermediate N. For finite N we observe a smooth crossover from the uniform state in the limit a/R ≫ 1 (weak attraction) to a localized state at small a/R (strong attraction). With increasing N this crossover narrows down to a discontinuous transition from the uniform state to a soliton of size ∼ R/√N. The two states are separated by an energy barrier, tunneling under which is exponentially suppressed at large N. The system behavior is marked by a peculiar competition between space-curvature effects and beyond-mean-field terms, both breaking the scaling invariance of a two-dimensional mean-field theory.

Direct experimental access to the most intriguing quantum phenomena is difficult due to their fragility to noise. To reach beyond the capabilities of very powerful numerical methods and approximations, leading theorists demand more than one-dimensional systems linked via interactions at long range. Trapped atomic ions are among the most promising candidates to provide a platform for experimental quantum simulations, featuring unique control and operational fidelities in one-dimensional systems of few-ions. We aim to extend this platform to larger size and dimensionality while preserving the unique controllability via trapping individual ions at individual sites of arrays - by trapping above surface traps with electrodes of the micrometer scale. We present results on coupling coherently excited ions within a basic-triangular array, observing transfer and interference. Recently, we achieved automated loading of single Mg+ ions within our array containing up to 13 trapping sites in a three-dimensional arrangement. Shuttling ions from a loading hub to multiple sites, we achieve a success rate >0.99 999 and demonstrate coherence preservation and coherent manipulation of superposition states of a hyperfine qubit during the process.

Ultracold dipolar systems, including atoms excited to Rydberg states and polar molecules, hold great potential for quantum simulation and computation. Rydberg atoms offer strong, long-range interactions, which enable the engineering of quantum entanglement and multi-qubit gates through the Rydberg blockade mechanism. Polar molecules also exhibit long-range interactions and possess multiple long-lived rotational states that can be coupled using microwave fields to achieve high-fidelity quantum operations. Optical tweezer arrays have enabled the trapping of both these systems, creating the possibility of a hybrid system that combines the advantages of both platforms. This hybrid system offers new capabilities, including non-destructive readout of the molecular state, cooling of molecules using Rydberg atoms, and photoassociation of giant polyatomic Rydberg molecules. In this talk, I will describe the first observation of Rydberg blockade due to the charge-dipole interaction between an atom and a polar molecule. Our experiment involves the creation of a hybrid system consisting of ultracold RbCs molecules and Rb atoms trapped in species-specific optical tweezers. We form weakly bound RbCs molecules by merging together optical tweezers containing Rb and Cs molecules. Strikingly, we find that weakly bound molecules can be produced without the widely used method of magnetoassociation across a Feshbach resonance but instead by exploiting an avoided crossing between atomic and molecular states as a function of tweezer separation. We transfer these weakly bound RbCs molecules to the rovibrational ground state using stimulated Raman adiabatic passage and achieve a one-way efficiency of 96.4(1)%. Finally, we observe blockade of the transition to the Rb(52s) Rydberg state due to the charge-dipole interaction with a RbCs molecule in the rovibrational ground state. The blockade we have observed provides a mechanism for conditional and non-destructive state readout of the molecule and opens up new research directions which I will briefly discuss.

When a Rydberg atom and a ground state “perturber” atom encounter one another in an ultracold gas, they interact via an oscillatory potential mediated by the scattering of the Rydberg electron off of the perturber. Sufficiently deep wells form in the oscillations of this potential that the perturber becomes trapped, binding the two atoms together into a molecule. This unusual mechanism is also able to bind several atoms together into trimers, tetramers, and even larger clusters. As the number of perturbers increases, it becomes impractical to describe this system within the framework of molecular physics, inviting a turn to the language of solid-state physics. In this talk, I will investigate how a dense environment of immobile perturbers modifies the spectrum of the Rydberg electron, which would otherwise be highly degenerate due to the SO(4) symmetry of the Kepler problem. I will show that this degeneracy leads to an exact mapping - familiar from supersymmetric quantum mechanics - between the perturbed Rydberg states and the states of a particle in a tight-binding lattice. The confluence of the infinite-ranged Coulomb potential and the zero-range electron-atom potentials leads to a plethora of possible lattice parameters. Using this mapping, I demonstrate how to realize a thermodynamic limit in the Rydberg electron and, as a result, the localization of the Rydberg electron in a disordered lattice.

Branched flow is a common phenomenon in wave dynamics, where deflections of particles, rays or waves due to random fluctuations in the surrounding medium eventually lead to a chaotic arborescent pattern. Branched flow has been reported in a remarkable variety of physical systems covering, e.g., pulsar-generated microwaves, ocean waves, beams of light and two-dimensional (2D) electron gas [1,2]. Here we report unexpected branched flow in electronic wave-packet dynamics of 2D periodic lattices without any randomized disorder [2]. The branching appears at wavelengths shorter than the typical length scale of the ordered periodic structure and for energies above the potential barrier. The branching is qualitatively similar in classical and quantum description. The strongest branches remain stable indefinitely and may create linear dynamical channels. In these channels the waves are not confined directly by potential walls as electrons in ordinary wires but rather, indirectly and more subtly by dynamical stability. We discuss the relevance of these "superwires" in 2D materials, especially in moiré superlattices, where the wavelengths are typically small compared to the lattice constant - thus creating an ideal system for branched flow. [1] For a review, see E. J. Heller, R. Fleischmann, and T. Kramer, Physics Today 74, 12, 44 (2021). [2] A. Daza, Eric J. Heller, A. M. Graf, and E. Rasanen, Proc. Nat. Acad. Sci. 118 (40), e2110285118 (2021).

Alkali dimers, such as Na$_2$ and K$_2$, residing on the surface of helium nanodroplets, are set into rotation and vibration, through the dynamic Stark effect, by a moderately intense femtosecond pump pulse. Coulomb explosion of the dimers [1,2], induced by an intense, delayed femtosecond probe pulse, is used to rec-ord the time-dependent nuclear motion. Concerning rotation, the measured alignment dynamics show a distinct, periodic structure that differs qualitatively from the well-known alignment dynamics of linear molecules in either the gas phase or dissolved in liquid helium [3]. Comparison to calculations using the time-dependent rotational Schrödinger equation shows that the deviation is due to the alignment-dependent interaction between the dimer and the droplet surface. Notably, this interaction pins dimers in the lowest-lying triplet state to the surface plane and as a result, the alignment dynamics becomes essentially identical to that of an elementary 2D quantum rotor [4]. Concerning vibration, the Coulomb explosion probe method enables us to measure the distribution of internuclear distances as a function of time. For K2, we observe a distinct oscillatory pattern caused by a two-state vibrational wave packet in the initial electronic state of the dimer. The wave packet is imaged for more than 250 vibrational periods with a precision better than 0.02~{\AA} on its central position and a resolution better than 1~{\AA} of its shape. Unlike the rotational motion, vibration of the dimers is essentially unaffected by the presence of the He droplet. References [1] H. H. Kristensen, L. Kranabetter, C. A. Schouder, C. Stapper, J. Arlt, M. Mudrich , and H. Stapel-feldt, Phys. Rev. Lett. 128, 093201 (2022). [2] H. H. Kristensen, L. Kranabetter, C. A. Schouder, J. Arlt, F. Jensen and H. Stapelfeldt, Phys. Rev. A 107, 023104 (2023). [3] A. S. Chatterley et al., Phys. Rev. Lett. 125, 013001 (2020). [4] L. Kranabetter et al., Phys. Rev. Lett. 131, 053201 (2023).

We explore the capabilities offered by attosecond extreme ultraviolet and x-ray pulses that can now be generated by free-electron lasers and high-order harmonic generation sources for probing photon-induced electron dynamics in molecules [1]. We theoretically analyze how spatial and temporal dependence of charge migration in a pentacene molecule can be followed by means of time-resolved photoelectron microscopy on the attosecond timescale. Performing the analysis, we accurately take into account that an attosecond probe pulse leads to considerable spectral broadening. We demonstrate that the excited-state dynamics of a neutral pentacene molecule in real space map onto unique features of photoelectron momentum maps. In the talk, I will also discuss capabilities of femtosecond photoelectron momentum microscopy of molecules adsorbed on surfaces that is already available. We reveal details of excited-state dynamics in a pentacene bilayer adsorbed on a silver substrate [2] and Copper phthalocyanine (CuPc) adsorbed on Titanium Diselenide (TiSe2) [3] from simulated photoelectron momentum maps compared to experimental data obtained at the Free-Electron Laser FLASH in Hamburg. [1] M. Reuner and D. Popova-Gorelova, Attosecond imaging of photo-induced dynamics in molecules using time-resolved photoelectron momentum microscopy, Phys. Rev. A 107, 023101 (2023) [2] K. Baumgärtner, M. Reuner, C. Metzger, D. Kutnyakhov, M. Heber, F. Pressacco, C.H. Min, T.R.F. Peixoto, M. Reiser, C. Kim, W. Lu, R. Shayduk, W. M. Izquierdo, G. Brenner, F. Roth, A. Schöll, S. Molodtsov, W. Wurth, F. Reinert, A. Madsen, D. Popova-Gorelova, and M. Scholz, "Ultrafast molecular orbital tomography of a pentacene thin film using timeresolved momentum microscopy at a free-electron laser", Nature Communications 13, 2741 (2022) [3] K. Baumgärtner, M. Nozaki, M. Reuner, N. Wind, M. Haniuda, C. Metzger, M. Heber,D. Kutnyakhov, F. Pressacco, L. Wenthaus, K. Hara, C.-H. Min, M. Beye, F. Reinert, F. Roth., S. Mahatha, A. Madsen, T. Wehling, K. Niki, D. Popova-Gorelova, K. Rossnagel, M. Scholz, Multiplex movie of concerted rotational motion of molecules on a 2D quantum materials, arXiv:2305.07773

I discuss quantitative bounds to propagation of quantum information in the Universe, which are formalized as constraints to quantum correlations in many-body systems. There exist quantum laws that dictate how much and what kind of information about quantum particles, conveyed by Entanglement and other kinds of quantum correlations, is accessible to imperfect, limited observers. While classical information (the outcome of a measurement) can be simultaneously shared by an arbitrary number of observers, broadcasting quantum information (the wavefunction of a system) is inevitably prohibited. These results elucidate how the emergence of a classical macroscopic reality is a consequence of quantum laws.

The high-fidelity storage of quantum information is crucial for quantum computation and communication. Many experimental platforms for these applications exhibit highly biased noise, with good resilience to spin depolarization undermined by high dephasing rates. In this work, we demonstrate that the memory performance of a noise-biased trapped-ion-qubit memory can be greatly improved by incorporating error correction of dephasing errors through teleportation of the information between two repetition codes written on a pair of qubit registers in the same trap. While the technical requirements of error correction are often considerable, we show that our protocol can be achieved with a single global entangling phase gate of remarkably low fidelity, leveraging the fact that the gate errors are also dominated by dephasing-type processes. By rebalancing the logical spin-flip and dephasing error rates, we show that for realistic parameters our memory can exhibit error rates up to two orders of magnitude lower than the unprotected physical qubits, thus providing a useful means of improving memory performance in trapped-ion systems where field-insensitive qubits are not available.

Whilst quantum computations can be viewed as abstract rotations on high dimensional Bloch spheres as physicists we must remember that these computations are still instantiated by physical processes with interesting thermodynamics. In this talk, I will describe how the computational content of a basic class of quantum algorithms has different thermodynamics depending on the computation being carried out. The deterministic quantum computation with one-clean-qubit model (DQC1) complexity class, or power-of-one qubit model, is examined as an open quantum system. We study the dynamics of a register of qubits carrying out a DQC1 algorithm and show that, for any algorithm in the complexity class, the evolution of the logical qubit can be described as an open quantum system undergoing a dynamics which is unital. Unital quantum channels respect the Tasaki-Crooks fluctuation theorem, and we demonstrate how this is captured by the thermodynamics of the logical qubit. As an application, we investigate the equilibrium and nonequilibrium thermodynamics of the DQC1 trace estimation algorithm. We show that different computational inputs, i.e., different traces being estimated, lead to different energetic exchanges across the register of qubits and that the temperature of the logical qubit impacts the magnitude of fluctuations experienced and quality of the algorithm.

Interference phenomena are often claimed to resist classical explanation. However, such claims are undermined by the fact that the specific aspects of the phenomenology upon which they are based can in fact be reproduced in a noncontextual ontological model [Catani et al., Quantum 7, 1119 (2023)]. This raises the question of what other aspects of the phenomenology of interference do in fact resist classical explanation. We answer this question by demonstrating that the most basic quantum wave-particle duality relation, which expresses the precise tradeoff between path distinguishability and fringe visibility, cannot be reproduced in any noncontextual model. We do this by showing that it is a specific type of uncertainty relation and then leveraging a recent result establishing that noncontextuality restricts the functional form of this uncertainty relation [Catani et al., Phys. Rev. Lett. 129, 240401 (2022)]. Finally, we discuss what sorts of interferometric experiment can demonstrate contextuality via the wave-particle duality relation.

The first part of the talk will circle around two-electron correlations we observe in ultrafast photoemission from sharp metal needle tips. We observe a large energy gap of 3 eV resulting from Coulomb repulsion of two electrons emitted into the small space time volume available. Furthermore, we will show how non-classical states of light, in particular bright squeezed vacuum light, changes the emission statistics of the emitted electrons. In the second part of the talk, we will give an update on strongly driven electrons in graphene. Based on our observation of Landau-Zener-Stückelberg-Majorana interferometry (repeated coherent Landau-Zener transitions between valence and conduction band), we can now go one step further and use this type of interferometry to read out band structure parameters that the driven electrons are sensitive to, most prominently the electron's Fermi velocity, i.e., the slope of the bands.

We study helium two-photon XUV+IR ionization through intermediate bound states. Using spectrally- and angularly-resolved attosecond electron interferometry, we characterize the complex-valued proba- bility amplitude towards the photoelectron quantum state. This allows reconstructing in space, time and energy the complete formation of the photoionized wavepacket.

We are interested in investigating theoretically ion-atom ionizing collisions. For this purpose, we have developed a parabolic quasi-Sturmian approach and applied it here to study the single ionization of helium by intermediate and high energy protons. The aim is to further understand this fundamental process by looking at fully differential cross sections for which measurements are available (on a relative scale for 1 MeV protons, and on an absolute scale for 75 keV protons). In the framework of our approach, the proton-electron interaction is treated as a perturbation, and the transition amplitude is extracted directly from the asymptotic behavior of the solution of an inhomogeneous Schrödinger equation for the Coulomb three-body system (e-, He+, p+). The driven equation is solved numerically by expanding in convolutions of quasi-Sturmians for the two-body (p+,He+) and (e-,He+) systems. Our calculated fully differential cross sections converge pretty fast as the number of terms in the expansions is increased, and are in reasonable agreement with experimental data and other theoretical results.

Mutual neutralization (MN) recently gained an increased focus, following the discovery of its effect on the late-type stars’ apparent metallicity. This led to a large number of theoretical studies dealing with $X^+$ + H$^-$ mutual neutralization, focusing on collision energies below 10 000 K, i.e., 1 eV. In order to confirm those calculations, recent measurements were also performed, but due to technical limitations of the merged beam setups both at UCLouvain and at DESIREE, the mass ratio between the anion and the cation was limited. Therefore $X^+$ + D$^-$ MN reactions were used as a proxy for $X^+$ + H$^-$, leading to difficulties in the comparison between theoretical and experimental results, as the isotope effect might have to be taken into account. The branching ratios of the He$^+$ + H−$^-$/D$^-$ mutual neutralization at a collision energy below 25 meV are correctly reproduced using the Landau-Zener model applied to potential energy curves computed by an anion-centered asymptotic model. The analyticity of both models allows getting a deeper insight into the reaction. It allows defining a low collision energy regime for the mutual neutralization at which the collision energy no longer affects the branching ratios. Using those models, we explain how heavier isotopes favor the production of higher excited states.

Vibronic coupling of electronic states leads to interesting spectroscopic phenomena represented for example by Jahn-Teller or Renner-Teller effects. It is well studied in the case of bound electronic states but much less is known for the metastable states. Recently we have studied a class of models for the vibronic dynamics of molecular anions created in the electron molecule collisions. We developed the numerical methods to treat vibronic interaction of several electronic states in continuum with up to four vibrational degrees of freedom. In series of three papers in Phys. Rev. Lett and Phys. Rev. A journals we applied this methodology to electron collisions with CO$_2$ molecule and explained the shape of high resolution two-dimensional electron energy loss spectra for vibrational excitation of this molecule by electron impact. The model includes Renner-Teller coupled dublet of $\Pi$ symmetry coupled vibronically to virtual state of $\Sigma$ symmetry.

My personal contribution to the research activities of the Pitaevskii Center on Bose-Einstein Condensation in Trento concerns the study of non-equilibrium states of ultracold gases at finite temperature. In this talk, I will provide an overview of the results obtained with the Stochastic Gross-Pitaevskii Equation for the behavior of temperature quenched Bose gases across the BEC transition in 3D and BKT transistion in 2D, as well as in the 2D-3D dimensional crossover. I will also briefly discuss the propagation of sound in the same systems and the extension to multicomponent condensate.

Recent technological progress has allowed for reliable coherent coupling of engineered few-photon states of non-classical light to individual molecules, as well as subsequent detection of the resulting photonic states in time-continuous as well as time-nonlocal spatiotemporal modal bases. This flow of quantum information via flying photonic states is expected to form the basis of emergent technologies, including quantum internet, quantum computing architectures, as well as quantum-enhanced microscopy and imaging. In this talk, I will turn to a more conventional application of light-matter interaction which is the spectroscopy of complex molecules; mediated by non-classical light and quantum nature of interaction, often referred to as quantum spectroscopy. I will present a quantum information-theoretic model for single-molecule quantum spectroscopy developed by us recently [1] that recasts spectroscopy as a quantum estimation problem, allowing us to evaluate fundamental quantum limits on the spectroscopic information that can be extracted from the setup. In particular, I will focus on the estimation of the interaction strength between the pulse and a two-level atom, equivalent to the estimation of the dipole moment. Our analysis of single-photon pulses highlights the interplay between the information gained from the absorption of the photon by the atom as measured in absorption spectroscopy, and the perturbation to the temporal mode of the photon due to spontaneous emission measured in fluorescence techniques. I will also show that for a vast class of biphoton states, time-frequency entanglement between the signal mode interacting with the atom and the idler mode provides no fundamental advantage and the same precision can be obtained with a separable input [2]. I will conclude by alluding to how we can bring to bear generalised open systems as well as tensor network techniques to evaluate fundamental limits in more involved spectroscopic scenarios, including multidimensional quantum spectroscopies of complex molecules [3]. [1] Albarelli, F., Bisketzi, E., Khan, A., & Datta, A. (2023). Fundamental limits of pulsed quantum light spectroscopy: Dipole moment estimation. Physical Review A, 107(6), 062601. [2] Khan, A., Albarelli, F., & Datta, A. (2023). Role of entanglement in single-molecule pulsed biphoton spectroscopy. arXiv:2307.02204v1. [3] Khan, A., Albarelli, F., & Datta, A. (2023) General formalism for Hamiltonian parameter estimation using pulsed quantum light spectroscopy. In preparation.

We investigate the contribution of negative-energy states on the $E2$ and $M1$ polarizabilities for the $^1S_0$ and $^3P_0$ states of the Sr clock and find that they dominate the $E2$-$M1$ polarizability difference. Our calculated result is $-7.74(3.92)\times10^{-5}$ a.u., which resolves the sign inconsistency between theory and experiment. Additionally, we extend our method to other optical clocks, confirming the crucial role of negative-energy states in determining the $M1$ polarizability.

A polaron is a quasiparticle created when an impurity strongly interacting with the environment in which it is embedded, dresses itself with the environmental excitations generated by such interactions. In our lab we investigate polaron physics in an ultracold system consisting of a Fermi sea of 6Li, in which either bosonic 41K or fermionic 40K impurities are embedded. In this talk I will present our recent results regarding impurities in strongly interacting imbalanced Fermi-Bose and Fermi-Fermi mixtures. We first investigated the Fermi-Bose mixture in the two cases of thermal impurities and of impurities forming a partial Bose-Einstein condensate [1]. In the thermal case, and for the thermal part of the partially condensed impurity cloud, we found good agreement between the recorded energy spectrum and the theory for isolated polarons. Moreover, we observed hints on mediated interactions between polarons for increasing impurity concentration. In the case of condensed impurities, we observed the emergence of an additional branch in the impurity energy spectrum, which can be explained by the presence of Bose polarons created by fermionic Li impurities in the K condensate. In our system we thus observed the coexistence of Fermi and Bose polarons. We then investigated more closely the effect of increasing the impurity concentration on the impurity energy spectrum and observed, for the first time, mediated interactions between polarons [2]. In order to understand how this effect is affected by different quantum statistics, we employed either bosonic or fermionic thermal impurities. In the single impurity limit, as expected, we did not observe any difference in the respective energy spectra. For higher impurity concentration we observed the experimentally challenging weak effect of the mediated polaron-polaron interaction. In particular, we detected the influence of the quantum statistics, which manifests itself as a sign inversion in the mediated polaron-polaron interaction. [1] I. Fritsche, C. Baroni, E. Dobler, E. Kirilov, B. Huang, R. Grimm, G. M. Bruun, and P. Massignan, Stability and breakdown of Fermi polarons in a strongly interacting Fermi-Bose mixture, Phys. Rev. A 103, 053314 (2021) https://link.aps.org/doi/10.1103/PhysRevA.103.053314; [2] C. Baroni, B. Huang, I. Fritsche, E. Dobler, G. Anich, E. Kirilov, R. Grimm M. Bastarrachea-Magnani, P. Massignan, and G. M. Bruun, Mediated interactions between Fermi polarons and the role of impurity quantum statistics, Nat. Phys. (2023) https://doi.org/10.1038/s41567-023-02248-4.

In the last few years, there have been remarkable experimental observations of the elusive supersolid state of matter—a state that exhibits properties of superfluidity and solidity simultaneously. Particular success has come from a system of ultracold atoms with intrinsic long-range dipolar interactions, cooled into the Bose condensed phase. These dipolar supersolids are formed with an array of high-density superfluid droplets and a superfluid bridge, creating the necessary crystal structure and global superfluidity. In this talk, I will discuss recent theoretical results on the nature of supersolidity in binary dipolar gases, where the inter-component interactions give rise to a plethora of new phases and provide a unique avenue in the hunt for long-lived supersolid states.

We explore the boundary time-crystal transition at the level of quantum trajectories which result from continuous monitoring. This is motivated by recent experiments [G. Ferioli, A. Glicenstein, I. Ferrier-Barbut, and A. Browaeys, Nat. Phys. 19, 1345 (2023)] realizing this many-body system and which allow one in principle to gain in situ information on its nonequilibrium dynamics. We find that the photon count signal as well as the homodyne current allow one to identify and characterize critical behavior at the time-crystal phase transition. In the time-crystal phase these quantities display persistent oscillations, resolvable in finite systems and in individual realizations. At the transition point the dynamics of the emission signals feature intermittent strong fluctuations, which can be understood through a simple nonlinear phase model. We furthermore show that the time-integrated homodyne current can serve as a useful dynamical order parameter. From this perspective the time crystal can be viewed as a state of matter in which different oscillation patterns coexist. Finally, we analyze the potential of these nonequilibrium phases for parameter estimation via continuous monitoring schemes. We discuss the theoretical bounds on sensitivity and present different protocols tailored to each phase.