arXiv subjects

Andreas W. Hauser

Publications and source records attributed to Andreas W. Hauser.

15 recordsLinked to original sources

arXiv2026

All-optical universal control of nuclear-spin qudits in trapped neutral atoms

Quantum systems with more than two levels - so-called qudits - offer increased computational density and reduced circuit complexity compared to qubit-based architectures, but achieving universal and scalable control remains challenging. We propose an all-optical scheme for universal qudit control in trapped neutral atoms in moderate to high magnetic fields, focusing on the fermionic isotope 173Yb (I = 5/2). The strong hyperfine interaction in the 3P1 manifold enables fast and selective Raman transitions between nuclear-spin states in the 1S0 ground-state manifold using a single linearly polarized laser. For each neighboring transition in the qudit manifold, we identify a magic polarization angle that enables coherent, state-selective control while suppressing off-resonant excitations, with operation frequencies exceeding 100 kHz. Combined with phase-shift operations, this provides universal control of the full single-qudit space. We further discuss compatible two-qudit gates based on the Rydberg blockade mechanism, completing a universal gate set, and analyze state-selective readout schemes compatible with the proposed protocol. Our results identify 173Yb as a promising platform for high-fidelity, all-optical qudit-based quantum information processing.

quant-ph↗

arXiv2026

Magnetic coupling between nuclear motion and nuclear spins in molecules

Among the possible types of magnetic dipole interactions in molecular systems, couplings between nuclear motion and the nuclear spin have probably received the least attention in molecular spectroscopy. Although very small in comparison to effects related to electron spin, this type of hyperfine interaction plays an important role in the NMR spectroscopy of molecular systems. While measurement and prediction of spin-rotation tensors are a common place, vibrationally induced effects still lack a comprehensive description. In this article we develop a generic, theoretical framework that is well embedded in modern electronic structure theory and inspired by the Breit-Pauli Hamiltonian for electronic interactions, distinguishing between nuclear spin-orbit and spin-other-orbit contributions. We show that the interaction of nuclear spins with pseudorotational excitations of highly symmetric molecules may lead to experimentally accessible hyperfine splittings in NMR spectra, triggered by infrared light.

physics.chem-ph↗

arXiv2025

Mapping the Optical Landscape of a Squaraine Molecule in the Visible and Ultraviolet Energy Range

Although squaraine dyes are commonly praised as candidates for light-based applications, little is known about their excited state landscape beyond the low-energy visible light region. Our work aims for an improved understanding of the photophysical properties of squaraines at the example of N-isobutyl substituted anilino-squaraine (SQIB) by extending ground-state and excited-state absorption spectroscopy of the molecule into the ultraviolet up to 6.5~eV. In addition, we distinguish the relative transition dipole moments of the excited state absorption peaks with the help of transient absorption anisotropy experiments. To relate experimental features to specific states, we employ a set of ab initio methods including time-dependent density functional theory (TDDFT), the Bethe-Salpeter equation (BSE) and n-electron valence perturbation theory on top of a self-consistent complete active space (CASSCF/NEVPT2). Our assignment is complemented by vibronic simulations and a discussion of two-photon absorption measurements. Through this joint effort, we are able to provide a consistent picture of the optical behavior of SQIB across the visible and ultraviolet light regime, and assign a total of twelve electronically excited states to our experimental data.

physics.chem-ph↗

arXiv2025

Lattice Instabilities Along the Transformation from Hexagonal to Cuboidal Structures in Hard- and Soft-Sphere Models

The diffusionless Burgers-Bain phase transition from a hcp arrangement to a cuboidal lattice (fcc and bcc) is analysed in great detail for Lennard-Jones solids. From the lattice vectors of an underlying bi-lattice smoothly connecting these phases, we are able to express the corresponding lattice sums for inverse power potentials in terms of fast converging Bessel function expansions resulting in an efficient evaluation to computer accuracy for cohesive energies. From the kissing hard-sphere limit we derive exact analytical expressions for the lattice parameters varying along the minimum energy path of the phase transition. This simple model suggests that for the Burgers-Bain transformation of a LJ solid requires a minimum of four lattice parameters, $(a,α,β,γ=c/a)$, describing the change in the base lattice lengths $a$ and $c$, the shear force acting on the hexagonal base plane ($α$), the sliding force of the middle layer of the original hexagonal packing arrangement($β$), and the cuboidal transformation ($γ=c/a$). This choice results in a two-step process: hcp$\to$fcc$\to$bcc. However, a further extension of the parameter space including an additional slide parameter for the middle layer, one suddenly observes a distinct symmetry-breaking effect along the hcp$\rightarrow$fcc transition path with a bifurcation point appearing joining the original Burgers with the Bain path of the bcc$\rightarrow$fcc cuboidal transition. Furthermore, for soft LJ potentials the bcc phase appears as a local minimum along the Burgers hcp$\rightarrow$fcc path with two transition states to either the hcp or fcc phase. The underlying topology of the Burgers-Bain phase transition also incorporates the rhombohedral distortion of the bcc phase, which is analyzed in detail.

cond-mat.mtrl-sci↗

arXiv2025

Kapitza's Pendulum as a Classical Prelude to Floquet-Magnus Theory

We present a pedagogical introduction to Floquet-Magnus theory through the classical example of Kapitza's pendulum - a simple system exhibiting nontrivial dynamical stabilization under rapid periodic driving. By deriving the equations of motion and analyzing the system using Floquet theory and the Magnus expansion, we obtain analytical stability conditions and effective evolution equations. While grounded in classical mechanics, the techniques are directly applicable to periodically driven quantum systems as well. The approach is fully analytical, using only tools from theoretical mechanics, linear algebra, and ordinary differential equations, and is suitable for instruction at the advanced undergraduate or graduate level.

physics.class-ph↗

arXiv2025

Single Qudit Control in $^{87}$Sr via Optical Nuclear Electric Resonance

Optical nuclear electric resonance (ONER) was recently proposed as a fast and robust single-qubit gate mechanism in $^{87}$Sr. Here, we demonstrate through numerical simulations that ONER can be extended to single-qudit control, addressing multiple one-level hyperfine transitions within the ten-dimensional nuclear-spin manifold. We identify suitable operating regimes and show that ONER enables high-fidelity spin manipulations, with simulated $\pi$-gate fidelities exceeding 99.9\%, while maintaining coherence under realistic parameter fluctuations. These results establish a proof-of-principle for optical qudit control in $^{87}$Sr and delineate practical parameter ranges for future experiments, highlighting ONER as a promising pathway toward high-dimensional quantum information processing.

quant-ph↗

arXiv2025

The Ridge Integration Method and its Application to Molecular Sieving, Demonstrated for Gas Purification via Graphdiyne Membranes

Eyring theory provides a convenient approximation to the rate of a chemical reaction, as it uses only local information evaluated near extremal points of a given potential energy surface. However, in cases of pronounced anharmonicity and particularly low-lying vibrational frequencies, deviations from the correct reaction rate can become substantial. Molecular Dynamics simulations, on the other hand, are very costly at higher levels of theory, and of limited use since molecular reactions are `rare' events and hence statistically less accessible. In this article, we present an alternative description for problems of gas separation and storage via two-dimensional materials such as porous graphene or flat metal-organic frameworks. Taking geometric advantage of the typical problem setting, our method is based on a statistical analysis of molecular trajectories near the so-called `ridge', a hypersurface which divides the reaction volume into a reactant and a product side. It allows for more realistic predictions of permeabilities and selectivities, e.g. derived from density functional theory, but without the considerable costs of a full molecular dynamics simulation on the corresponding Born-Oppenheimer potential energy surface. We test our method on the example of methane separation from nitrogen and carbon dioxide via a graphdiyne membrane.

physics.chem-ph↗

arXiv2025

Molecular Pseudorotation in Phthalocyanines as a Tool for Magnetic Field Control at the Nanoscale

Metal phthalocyanines, a highly versatile class of aromatic, planar, macrocyclic molecules with a chelated central metal ion, are topical objects of ongoing research and particularly interesting due to their magnetic properties. However, while current focus lies almost exclusively on spin-Zeeman-related effects, the high symmetry of the molecule and its circular shape suggests the exploitation of light-induced excitation of twofold degenerate vibrational states in order to generate, switch and manipulate magnetic fields at the nanoscale. The underlying mechanism is a molecular pseudorotation that can be triggered by infrared pulses and gives rise to a quantized, small but controllable magnetic dipole moment. We investigate the optical stimulation of vibrationally-induced molecular magnetism and estimate changes in the magnetic shielding constants for confirmation by future experiments.

physics.chem-ph↗

arXiv2025

Nuclear Electric Resonance for Spatially-Resolved Spin Control via Pulsed Optical Excitation in the UV-Visible Spectrum

Nuclear electric resonance (NER) spectroscopy is currently experiencing a revival as a tool for nuclear spin-based quantum computing. Compared to magnetic or electric fields, local electron density fluctuations caused by changes in the atomic environment provide a much higher spatial resolution for the addressing of nuclear spins in qubit registers or within a single molecule. In this article, we investigate the possibility of coherent spin control in atoms or molecules via nuclear quadrupole resonance from first principles. An abstract, time-dependent description is provided which entails and reflects on commonly applied approximations. This formalism is then used to propose a new method we refer to as `optical' nuclear electric resonance (ONER). It employs pulsed optical excitations in the UV-visible light spectrum to modulate the electric field gradient at the position of a specific nucleus of interest by periodic changes of the surrounding electron density. Possible realizations and limitations of ONER for atomically resolved spin manipulation are discussed and tested on $^9$Be as an atomic benchmark system via electronic structure theory.

quant-ph↗

arXiv2025

Symmetry- and Gradient-enhanced Gaussian Process Regression for the Active Learning of Potential Energy Surfaces in Porous Materials

The theoretical investigation of gas adsorption, storage, separation, diffusion and related transport processes in porous materials relies on a detailed knowledge of the potential energy surface of molecules in a stationary environment. In this article, a new algorithm is presented, specifically developed for gas transport phenomena, which allows for a highly cost-effective determination of molecular potential energy surfaces. It is based on a symmetry-enhanced version of Gaussian Process Regression with embedded gradient information and employs an active learning strategy to keep the number of single point evaluations as low as possible. The performance of the algorithm is tested for a selection of gas sieving scenarios on porous, N-functionalized graphene and for the intermolecular interaction of CH$_4$ and N$_2$.

physics.chem-ph↗

arXiv2025

Optical Nuclear Electric Resonance in LiNa: Selective Addressing of Nuclear Spins Through Pulsed Lasers

Optical nuclear electric resonance (ONER), a recently proposed protocol for nuclear spin manipulation in atomic systems via short laser pulses with MHz repetition rate, exploits the coupling between the nuclear quadrupole moment of a suitable atom and the periodic modulations of the electric field gradient generated by an optically stimulated electronic excitation. In this theory paper, we extend the scope of ONER from atomic to molecular systems and show that molecular vibrations do not interfere with our protocol. Exploring the diatomic molecule LiNa as a first benchmark system, our investigation showcases the robustness with respect to molecular vibration, and the ability to address and manipulate each of the two nuclear spins independently, simply by adjusting the repetition rate of a pulsed laser. Our findings suggest that it might be possible to shift complicated spin manipulation tasks required for quantum computing into the time domain by pulse-duration encoded laser signals.

quant-ph↗

arXiv2025

Optical nuclear electric resonance as single qubit gate for trapped neutral atoms

The precise control of nuclear spin states is crucial for a wide range of quantum technology applications. Here, we propose a fast and robust single-qubit gate in $^{87}$Sr, utilizing the concept of optical nuclear electric resonance (ONER). ONER exploits the interaction between the quadrupole moment of a nucleus and the electric field gradient generated by its electronic environment, enabling spin level transitions via amplitude-modulated laser light. We investigate the hyperfine structure of the 5s$^2$~$^1S_{0}\rightarrow{}$~5s5p~$^3P_1$ optical transition in neutral $^{87}$Sr, and identify the magnetic field strengths and laser parameters necessary to drive spin transitions between the $m_I$ = -9/2 and $m_I$ = -5/2 hyperfine levels in the ground state. Our simulations show that ONER could enable faster spin operations compared to the state-of-the-art oscillations in this 'atomic qubit'. Moreover, we show that spin-flip operations exceeding 99.9\% fidelity can be performed even in the presence of typical noise sources. These results pave the way for significant advances in nuclear spin control, opening new possibilities for quantum memories and other quantum technologies.

quant-ph↗

arXiv2018

Femtosecond photoexcitation dynamics inside a quantum solvent

The observation of chemical reactions on the time scale of the motion of electrons and nuclei has been made possible by lasers with ever shortened pulse lengths. Superfluid helium represents a special solvent that permits the synthesis of novel classes of molecules that have eluded dynamical studies so far. However, photoexcitation inside this quantum solvent triggers a pronounced response of the solvation shell, which is not well understood. Here we present a mechanistic description of the solvent response to photoexcitation of indium (In) dopant atoms inside helium nanodroplets (He$_\mathrm{N}$), obtained from femtosecond pump-probe spectroscopy and time-dependent density functional theory simulations. For the In-He$_\mathrm{N}$ system, part of the excited state electronic energy leads to expansion of the solvation shell within 600 fs, initiating a collective shell oscillation with a period of about 30 ps. These coupled electronic and nuclear dynamics will be superimposed on intrinsic photoinduced processes of molecular systems inside helium droplets.

physics.atm-clus↗

arXiv2011

Homo- and heteronuclear alkali metal trimers formed on helium nanodroplets. II. Femtosecond spectroscopy and spectra assignments

Homo- and heteronuclear alkali quartet trimers of the type K_{3-n}Rb_{n} (n=0,1,2,3) formed on helium nanodroplets are probed by one-color femtosecond photoionization spectroscopy. The obtained requencies are assigned to vibrations in different electronic states by comparison to high level ab initio calculations of the involved potentials including pronounced Jahn-Teller and spin-orbit couplings. Despite the fact that the resulting complex vibronic structure of the heavy alkali molecules complicates the comparison of experiment and theory we find good agreement for many of the observed lines for all species.

physics.atm-clus↗

arXiv2008

Extended Hartree-Fock method based on pair density functional theory

A practical electronic structure method in which a two-body functional is the fundamental variable is constructed. The basic formalism of our method is equivalent to Hartree-Fock density matrix functional theory [M. Levy in {\it Density Matrices and Density Functionals}, Ed. R. Erdahl and V. H. Smith Jr., D. Reidel, (1987)]. The implementation of the method consists of solving Hartree-Fock equations and using the resulting orbitals to calculate two-body corrections to account for correlation. The correction terms are constructed so that the energy of the system in the absence of external potentials can be made to correspond to approximate expressions for the energy of the homogeneous electron gas. In this work the approximate expressions we use are based on the high-density limit of the homogeneous electron gas. Self-interaction is excluded from the two-body functional itself. It is shown that our pair density based functional does not suffer from the divergence present in many density functionals when homogeneous scaling is applied. Calculations based on our pair density functional lead to quantitative results for the correlation energies of atomic test cases.

cond-mat.other↗