03.12.24 – Elisabeth Gruber, University of Innsbruck, Austria
10.12.24 – Andrés Ordonez, Imperial College London, UK
14.01.25 – Aurelia Chenu, Université de Louxembourg
28.01.25 – Jale Schneider, ISE Fraunhofer, Freiburg
10.12.24 – Andrés Ordonez, Imperial College London, UK
Laser-induced electronic dynamics in chiral molecules
Chiral molecules are ubiquitous in nature, from the building blocks of life (sugars and aminoacids) to unidirectional molecular motors. In light-matter interactions, the common knowledge is that molecular chirality is expressed via the magnetic-dipole interaction. This is usually very weak and leads to very small corrections on top of the dominant electric-dipole interaction. However, the last two decades have seen the discovery a variety of chiral phenomena that are expressed solely via the electric-dipole interaction, and can result in very strong effects [1,2]. Such strong effects are not only interesting from a fundamental point of view, but also find application in the detection and manipulation of chiral molecules. In this seminar, I will explain how molecular chirality affects the phase of photoelectron wave packets, their angular distribution, the resulting orientation of the parent ions, and the population of electronically excited states upon multiphoton excitation. I will start by discussing the generation of photoelectron vortices – photoelectron waves with a helical phase front – upon strong-field ionization of chiral molecules with linearly polarized light [3]. Then I will discuss the asymmetry of the photoelectron angular distributions in a variety of multiphoton schemes [4], and its connection to molecular orientation [5,6]. And finally, I will discuss a new type of chiral light [7] and its connection to the coherent control of excited state populations in chiral molecules [8].
[1] Ordonez, Smirnova. Generalized Perspective on Chiral Measurements without Magnetic Interactions. Phys Rev A 98, 063428 (2018) [2] Ayuso, Ordonez, Smirnova. Ultrafast Chirality: The Road to Efficient Chiral Measurements. Phys Chem Chem Phys 24 26962 (2022) [3] Planas, Ordóñez, Lewenstein, Maxwell. Ultrafast Imaging of Molecular Chirality with Photoelectron Vortices. Phys Rev Lett 129, 233201 (2022) [4] Ordonez, Smirnova. Disentangling Enantiosensitivity from Dichroism Using Bichromatic Fields. Phys Chem Chem Phys 24, 7264 (2022) [5] Ordonez, Ayuso, Decleva, Smirnova. Geometric Magnetism and Anomalous Enantio-Sensitive Observables in Photoionization of Chiral Molecules. Comm Phys 6, 1 (2023) [6] Wanie, et al. Capturing Electron-Driven Chiral Dynamics in UV-Excited Molecules. Nature 630, 109 (2024) [7] Ayuso, et al. Synthetic Chiral Light for Efficient Control of Chiral Light–Matter Interaction. Nat Phot 13, 866 (2019) [8] Ordóñez, Vindel-Zandbergen, Ayuso. Chiral Coherent Control of Electronic Population Transfer. arXiv:2309.02392 (2023)
Chair: Lukas Bruder
03.12.24 – Elisabeth Gruber, University of Innsbruck, Austria
Probing singly and multiply charged molecular ions within helium nanodroplets
Helium nanodroplets offer an exciting and versatile matrix for trapping and cooling dopants in the gas phase, providing an exceptional platform for studying the spectroscopic properties of atoms, molecules, and clusters – both neutral and charged – at temperatures below 1 K. Furthermore, these droplets allow the stabilization and exploration of transient and metastable species, which are usually challenging to investigate in conventional experimental setups due to their short lifetimes.
In this presentation, I will discuss how highly charged helium nanodroplets [1] enable the efficient formation of singly and multiply charged molecular ions, which we characterize using mass spectrometry [2]. In addition, I will introduce new methods we have developed to gently extract these molecular ions from the helium matrix, allowing the formation of helium-tagged molecular ions [3,4]. These tagged ions are particularly valuable for messenger spectroscopy, as they allow us to perform electronic and ro-vibrational spectroscopy on cold molecular ions with minimal interference from the helium tag.
[1] F. Laimer et al., Phys. Rev. Lett.123, 165302 (2019)
[2] L. Ganner et al., Phys. Rev. Lett.133, 023001 (2024)
[3] P. Martini et al., Phys. Rev. Lett.127, 263401 (2021)
[4] S. Bergmeister et al., Rev. Sci. Instrum.94, 055105 (2023)
Using X-ray Free-Electron Lasers to study ultrafast dynamics in molecules
X-ray Free-Electron Lasers (FELs) have developed into powerful tools to study ultrafast dynamics in atoms, molecules, nanoparticles and solids. In recent years, FELs have made remarkable progress in producing novel beam modes, like multi-color X-ray pulses and attosecond X-ray pulses, to enable new types of experiments. The soft X-ray branch Athos of SwissFEL is one of the newest FELs worldwide and was designed with the goal to produce highly flexible beam parameters ranging from full polarization control to widely tuneable multi-color pulses. Within this talk, I will introduce Athos and focus on results from the Maloja experimental station for atomic, molecular, non-linear and chemical sciences. Experiments on prototypical reactions in gas-phase molecules induced by optical lasers as well as X-rays will be discussed.
Chair: Giuseppe Sansone
21.11.24 – Andrea Pizzi, University of Cambridge, UK
Quantum scars in many-body systems
Chaos makes isolated systems of many interacting particles quickly thermalize and forget about their past. Here, we show that quantum mechanics hinders chaos in many-body systems: although the quantum eigenstates are thermal and strongly entangled, exponentially many of them are scarred, that is, have an enlarged weight along underlying classical unstable periodic orbits. Scarring makes the system more likely to be found on an orbit it was initialized on, retaining a memory of its past and thus weakly breaking ergodicity, even at long times and despite the system being fully thermal. We demonstrate the ubiquity of quantum scarring in many-body systems by considering a large family of spin models, including some of the most popular ones from condensed matter physics. Our findings, at hand for modern quantum simulators, prove structure in spite of chaos in many-body quantum systems.
Chair: Andreas Buchleitner
12.11.24 – Barbara Soda, Perimeter Institute, Ontario, Canada
Trajectory-protected quantum computing
We demonstrate a quantum computing model that utilizes a qubit’s motion to protect it from decoherence. We model a qubit interacting with a quantum field via the standard light-matter interaction model: an Unruh-DeWitt detector, i.e. the qubit, follows a prescribed classical trajectory while interacting with a scalar quantum field. We switch off the rotating-wave terms, i.e. the resonant transitions, by controlling the trajectory of the qubit, thereby eliminating decoherence via dominant channels of decoherence. This phenomenon is known as acceleration-induced transparency. We use the stimulated counter-rotating wave terms (i.e. the non-resonant transitions) to perform one-qubit gates. The two-qubit, entangling, gates are performed by vacuum interaction related to entanglement harvesting, where we make use of control via squeezed state stimulation of the field. We find that the error protection due to trajectory control hinders entanglement creation via two-qubit gates. Finally, we discuss the fundamental limits on the trade-off between isolating a quantum computer, and the speed with which entangling gates may be applied.
Chair: Andreas Buchleitner
31.10.24 – Olga Lushchikova, Kyushu University, Japan
Structure and reactivity of copper clusters
Gas-phase metal clusters provide a powerful platform for investigating catalytic processes at the molecular level. Their tunable structure, charge, and composition enable precise control over catalytically active sites, leading to enhanced activity compared to bulk metals due to their small size, high surface-to-volume ratio, and size-dependent physical and chemical properties.
Given the high potential of copper (Cu) clusters in methanol synthesis from CO₂, understanding the reaction mechanisms at the molecular level is crucial for the rational design of high-performance catalysts. Gas-phase clusters can effectively mimic active sites, providing key insights into these processes. The electronic and geometric properties of Cu clusters were explored through IRMPD spectroscopy at FELIX free electron laser facility1, helium solvation studies at the University of Innsbruck2, and UV-VIS photodissociation spectroscopy of trapped ions at Kyushu University, with all experiments complemented by theoretical calculations.
Moreover, the reactivity of Cu clusters toward CO₂ was investigated, and the resulting complexes were characterized using IRMPD spectroscopy.3,4It was found that in anionic clusters, charge and size significantly influenced reactivity, leading to CO₂ activation or dissociation.3 In contrast, cationic clusters exhibited uniform behavior, with CO₂ consistently physiosorbed in an end-on configuration.4 Further insights were obtained through IR photodetachment spectroscopy on helium-tagged complexes, which provided narrower bandwidths. These measurements revealed that the binding energy between cationic Cu clusters and CO₂ decreases with increasing cluster size, identifying Cu⁺ as the most reactive among the cations as illustrated in Figure 1.5
This comprehensive study advances the understanding of Cu clusters at the nanoscale and provides valuable insights into their catalytic behavior, contributing to the rational design of efficient and selective catalysts for chemical transformations.
Figure 1: The frequency of the asymmetric stretch vibration of CO2 bound to copper clusters of different sizes (n), obtained experimentally (blue circles) and computationally (black triangles). The dashed line represents the frequency for free CO2. Additionally, the binding energy of CO2 to each cluster is depicted with red squares. Geometric structures for selected configurations are also presented.
Lushchikova, O. V. et al.J. Phys. Chem. Lett. 10, 2151–2155 (2019).
Lushchikova, O. V. et al. Phys. Chem. Chem. Phys.25, 8463–8471 (2023).
Lushchikova, O. V., Szalay, M., Höltzl, T. & Bakker, J. M. Farad. Discuss.52, (2022).
Lushchikova, O. V. et al.Phys. Chem. Chem. Phys.23, 26661–26673 (2021).
Reider, A. M. et al.Phys. Chem. Chem. Phys. 26, 20355–20364 (2024).
Chair: Frank Stienkemeier
29.10.24 – Dietrich Leibfried, National Institute of Standards and Technology (NIST), USA
How to train your Molecule
An amazing level of quantum control is routinely reached in modern experiments with atoms, but similar control over molecules has been an elusive goal. A method based on quantum logic spectroscopy [1] can address this challenge for a wide class of molecular ions [2,3]. We have now realized many basic aspects of these proposals.
In our demonstrations, we trap a calcium ion together with a calcium hydride ion (CaH+) that is a convenient stand-in for more general molecular ions. We laser-cool the two-ion crystal to its motional ground state and then drive stimulated Raman transitions in the molecular ion. Laser-based transitions in the molecule can deposit a single quantum of excitation in the motion of the ion pair when a motional „sideband“ is driven. We can efficiently detect this single quantum of excitation with the calcium ion, which non-destructively projects the molecule into the final state of the sideband transition, a known, pure quantum state.
The molecule can be coherently manipulated after this first projection by driving further stimulated Raman, mm-wave or vibrational overtone transitions. After attempting a transition, the resulting molecular state can be read out by another quantum logic state detection. We demonstrate this by driving Rabi oscillations between different rotational and vibrational states [4-6] and by active feedback on the rotational state of the molecule [7]. Transitions in the molecule are either driven by a single, far off-resonant continuous-wave laser, by a far-off-resonant frequency comb or a single frequency comb tooth resonant with a certain vibrational overtone transition. This makes the approach suitable for quantum control and precision measurement of a large class of molecular ions. Controlled transitions to excited vibrational levels open avenues to precise characterization of the electronic ground state potential surface and to coherent dissociation along a specific bond.
[1] P.O. Schmidt, et al. Science 309, 749 (2005).
[2] S. Ding, and D. N. Matsukevich, New J. Phys. 14, 023028 (2012).
[3] D. Leibfried, New J. Phys. 14, 023029 (2012).
[4] C.-W. Chou et al., Nature 545, 203 (2017).
[5] C.-W. Chou et al., Science 367, 1458 (2020).
[6] A. L. Collopy et al., Phys. Rev. Lett. 130, 223201 (2023).
[7] Y. Liu et al., Science 385, 790 (2024).
Chair: Tobias Schätz
22.10.24 – Frederike Doerr & Felix Selz, University of Freiburg
Frederike Doerr
Stroboscopic Control Techniques in Trapped Atomic Ions
Trapped ions are a widely used platform for quantum simulations, offering insights into the interaction of light and matter. A recent method [1] enables superresolved observation of ion dynamics, allowing for precise measurements of position and momentum with high temporal and spatial resolution, contributing to advancements in quantum metrology and control. Building on this, my work presents the starting point for extending the technique from coherent displaced states to cyclic non-classical states, such as squeezed states, which are key for observing the transfer of spatial entanglement into robust electronic degrees of freedom in multiple ions [2,3]. To overcome the limitations mentioned in [1], notably the switching time of acousto-optic modulators (AOMs), which is crucial for working at higher frequencies needed for squeezed states, we have designed a new setup that focuses the laser beam in the AOM, reducing pulse lengths and response times by a factor of three. This setup is currently being tested within stroboscopic measurements, and simulations suggesting it may enable the use of squeezed states, pushing the boundaries of quantum control and aiding in the study of fundamental phenomena such as early-universe physics.
[1] Florian Hasse et al., Phys. Rev. A 109, 053105 (2024) [2] M. Wittemer et al., Phys. Rev. Lett. 123, 180502 (2019). [3] M. Wittemer et al., Philos. Trans. R. Soc. A 378, 20190230 (2020)
Chair: Tobias Schätz
Felix Selz
Investigation of Spintronic Terahertz Emitters and their Application in Terahertz Spectroscopy
Spintronic terahertz emitters offer terahertz sources with an unmatched broad frequency bandwidth, while being easy to fabricate and operate. In this presentation, I first introduce my work on developing fiber-tip spintronic terahertz emitters and the investigation of their optical damage threshold. In the second part of the talk, I present an optical pump-terahertz probe setup as a novel approach to study electron and phonon temperatures following optical pump excitation in spintronic terahertz emitters. Finally, a study of the nonlinear response of ZnTe in the terahertz frequency range using time-resolved terahertz pump-terahertz probe spectroscopy is presented.
