Atomic Summer Camp 2021

The one-body reduced density matrix (1RDM) plays a fundamental role in describing and predicting quantum features of ultra-cold bosonic systems, such as Bose-Einstein condensation or quantum correlations. The recently proposed reduced density matrix functional theory for bosonic ground states [1] establishes the existence of a universal functional that recovers quantum correlations exactly. Based on a novel decomposition of the 1RDM, we have developed a method to design reliable approximations for such universal functionals [2]: our results suggest that for translational invariant systems the constrained search approach of functional theories can be transformed into an unconstrained problem through a rather simple parametrization of an Euclidian space. This simplification of the search approach allows us to use standard machine-learning methods to perform a quite efficient computation of universal functional for bosonic systems, its derivative and the ground-state energy. For the Bose-Hubbard model, we present a comparison between our approach and Quantum Monte Carlo. [1] CL Benavides-Riveros et al., Phys. Rev. Lett. 124, 180603 (2020). [2] J Schmidt, M Fadel and CL Benavides-Riveros, arXiv:2104.03208.

We show that $k$-point correlation measurements on output of a non-interacting, multimode random unitary allow to quantify the $k$-particle coherence of $N\geq k$ identical (bosonic or fermionic) particles. We establish a strict monotonic relationship between $k$-particle coherence, the interference contrast in the experimentally accessible counting statistics, and the degree of the particles' mutual distinguishability, as controlled by their internal degrees of freedom, given separable many-particle input states. Non-separability on input can be unveiled by comparison of correlation measurements of different orders.

Identifying a quantum system as chaotic simplifies its description, since some of its properties display the universal statistical features of random matrices. Nevertheless, even a fully chaotic system is expected to show fingerprints of its own specific nature, which distinguish it from random matrix statistics. We investigate energy statistics and eigenstate fractal dimensions for the chaotic phase of the Bose-Hubbard Hamiltonian in relation to Gaussian orthogonal random matrices and to the bosonic embedded random-matrix ensemble. The latter, in contrast to the Gaussian ensemble, mirrors the few-body nature of interactions and is therefore expected to better approximate actual quantum many-particle systems. We show how the onset of chaos is clearly signalled by the asymmetry of the distribution of the eigenstates’ fractal dimensions. When chaos has fully developed, mean and eigenstate-to-eigenstate fluctuations of the fractal dimensions of Bose-Hubbard eigenstates show clear fingerprints of ergodicity and are well described by the two random-matrix ensembles. Despite such agreement, the full distributions of the fractal dimensions for these three models are ever more distinguishable from each other as Hilbert space grows.

The hydrogen atom possesses an infinite number of bound Rydberg states. As Pauli revealed nearly a century ago, each Rydberg level is highly degenerate due to the conservation of the Runge-Lenz vector. The degenerate energy levels can be split apart by immersing an array of neutral ground state atoms within the Rydberg wave function. In this talk I will show how we solve for the perturbed Rydberg spectrum by reformulating the Hamiltonian as a tight-binding Hamiltonian describing the hopping of a particle from atom to atom. The effective tunneling rates and on-site energies are determined by the interplay of the zero-range electron-atom interactions and the long-ranged Coulomb interaction. We utilize this direct parallel with a solid-state system to study the possibility that the Rydberg electron undergoes Anderson localization. The fundamental properties of the Rydberg atom make it possible to take an appropriate thermodynamic limit by going to higher Rydberg levels. Furthermore, we exploit the variety of possible hoppings and types of disorder to probe a range of novel phenomena.

Paraphrasing the known novel "Alice's adventures in wonderland" by Lewis Carroll, this talk will focus on various motifs that arise when Rydberg atoms are placed nearby neutral ones. More specifically, the aim is to illustrate and highlight the importance of Rydberg electrons which in such systems can play a multifaceted role. The first half of the talk focuses on a novel class of molecular states (dressed ion-pair states) that combine features of the ultralong-range Rydberg molecules, i.e. formed by a Rydberg atom and a ground state atom, and heavy Rydberg states, i.e. the molecular analogue of a Rydberg atom. A connection between these two molecular types is achieved by the dressed ion-pair model introducing a new perspective where ultralong-range Rydberg molecules can be thought of as a positive ion interacting with an anion of fractional charge. The reminder of the talk will focus on the possibility to use Rydberg electrons as probes. For example, bound Rydberg electrons have been used as a probe to extract the atom pair correlations in systems where a Rydberg atom is coupled to a Bose-Einstein condensate. Here, we explore the option to utilize an unbound Rydberg electron a structural tool in order to extract information about the details of Rydberg molecules, such as bond length, electron-atom phase shifts etc. Namely, we study of photoionization process where a Rydberg molecule is used as an initial state. The corresponding photoionization spectra posses a fine structure at low energies which results from the interference of the direct pathway of the photoelectron to the continuum with an indirect one. The indirect pathway accounts for scattering processes between the photoelectron and the neighboring constituent of the Rydberg molecule, i.e. the neutral atom. To illustrate the application of this interference phenomenon, we propose a scheme to extract low-energy electron-atom scattering phase shifts utilizing an unbound Rydberg electron.

The two-step mechanism of dissociative ionization has proven to be a useful and an easy-to-handle model for many theoretical simulations. In my talk I present experimental data and theoretical calculations that suggest a different model for photoionization with femtolaser pulses with a central wavelength of 400nm. When singly-excited Rydberg states are resonantly excited during the process of dissociation, the dominat mechanism might follow a direct transition from the Rydberg state to the first excited 2p$\sigma_u$ state of the H2+ cation without any involvement of the 1s$\sigma_g$ ground state. This talk provides observations that confirm this mechanism.

Aggregates of fluorescent dye molecules on dielectric surfaces are of great interest for various technological applications. Due to strong interactions between the molecular transition dipoles, the excitonic eigenstates are coherently delocalized over many molecules [1]. These eigenstates and the corresponding optical transitions are determined by the molecular arrangement. We discuss the dependence of dark and bright states on the spatial shape of the electromagnetic radiation used to probe the aggregate [2]. Strongly inhomogeneous fields can be generated via radiation from the apex of a metallic tip, which allows also scanning across the aggregate. Resulting spatially resolved spectra provide extensive information on the eigenenergies and wave functions [2, 3]. We find that these delocalized eigenfunctions can be directly reconstructed using convolutional neural networks [3]. [1] A. Eisfeld, C. Marquardt, A. Paulheim, and M. Sokolowski, Phys. Rev. Lett. 119, 097402 (2017). [2] X. Gao and A. Eisfeld, J. Phys. Chem. Lett. 9, 6003 (2018). [3] F. Zheng, X. Gao and A. Eisfeld, Phys. Rev. Lett. 123, 163202 (2019).

Free-electron lasers (FELs) provide intense bursts of XUV light on the femtosecond timescale. With these pulses it is possible to directly and resonantly excite atom-specific transitions, and doing so at a sufficiently high intensity (on the order of 10E14 to 10E15 W/cm^2), which leads to the onset of strong-coupling effects within these transitions. Hereby one can also make use of the partially coherent stochastic nature of the FEL pulses, which are generated through the process of self-amplified spontaneous emission. The characteristic of strongly coupled transitions is imprinted on the measured absorption spectra of the FEL pulses after being transmitted through a moderately dense atomic medium. In this talk I will discuss, for the example of an autoionizing two-electron transition in helium, how the modified absorption spectra can be understood and connect to transient energy shifts of dressed states, as well as to the population dynamics of this two-electron system.

The ultrafast structural dynamics of water following inner-shell ionization are a crucial issue in high-energy radiation chemistry in liquid media. We have exposed isolated water molecules to a short x-ray pulse from a free-electron laser, the EuXFEL in Hamburg, Germany, and detected momenta of all produced ions in coincidence. By combining experimental results and theoretical modeling, we can image dissociation dynamics of individual molecules in unprecedented detail. We reveal significant molecular structural dynamics in H2O2+, such as asymmetric deformation and bond-angle opening, leading to two-body or three-body fragmentation on a timescale of a few femtoseconds. We thus record a molecular movie of a water molecule, which reveals dynamical patterns relevant as initiating steps of subsequent radiation damage processes in a liquid environment.

In this talk I will present preliminary results of CH2I2 molecular dissociation following ionisation by intense XUV radiation. In iodine, the resonant ionisation of an inner-shell 4d electron deposits a large amount of energy in the molecule, precipitating Auger decay. The fragments of the dissociating molecules are measured in coincidence using a reaction microscope at FLASH2 in Hamburg. Observing the momenta of all ions from two-body or thee-body break ups allows to reveal different dissociation channels. By employing a pump-probe technique, irradiating the molecule with two XUV pulses separated by a defined and variable time delay, the intramolecular dynamics after ionisation become accessible.

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We have successfully added an XUV photon spectrometer to the reaction microscope (REMI) installed at the Free-Electron-Laser in Hamburg (FLASH). We have measured the recoil-ion momentum distribution. Simultaneously we recorded the incoming FEL spectra on a shot-to-shot basis at up to 250 kHz repetition rate in burst mode. I will present first results of a recent experiment on two-photon double-ionization of Helium at FEL photon energies between 39.7eV and 57.0eV. In the future, we plan to include the spectral information to improve resolution of event-based detection in the REMI.

The phenomenon of superfluorescence – or collective emission of fluorescent light by atomic systems – was predicted by Dicke [1] in 1954 and was experimentally observed in the optical regime in the 70s [2]. In this process, the atoms are initially prepared in an incoherent excited state, which triggers the spontaneous emission. Under certain conditions, the produced field does not simply escape the system but makes a considerable impact on the neighboring atoms thus coupling their phases. This synchronization of atoms leads to a collective emission of a highly intensive superfluorescence pulse [3]. Recent experiments demonstrate that the same phenomenon can be observed in the x-ray domain [4,5]. New conditions require a certain revision of rather qualitative models used before for the analysis of this fine effect. A correct description of incoherent processes, as well as accounting for realistic atomic-level structure and field propagation are crucial for a quantitative description of x-ray superfluorescence, which challenges previously existing methodologies. We developed a novel formalism based on rigorous derivations from the first principles, capable of a detailed characterization of the superfluorescence process under realistic conditions without any significant loss of both accuracy and clarity. The resulting stochastic differential equations resemble traditional Maxwell-Block equations and in addition accommodate noise terms. The obtained equations enable an intuitive illustration of the process: noise terms describe the influence of the spontaneous emission, the traditional Maxwell-Block part is responsible for its propagation and interaction with the other parts of the system. It turns out that the developed formalism -- where noise terms play the role of the quantum corrections of classical physics -- can be used not only in the problem of the superfluorescence but believed to be extended to many other light-matter interaction problems. As an example of the application of the derived stochastic differential equations, the description of superfluorescence in systems with a size smaller than the radiation wavelength will be demonstrated. Even for this model system, the well-known Dicke model leads to a strong dependence of the number of involved equations on the number of atoms [3]. In contrast, our formalism does not suffer from it and can be successfully used for a description of both spontaneous emission and superfluorescence, covering two qualitatively different processes on the same footing. [1] Dicke, R. H. (1954). Coherence in Spontaneous Radiation Processes. Physical Review, 93(1), 99–110. https://doi.org/10.1103/physrev.93.99 [2] Skribanowitz, N., Herman, I. P., MacGillivray, J. C., & Feld, M. S. (1973). Observation of Dicke Superradiance in Optically Pumped HF Gas. Physical Review Letters, 30(8), 309–312. https://doi.org/10.1103/physrevlett.30.309 [3] Gross, M., & Haroche, S. (1982). Superradiance: An essay on the theory of collective spontaneous emission. Physics Reports, 93(5), 301–396. https://doi.org/10.1016/0370-1573(82)90102-8 [4] Rohringer, N., Ryan, D., London, R. et al. Atomic inner-shell X-ray laser at 1.46 nanometres pumped by an X-ray free-electron laser. Nature 481, 488–491 (2012). https://doi.org/10.1038/nature10721 [5] Mercadier, L., et al. (2019). Evidence of Extreme Ultraviolet Superfluorescence in Xenon. Physical Review Letters, 123(2). https://doi.org/10.1103/physrevlett.123.023201

Quantum physics is one of the most important disciplines of modern physics and a world without its results is hardly imaginable. It touches a wide spectrum from the fundamental understanding such as the hydrogen atom or quantum teleportation up to daily applications, for instance, microprocessor or magnetic resonance imaging. A crucial result of the quantised description of the electromagnetic field is the zero-point energy, which for instance results in an improved theoretical description of the heuristically introduced van der Waals force between neutral particles, or the Casimir effect. These forces belong to the dispersion interaction and have been well studied in theory and experiments during recent years. However, most of these investigations occurred under well-controlled laboratory conditions and the dispersion forces dramatically change their behaviour when considering an environment. In this talk, we will introduce the dispersion forces and demonstrate the impact of environmental media. To illustrate such impacts, we will discuss the van der Waals interaction between dissolved particles and the fluorescence spectroscopy of molecules near noble gas clusters.

TBA

Information will follow

We present kinematically complete experiments on photodissociation of H 2 using a combination of XUV and IR pulses and demonstrate how the interplay of entangled two-electron states leads to symmetry breaking in the emission direction of the photoelectron with respect to the ejected proton. We show that the symmetry breaking can be controlled with the delay between two pulses on a sub-femtosecond time scale. Our experimental results are supported by a semiclassical simulation based on the WKB approximation.

Preliminary results of the fully differential cross section of lithium after single ionization by 110 eV electron impact will be presented. Features in the FDCS like considerable emission in perpendicular plane shows the higher order effects in the projectile-target interactions. Furthermore, we motivate for the ongoing experiments of double ionization studies producing one K- and one L-shell vacancy.

Electron correlation plays a fundamental role in light-matter interaction. To gain new insights into the role of the initial state for the ionization process, we conducted a two-color extreme ultraviolet (XUV)-infrared (IR) experiment using a reaction microscope (ReMi) to study IR strong-field ionization out of selectively prepared doubly excited states in helium in the XUV energy region between 59 eV and 80 eV, with XUV light provided by the free-electron laser in Hamburg FLASH. Both single- and double-ionization have been observed and the impact of different ionization mechanisms will be discussed, also in comparison with model calculations.

Theoretical and experimental studies on intense laser atom interaction have drawn many interests over the past few decades. We consider strong-field tunnel ionization and explore two problems dealing with the ionized electron’s dynamics in the presence of infrared, high-intensity, elliptically-polarized laser pulses applied to an attoclock setup. In the first problem, we obtain the electron’s dynamics from a static potential, describing the complicated field-driven dynamics by a simple time-independent Hamiltonian. For the second problem, we set up an analytical expression for the ‘attoclock offset angle’. We use the time-dependent Kramers-Henneberger (KH) potential, which provides an exact description of the electron’s trajectory, and show how some approximations within the KH potential lead to the static potential and the analytical offset angle. We also elucidate good agreement of our theory with the numerical results obtained from classical equations of motions. Finally, the comparison with available experimental data has led to interestingly new tunnel-exit conditions different from the conventional models.

While electron-electron interactions play a fundamental role in any atom beyond hydrogen, they also govern molecular structure and reactivity. We introduce and experimentally demonstrate a general concept to control multi-electron interaction by intense, ultrashort laser fields. In particular, strong coupling to excited states allows to modify the effective exchange energy by infrared (IR)-induced valence orbital mixing. For a proof-of-principle, we focus on the sulfur hexafluoride molecule SF$_6$, considering the coupling of a sulfur 2p core hole with a valence excited electron, using a combination of soft x-ray and IR laser pulses. The IR laser intensity represents a control knob to tune the effective exchange interaction energy, resulting in a characteristic change in the spin-orbit-split oscillator strength ratio that is directly quantified in the experiment. Such direct control of effective electronic interactions and correlation is a significant step towards laser-directed chemistry on the fundamental electronic level with single-atomic site selectivity.

We consider the process of tunnel ionization of atoms driven by circularly polarized (CP) pulses. An intuitive semiclassical picture of tunnel ionization by a static laser field is an electron ionizing through the potential barrier induced by the laser field with constant energy, referred to as the adiabatic ionization. When the laser field alternates in time, such as it is the case for CP pulses, the energy of the electron changes in time, and at the tunnel exit, it is distributed in a range of energy on the order of the ponderomotive energy of the laser. The adiabatic picture no longer holds, and the ionization process is referred to as nonadiabatic. Extensive theoretical and experimental studies are performed in the attosecond community to probe and understand these nonadiabatic effects in photoionization. Our goal is to understand nonadiabatic processes in CP pulses using electron trajectories in the combined laser and Coulomb fields. We map the electron dynamics in a frame which rotates with the laser field, referred to as the rotating frame (RF). Our results show that in the RF, counter-intuitively, the energy of the electron is constant during tunnel ionization, and as a consequence follows the picture of adiabatic ionization. This allows us to understand and predict, for instance, the role played by ring currents in atoms and the shape of the laser envelope, and to shed light on classical-quantum correspondence. J. Dubois, U. Saalmann, J.-M. Rost.

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The coupling between a moving atom and a laser beam can strongly modify the dynamical properties of the atom. A complete description of the state of the atom must take both its center-of-mass as well as its internal dynamics into account, and may thus be rather challenging to achieve. In this talk we discuss how the dynamical properties of the atom can be conveniently studied within the framework of matter-wave absorption. We present a model that describes the interaction between a nonrelativistic structureless quantum particle and a thin time-dependent absorbing barrier. The barrier represents a localized laser beam with a time-dependent intensity, and is characterized by a time-dependent aperture function that embeds the absorbing properties of the barrier. The main advantage of this model is that it is exactly solvable. We derive in particular a simple analytical expression of the Husimi distribution of the particle in the case where the aperture function consists of a succession of two narrow Lorentzians, hence representing a double slit in time. This allows us to analytically investigate the phase-space structure of the resulting diffraction pattern.

I will talk about a new algorithm that finds quickly the best optimal configuration of s-type Gaussians that represent the wave-function of single-electron systems with high accuracy.

The effect of a dissipative environment on the ultrafast nonadiabatic dynamics at conical intersections is nowadays well understood. Yet, the molecular wavepacket dynamics at conical intersection has not been directly characterized and monitored via femtosecond nonlinear spectroscopy. We have combined the hierarchy equation of motion, the multiple Davydov ansatz, and the Multiconfigurational Ehrenfest dynamics with the nonlinear response function formalism to simulate four-wave-mixing (4WM) signals for a set of conical intersection systems which are coupled to the thermal bath. Our results show that the signals indeed visualize evolutions of the wave packet at conical intersections, and these evolutions are quite sensitive to the system-bath coupling strengths.

Is time fundamental or emergent? This is an old question but to date the answer remains elusive. Advocating for the latter, we put forth a set of basic prerequisites necessary for its emergence and examine their implications in quantum and classical mechanics. In such timeless approaches, the unitary dynamics are derived from the description of a constrained global state of a bipartite system, comprised of a quantum `clock' and a generic quantum `system’. Guided by the fact that every determination of change in time is made in relation to the reading of a clock, the central notion is that of a system state conditioned on the state of a clock. As a result, the concept of time or, more concretely, dynamics emerges from the inherent correlation between both subsystems. In particular, we demonstrate the emergence of the Schrödinger, von Neumann and Liouville equation for pure quantum states, quantum density operators and classical phase space densities, respectively. A numerical illustration is given for a paradigmatic model from atomic physics.

Second quantization is traditionally applied to model closed quantum systems of identical particles. There are several benefits associated with this approach, among them dimensionality reduction due to permutation symmetry and representation of physical operators in terms of bosonic (or fermionic) creation and annihilation operators. While the advantages of the first point are evident, the bosonization is crucial when complexity of the chosen method of description directly depends on commutation relations of operators (as in Heisenberg equations of motion, phase-space methods, etc). In order to describe realistic quantum dynamics, one has to account for the effects stemming from the interaction with the environment. The interaction of real quantum systems with the environment gives rise to numerous incoherent phenomena (decoherence, dissipation, incoherent pumping, damping, etc.), that in many situations have a significant impact on the behaviour of the system. In this case, one has to adopt the density matrix formalism, which is capable of describing incoherent processes. However, inclusion of incoherence leads to formation of states, which cannot be constructed out of symmetric pure states. This is the point where the traditional second quantization fails, since it applies only to pure quantum states. In several works, for instance in [1-2], it was discovered that under the assumption that dissipation preserves the permutation symmetry (i.e. particles are still identical), one may naturally introduce occupation number representation for the density matrix. However, as we have mentioned before, the traditional second quantization is not applicable. This fact motivates extension of the second quantization to open quantum systems. In this talk, we present the method of the generalized second quantization, applicable to an open quantum system of multilevel particles. To that end, we supplement the Liouville space of the density matrix with additional algebraic objects, that enable us to formulate the procedure of quantization similar to the traditional one. As a result, we get numerous benefits, such as size reduction, bosonization and occupation number representation – all of which greatly simplify the analysis of a wide range of quantum master equations. The method proves to be a powerful and intuitive tool for the analytical and numerical analysis of various quantum master equations. [1] M. Gegg and M. Richter, Scientific reports 7, 1 (2017). [2] M. Bolaños and P. Barberis-Blostein, Journal of Physics A: Mathematical and Theoretical 48, 445301 (2015).

The dynamics of one-dimensional few-particle systems are key to understand the phenomenological differences between single- and many-body systems, and ultimately the transition to the thermodynamic limes. While experimentally such systems become increasingly controllable, with particle interactions tunable from strongly attractive to strongly repulsive and short- (contact) to long-range (Coulomb), exact numerical approaches are feasible but challenging. In the first part of this talk we briefly comment on our exact treatment of two indistinguishable, Coulomb-interacting bosons in a one-dimensional trapping potential, by numerical diagonalization of the many-body Hamiltonian after discretization in an appropriate finite element basis. In the second part we show how the dynamics of a simple single-particle observable allows us to distinguish long-ranged Coulomb from short-ranged contact interactions. This bears new insights in the interaction-dependence of few-body dynamics, and can be verified via state-of-the-art experiments with ultracold atoms.

Cherenkov and transition radiation are radiation phenomena due to a charged particle that crosses a dielectric medium or the interface of two of these. They have served to understand deeper the interaction of light and matter and have provided several applications. In this talk, we consider how both phenomena are modified when they occur in presence of a three-dimensional topological insulator with planar symmetry. To this aim, we employ the Green's function approach to find the electromagnetic field, the angular distribution of the radiation and the frequency distribution for both kinds of radiation. After analyzing the new interesting modifications, our results are compared favorably with recent experimental measurements.

I will talk about our theoretical studies on ultrafast electron dynamics accessible via the interaction of ultrafast and intense laser pulses with 2D materials. Strong field dynamics of solid has significant potential in fundamental science, such as understanding states of matter and discovering new states, materials characterization, engineering novel hybrid materials. Besides it has application in meteorology and imaging electronic processes, on-chip attosecond laser sources, and signal processing with THZ to PHz datarate. As an emerging direction of 2D systems in strong-field, I mention the Valleytronics, the maximality condition of valley polarization and discuss the scaling factors by which we can coherently control the carriers and store data via valley degree of freedom. Particular emphasis is put on the gapless Dirac fermions where we have shown such system may processes valley polarization to almost 100 % with the linearly polarized shaped pulse.

With rapidly growing abilities to fabricate systems with submicron dimensions it is possible to explore the interplay of electronic and mechanical properties at the nanoscale. These so called nanoelectromechanical systems (NEMS) allow to study fundamental phenomena like phonon-assisted electron transport and have many applications including signal processing and high-precision measurements of force and electric charge. One particularly intriguing effect is the shuttling of electrons, where the mechanical oscillation of a metallic grain is achieved by sequential electron tunneling between the grain and two connecting leads. The coupled system of mechanical and electronic degrees of freedom has drawn considerable theoretical and experimental attention since its original proposal. However, many aspects of electron shuttling are still largely unexplored. In the first part of my talk, I will discuss the interplay of synchronization and topological protection in a chain of electron shuttles. By mechanically coupling the systems together synchronized motion of the shuttles at the ends of the chain can be observed. These synchronized states are topologically protected against local disorder and, furthermore, can be directly measured by the electron current across each shuttle with current technology [1]. In the second part, I will show theoretically that rotor shuttles, fundamental NEMS without intrinsic frequencies, are capable of working as an engine or electron pump [2], and furthermore, are able to rectify an oscillatory bias voltage over a wide range of external parameters in a highly controlled manner [3], even if subject to the stochastic nature of electron tunneling and thermal noise. These examples demonstrate that there are still many phenomena to explore in the field of electron shuttling and NEMS in general. [1] CWW, V. M. Bastidas, G. Schaller, and W. J. Munro, Phys. Rev. B 102, 014309 (2020). [2] CWW, P. Strasberg, and G. Schaller, Phys. Rev. Appl. 12, 024001 (2019). [3] CWW, A. Celestino, A. Croy, and A. Eisfeld, arXiv:2103.11380.

Already in 1926, Schroedinger had pointed out the importance of Gaussian wavepackts in the transition from micro- to macro-mechanics [1]. We discuss the connections of different approximate and numerically exact ways, based on Gaussian wavepackets, to solve the time-dependent Schroedinger equation for a many-particle system. Special emphasis will be laid upon trajectory-guided coherent states that are, e.g., used in semiclassical initial value representations of the propagator [2]. Furthermore, some recent progress in numerical implementations of the Davydov-Ansatz for spin-boson models will be presented [3,4]. Finally, we elaborate on the use of generalized coherent states in particle number conserving dynamics in Bose-Hubbard systems [5]. References: [1] E. Schroedinger, Die Naturwissenschaften 14, 664 (1926) [2] M. Herman and E. Kluk, Chem. Phys. 91, 27 (1984) [3] R. Hartmann et al., J. Chem. Phys. 150, 234105 (2019) [4] M. Werther and F. Grossmann, Phys. Rev. A 102, 063710 (2020) [5] Y. Qiao and F. Grossmann, Phys. Rev. A 103, 042209 (2021)

We formulate a general theory of wave-particle duality for many-body quantum states, which quantifies how wave- and particle-like properties balance each other. Much as in the well-understood single-particle case, which-way information — here on the level of many-particle paths — lends particle character, while interference — here due to coherent superpositions of many-particle amplitudes — indicates wave-like properties. We analyze how many-particle which-way information, continuously tunable by the level of distinguishability of fermionic or bosonic, identical and possibly interacting particles, constrains interference contributions to many-particle observables and thus controls the quantum-to-classical transition in many-particle quantum systems. The versatility of our theoretical framework is illustrated for Hong-Ou-Mandel- and Bose-Hubbard-like exemplary settings.