Over the last twenty years, physicists have learned to manipulate individual quantum objects: atoms, ions, molecules, quantum circuits, electronic spins... It is now possible to build "atom by atom" a synthetic quantum matter. By controlling the interactions between atoms, one can study the properties of these elementary many-body systems: quantum magnetism, transport of excitations, superconductivity... and thus understand more deeply the N-body problem. More recently, it was realized that these quantum machines may find applications in the industry, such as finding the solution of combinatorial optimization problems. This seminar will present an example of a synthetic quantum system, based on laser-cooled ensembles of individual atoms trapped in microscopic optical tweezer arrays. By exciting the atoms into Rydberg states, we make them interact, even at distances of more than ten micrometers. In this way, we study the magnetic properties of an ensemble of more than a hundred interacting ½ spins, in a regime in which simulations by usual numerical methods are already very challenging. Some aspects of this research led to the creation of a startup, Pasqal.

The striking imbalance of matter and antimatter in our universe inspires experiments to compare the fundamental properties of matter/antimatter conjugates with high precision. The BASE collaboration at the antiproton decelerator of CERN is performing such high-precision comparisons with protons and antiprotons. Using advanced cryogenic Penning traps, we have performed the most precise comparison of the proton-to-antiproton charge-to-mass ratio with a fractional uncertainty of 16 parts in a trillion [1], and have invented a novel spectroscopy technique, that allowed for the first direct high-precision measurement of the antiproton magnetic moment with a fractional accuracy of 1.5 parts in a billion [2]. Together with our last measurement of the proton magnetic moment [3] this improves the precision of previous magnetic moment based tests of the fundamental CPT invariance by more than a factor of 3000. A time series analysis of the sampled magnetic moment resonance furthermore enabled us to set first direct constraints on the interaction of antiprotons with axion-like particles (ALPs) [4], and most recently, we have used our ultra-sensitive single particle detection systems to derive constraints on the conversion of ALPs into photons [5]. In parallel we are working on the implementation of new measurement technology to sympathetically cool antiprotons [6] and to apply quantum logic inspired spectroscopy techniques [7]. In addition to that, we are currently developing the transportable antiproton-trap BASE-STEP, partly developed at Mainz, to relocate antiproton spectroscopy experiments from CERN’s accelerator environment to dedicated precision laboratory space at Heinrich Heine University Düsseldorf, very recently, the first loaded transport of this trap has been demonstrated successfully I will give a general introduction to the topic, will review the recent results produced by BASE, with particular focus on recent developments towards an at least 10-fold improved coherent measurement of the antiproton magnetic moment, and towards the first antiproton transport. [1] M. J. Borchert et al., Nature 601, 35 (2022). [2] C. Smorra et al., Nature 550, 371 (2017). [3] G. Schneider et al., Science 358, 1081 (2017). [4] C. Smorra et al., Nature 575, 310 (2019). [5] J. A. Devlin et al., Phys. Rev. Lett. 126, 041321 (2021). [6] M. A. Bohman et al. Nature 596, 514 (2021) [7] J. M Conrejo et al., New J. Phys. 23 073045

Precision measurements of atomic transition frequencies have become a promising path for testing theories for new physics beyond the standard model. To achieve even higher precision more stable and narrow-linewidth laser sources are required. Superradiant lasers are a candidate for realising a narrow-linewidth, high-bandwidth active frequency reference. They shift the phase memory from the optical cavity, which is subject to technical and thermal vibration noise, to an ultra-narrow optical atomic transition of an ensemble of cold atoms trapped inside the cavity. Our previous demonstration of pulsed superradiance on the mHz transition in 87Sr achieved a fractional Allan deviation of 6.7*10−16 at 1s of averaging. Moving towards continuous-wave superradiance promises to further improve the short-term frequency stability by orders of magnitude. A key challenge is the continuous supply of cold atoms into a cavity, while staying in the collective strong coupling regime. We demonstrate continuous loading and transport of cold 88Sr atoms inside a ring cavity, after several stages of laser cooling and slowing. We further describe the emergence of distinct zones of collective continuous lasing of the atoms on the 7.5kHz transition, 7x narrower than the cavity linewidth, and pumped by the cooling lasers via inversion of the motional states. The lasing is supported by self-regulation of the number of atoms inside the cavity that pins the dressed cavity frequency to a fixed value over >3MHz of raw applied cavity frequency. In the process up to 80% of the original atoms are expelled from the cavity. I will also present a new project in Heidelberg aiming to use precision spectroscopy of highly charged ions to search for a variation of the fine-structure constant.

Compressible computational fluid dynamics (CFD) is an active and fruitful area of research. In this talk, we will focus on time integration methods optimized for CFD applications. We will briefly review the classical Courant-Friedrichs-Lewy (CFL) constraint and present error-based time step size control as an alternative. In particular, we will discuss how the design of the methods influences their efficiency and robustness. Combining theoretical analysis with a data-driven approach, we will present new optimized time integration methods for compressible CFD applications that are available in open-source software.

From dark matter and dark energy to neutrino oscillations and the lack of antimatter in the universe, there is growing evidence that the Standard Model is incomplete. Tests of Quantum Electrodynamics (QED) with few-electron systems offer a promising avenue for looking for new physics, as QED is the best understood quantum field theory and extremely precise predictions can be obtained for few-electron systems. Unfortunately, despite decades of effort, QED is poorly tested in the regime of strong coulomb fields, precisely the region where new exotic physics may be most visible. I will present a new paradigm for probing higher-order QED effects using spectroscopy of Rydberg states in exotic atoms, where orders of magnitude stronger field strengths can be achieved while nuclear uncertainties may be neglected. Such tests are now possible due to the advent of quantum sensing microcalorimeter x-ray detectors and new facilities providing low-energy intense beams of exotic particles for precision physics. First measurements have been successfully conducted at J-PARC with muonic atoms, but antiprotonic atoms offer the highest sensitivity to strong-field QED. I will present an overview of the PAX project, a new experiment for antiprotonic atom x-ray spectroscopy with a large-area transition edge sensor (TES) x-ray detector and low-energy cyclotron trap at ELENA. Finally, the experimental paradigm can also be reversed such to study low-lying states and access nuclear properties, such as those pursued in the QUARTET collaboration at Paul Scherrer Institute to improve the charge radii of light nuclei. I will present first results from QUARTET, and discuss synergies between atomic and nuclear physics accessible with these experiments.

The field of ultra-cold chemistry of ions and atoms has been launching the fundamental quest for investigating its quantum regime for decades. A simplifying summary of the quest might be: How do interactions and chemical reactions proceed at extremely low temperatures? The classical picture predicts that all dynamics comes to a standstill as zero velocity is approached. However, deviations are expected since the classical model ceases to be appropriate at microscopic scales and at low temperatures, where particle-wave dualism of matter get’s important. In this regime, quantum effects dominate and reactions are predicted to obey fundamentally different rules. Examples are: (i) collisions of atoms, necessary for a reaction, cannot be described as a billiard-like impact between hard spheres anymore, but rather as interfering waves, interacting at long range, which can coherently amplify or even decoherently annihilate each other. (ii) energy barriers can exceed the available kinetic energy, but nevertheless be efficiently passed via quantum tunnelling, ruling the dynamics. Experimentally, we immerse a single barium (Ba+) ion in a bath of fermionic lithium (Li) atoms. We span temperatures from far above room temperature down deep into the s-wave regime of nano-Kelvin. We report our results on exploiting the collision energy dependence of magnetically tunable atom-ion scattering (Feshbach) resonances and explain how to assign their partial-wave-classification experimentally. In the first half, we will give a basic tutorial on quantum scattering of atom-ion ensembles and distill the substantial differences to atom-atom dynamics. We aim to discuss how to gain control and state-sensitive detection on the level of individual quanta within the merged ion-atom system and to study and establish optically trapping of ions and atoms in general - for example to reveal the quantum dynamics of ion-atom and ion-molecule reactions in absence of any detrimental radio-frequency fields.

This talk will outline three revolutions that happened in the past decades in the development of Earth system models that are used to perform weather and climate predictions. The quiet revolution has leveraged better observations and more compute power to allow for constant improvements of prediction quality of the last decades, the digital revolution has enabled us to perform km-scale simulations on modern supercomputers that further increase the quality of our models, and the machine learning revolution has now shown that machine learned weather models are often competitive with physics based weather models for many forecast scores while being easier, smaller and cheaper. This talk will summarize the past developments, explain current challenges and opportunities, and outline how the future of Earth system modelling will look like. In particular, regarding machine-learned foundation models in a physical domain such as Earth system science.

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In my talk, I will present a novel approach to magnetic resonance microscopy that exploits nitrogen-vacancy (NV) centers in diamond for optically detected magnetic resonance (ODMR). The fusion of optical microscopy and nuclear magnetic resonance (NMR) spectroscopy bypasses the conventional reliance on k-space sampling and magnetic field gradients for spatial encoding of NMR signals, enabling real-space magnetic resonance imaging (MRI). We demonstrate the capabilities of our widefield optical NMR microscopy technique by imaging NMR signals within a model microstructure, achieving a spatial resolution of approximately 10 μm over an area of ~235 × 150 μm². Each camera pixel captures a complete NMR spectrum, providing comprehensive information on signal amplitude, phase, local magnetic field strengths, and gradients. The integration of optical microscopy and NMR opens up new possibilities for a wide range of applications in the physical and life sciences, which I will discuss in the last part of my talk. These applications include imaging metabolic activity in single cells or tissue slices, analyzing battery materials, and facilitating high-throughput NMR analysis.

Title: Circuits of Microfluidic Memristors: Computing with Aqueous Electrolytes Speaker: René van Roij, Institute for Theoretical Physics, Utrecht University, The Netherlands Abstract: In this online talk we will discuss recent advances in our understanding of the physics of cone-shaped microfluidic channels under static and pulsatile voltage- and pressure drops. On the basis of Poisson-Nernst-Planck-Stokes equations for transport of aqueous electrolytes through channels carrying a surface charge, we will provide a theoretical explanation for the experimentally observed diode-like current rectification of these channels. At steady electric driving this rectification involves salt depletion or accumulation in the channel depending on the sign of the applied voltage [1], and this effect also explains the observed pressure-sensitivity of the electric conductance. An extension towards an applied AC voltage predicts these channels to be tunable between diodes at low frequencies ωτ<<1, memristors (resistors with memory) at intermediate frequencies ωτ ~ 1, and Ohmic resistors at high frequency ωτ>>1 , with a characteristic (memory retention) time τ proportional to the square of the channel length [2]. We predict that Hodgkin- Huxley-inspired iontronic circuits of short (fast) and long (slow) conical channels yield neuromorphic responses akin to (trains of) action potentials [2] and several other neuronic spiking modes [3]. Next, we show theoretically and experimentally that a tapered microfluidic channel filled with an aqueous nearly close-packed dispersion of colloidal charged spheres is a much stronger memristor than the channel with only surface charges on the channel wall [4]. Upon applying a train of four positive (negative) voltage pulses, each pulse representing a binary “1” (“0”), we map the hexadecimal number represented by this train on an analog channel conductance, which offers opportunities for reservoir computing -we give a proof of principle for the case of recognizing hand-written digits [4]. Finally we will also discuss recent and ongoing work on iontronic information processing. We exploit the mobility of the medium (water) by considering simultaneously applied pulsatile pressure and voltage signals to increase the bandwidth [5]. Finally, the versatile ionic nature of the charge carriers allows for Langmuir-like ionic exchange reaction kinetics on the channel surface [6]. We show that this can give rise to direct iontronic analogues of synaptic long-term potentiation and coincidence detection of electric and chemical signals [7], which are both ingredients for brain-like (Hebbian) learning. References: [1] W.Q. Boon, T. Veenstra, M. Dijkstra, and R. van Roij, Pressure-sensitive ion conduction in a conical channel: optimal pressure and geometry, Physics of Fluids 34, 101701 (2022). [2] T.M. Kamsma, W.Q. Boon, T. ter Rele, C. Spitoni, and R. van Roij, Iontronic Neuromorphic Signaling with Conical Microfluidic Memristors, Phys. Rev. Lett. 130, 268401 (2023). [3] T.M Kamsma, E. A. Rossing, C. Spitoni, and R. van Roij, Advanced iontronic spiking modes with multiscale diffusive dynamics in a fluidic circuit, Neuromorph. Comput. Eng. 4 024003 (2024). [4] T.M. Kamsma, J. Kim, K. Kim, W.Q. Boon, C. Spitoni, J. Park, and R. van Roij, Brain-inspired computing with fluidic iontronic nanochannels, PNAS 121, e23202242121 (2024). [5] A. Barnaveli, T.M. Kamsma, W.Q. Boon, and R. van Roij, Pressure-gated microfluidic memristor for pulsatile information processing, arXiv:2404.15006. [6] W.Q. Boon. M. Dijkstra, and R. van Roij, Coulombic Surface-Ion Interactions Induce Nonlinear and Chemistry-Specific Charging Kinetics, Phys. Rev. Lett. 130, 058001 (2023). [7] T.M. Kamsma, M. Klop, W.Q. Boon, C. Spitoni, and R. van Roij, arXiv:2406.03195

Our understanding of the origin of heavy elements by the r-process (rapid neutron capture process) has made great progress in the last years. In addition to the gravitational wave and kilonova observations for GW170817, there have been major advances in the hydrodynamical simulations of neutron star mergers and core-collapse supernovae, in the microphysics included in those simulations (neutrinos and high density equation of state), in galactic chemical evolution models, in observations of old stars in our galaxy and in dwarf galaxies. This talk will report on recent breakthroughs in understanding the extreme environments in which the formation of the heavy elements occurs, as well as open questions regarding the astrophysics and nuclear physics involved.

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