## Events at Physics |

### Events During the Week of February 18th through February 25th, 2024

### Sunday, February 18th, 2024

**Wonders of Physics****The Wonders of Physics 41st annual show****Time:**1:00 pm - 2:15 pm**Place:**2103 Chamberlin**Abstract:**A fun, fast-paced physics demonstration show for all ages!**Host:**Haddie McLean**Wonders of Physics****The Wonders of Physics 41st annual show****Time:**4:00 pm - 5:15 pm**Place:**2103 Chamberlin**Abstract:**A fun, fast-paced physics demonstration show for all ages!**Host:**Haddie McLean### Monday, February 19th, 2024

**Atomic Physics Seminar****Using precision measurements to study the interplay of quantum mechanics and gravity and explore the universe****Time:**12:00 pm - 1:00 pm**Place:**5310 Chamberlin Hall**Speaker:**Tejas Deshpande, Northwestern University**Abstract:**Gravity is the least understood of the four fundamental forces at a microscopic level. Moreover, quantum phenomena are typically challenging to observe in macroscopic systems. An overlap between gravitational and quantum phenomena is expected to occur at very high energies like those inside a black hole or close to the big bang. Precision measurements offer an alternate low-energy route to studying such phenomena in tabletop experiments. Furthermore, precision measurement tools can be used for cosmic exploration through detection of gravitational waves (GWs) and direct detection of dark matter (DM). In this talk, I will discuss three current experiments aiming to investigate such phenomena of great importance to fundamental physics research. The first experiment involves performing light-pulse atom interferometry (LPAI) over a 100 m vertical baseline and is part of an international collaboration called Matter-wave Atomic Gradiometer Interferometric Sensor (MAGIS). I will discuss the work done by our group at Northwestern University (NU) on the design, construction, and testing of the laser system for MAGIS. I will present empirical and simulation results demonstrating the potential of MAGIS to detect GWs and ultralight DM at mid-band frequencies (0.03-3 Hz) with unprecedented sensitivity. Moreover, I will discuss how MAGIS can study macroscopic quantum phenomena by creating quantum superpositions of massive systems separated by tens of meters. The second experiment aims to use LPAI to measure Newton's gravitational constant, to a precision of less than 10 parts per million, using a 2 m vertical baseline at NU. I will present some recent results from the first stage of the NU apparatus utilizing resonant LPAI, with a record 504 loops, and discuss its implications for MAGIS. The third experiment involves detecting ultralight DM using cryogenic Fabry-Perot cavities at NU. I will conclude with some ideas for near-future experiments which aim to study the interplay of quantum mechanics and gravity using a combination of technologies associated with LPAI, laser interferometry, and kilogram-scale levitation.**Host:**Thad Walker**Plasma Physics (Physics/ECE/NE 922) Seminar****"Coherent structures in non-neutral electron plasmas, 2D fluids, and beyond"****Time:**12:00 pm - 1:15 pm**Place:**1227 Engineering Hall**Speaker:**Noah Hurst, WiPPL, UW-Madison Physics Department**Abstract:**The self-organization of fluids and plasmas into coherent structures has important consequences for their dynamics and transport, particularly with regard to fusion plasmas and geophysical fluids. Dedicated efforts to understand the life-cycle of these structures are needed, including their formation, dynamics, and stability. Discussed here are a series of experiments with non-neutral pure electron plasmas that follow an interesting trajectory from antimatter technology to studies of coherent vortices in two-dimensional (2D) fluids. The apparatus used for electron plasma vortex dynamics relies on an isomorphism between the plasma ExB drift motion and the 2D ideal fluid equations. The vortices are subjected to external flows to investigate their dynamics and stability, thus recreating in a controlled environment the conditions they might experience in the wild. Experiments are presented regarding vortex oscillations due to applied strain flows, adiabatic vortex behavior in time-dependent strain, spatial Landau damping of vortex modes, the Kelvin-Helmholtz instability within a background strain flow, and the splitting of initially elliptical vortices into multiple pieces. The experimental results are compared to particle-in-cell simulations and simple theoretical models when possible. Finally, opportunities for future work are described involving vortex dynamics, other types of coherent structures in plasmas and fluids, and non-neutral plasma physics.**Host:**Physics**Theory Seminar (High Energy/Cosmology)****Hierarchies from string theory****Time:**4:00 pm - 5:00 pm**Place:**5280 Chamberlin Hall**Speaker:**Jakob Moritz, CERN**Abstract:**I will explain a series of my works that has resulted in the first ever construction of a controlled cosmological solution of string theory in which the vacuum energy — though negative in sign — is exponentially small, while extra dimensions are microscopic. I will also, in brief terms, outline some of my ongoing efforts to construct solutions with positive vacuum energy. Finally, I will turn to another series of my works about probing string theoretic axions — the string axiverse. I will argue that the strong CP problem is generically absent in string compactifications, and that future observation might well detect string theoretic axions through their couplings to photons.**Host:**Lisa Everett### Tuesday, February 20th, 2024

**No events scheduled**### Wednesday, February 21st, 2024

**Physics ∩ ML Seminar****Diffusion Models - A Thermodynamic Perspective****Time:**12:00 pm - 1:00 pm**Place:**Chamberlin 2104**Speaker:**Akhil Premkumar, KICP Chicago, University of Chicago**Abstract:**Diffusion models have found immense success in modeling complex, high dimensional data distributions. Most recently, the text-to-video generation tool, Sora by OpenAI, uses such models to produce extremely high fidelity videos from simple text prompts. In this talk I will introduce a physicist-friendly intuition for diffusion models. Starting with first principles I will demonstrate how diffusion models can be understood as a variational problem, like the ones we come across in physics. I will give a thermodynamic interpretation to the these models, connecting them back to the fluctuation theorems that originally inspired their invention. Based on: arXiv:2310.04490**Host:**Moritz Muenchmeyer**Atomic Physics Seminar****Cavity-enabled measurements and interactions in neutral atoms****Time:**12:00 pm - 1:00 pm**Place:**5310 Chamberlin Hall**Speaker:**Zhenjie Yan, UC Berkeley**Abstract:**Control over interactions and measurements in quantum systems is crucial for applications such as quantum simulation and computation. In this talk, I will highlight our recent progress in realizing nondestructive readout and long-range interactions in atomic tweezer arrays using a strongly coupled optical cavity. Through selectively coupling a single atom with the cavity mode, we achieve a rapid mid-circuit measurement without perturbing the quantum coherence of the other atoms. Conversely, the collective emission from multiple atoms into the cavity can be coherently enhanced or suppressed. By controlling the atom-cavity interaction at the single-atom level, we observe both super- and subradiant cavity emissions from the constructed atomic ensembles. I will then discuss how we engineer long-range mechanical interactions via photon exchange and present our recent observation of a self-organization phase transition in a mesoscopic system. Finally, I will discuss how the cavity can be used to monitor and manipulate strongly interacting quantum gases, opening new avenues for experimental research in quantum many-body physics.**Host:**Thad Walker**Theory Seminar (High Energy/Cosmology)****Axion Dark Matter: From the Laboratory to the Cosmos****Time:**4:00 pm - 5:00 pm**Place:**5280 Chamberlin Hall**Speaker:**Joshua Foster, MIT**Abstract:**The quantum chromodynamics (QCD) axion, a possible solution to the Strong CP Problem, and more general axion-like particles, with their intrinsic connection to high-scale theories like Grand Unification and String Theory, represent uniquely well-motivated dark matter candidates. As an ultralight particle, the phenomenology of the axion is fundamentally wave-like, requiring paradigmatically different approaches in the search for dark matter. In this talk, I will describe an ongoing multi-pronged search effort targeting the discovery of the axion through precision measurement with the ABRACADABRA and DMRadio detectors in conjunction with observation of extreme astrophysical systems that may generate smoking-gun signals of their interactions with axions. I will then show how state-of-the-art techniques in high-performance computing can be applied to simulate the nonlinear processes that generate axions in the early universe, leading to sharp predictions of the QCD axion mass and shedding light on the dynamics and observable signals associated with networks of topological defects.**Host:**Lisa Everett### Thursday, February 22nd, 2024

**R. G. Herb Condensed Matter Seminar****Superconductor — Insulator Transition and Coulomb drag in non-Fermi-Liquid modelled by an array of SYK grains****Time:**10:00 am - 11:00 pm**Place:**5310 Chamberlin**Speaker:**Alexander Chudnovskiy, Hamburg**Abstract:**We present a model of a strongly correlated system with a non-Fermi liquid high temperature phase. Its ground state undergoes an insulator to superconductor quantum phase transition (QPT) as a function of a pairing interaction strength. Both the insulator and the superconductor are originating from the same interaction mechanism. The resistivity in the insulating phase exhibits the activation behavior with the activation energy, which goes to zero at the QPT. This leads to a wide quantum critical regime with an algebraic temperature dependence of the resistivity. Upon raising the temperature in the superconducting phase, the model exhibits a finite temperature phase transition to a Bose metal phase, which separates the superconductor from the non-Fermi liquid metal. Furthermore, in the high-temperature non-Fermi-liquid phase, we analyse Coulomb drag and near-field heat transfer in a double-layer system of incoherent metals. The absence of quasiparticles in the strange metal leads to temperature-independent drag resistivity, which is in strong contrast with the quadratic temperature dependence in the Fermi liquid regime. We show that all parameters of the theoretical model can be independently measured in near-field heat transfer experiments, performed in Fermi liquid and strange metal regimes.**Host:**Alex Levchenko**Wisconsin Quantum Institute Colloquium****Exploring many-body problems with arrays of individual atoms****Time:**3:30 pm - 5:00 pm**Place:**Discovery Building, DeLuca Forum**Speaker:**Antoine Browaeys, Laboratoire Charles Fabry, Institut d’Optique, CNRS**Abstract:**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.This event starts at 3:30pm with refreshments, followed at 3:45pm by a short presentation by Atharva Vidwans (MSPQC student Micheline Soley group) titled "qDRIVE: A Variational Quantum Eigensolver for Resonance Identification on Near-Term Computers". The invited presentation starts at 4pm.

**Host:**Mark Saffman**Astronomy Colloquium****Building Planetary Systems: The Formation of Planetesimals****Time:**3:30 pm - 4:30 pm**Place:**4421 Sterling Hall**Speaker:**Jake Simon, Iowa State University**Abstract:**Planetesimals are small rocky (and sometimes icy) objects, typically 1-100 km in diameter (e.g., asteroids and comets in the Solar System). One of the largest unanswered questions in all of planetary astrophysics is: how do these planetesimals form in the disks that orbit young, newly forming stars (“circumstellar disks”)? In this talk, I will discuss my group’s research projects devoted to answering this important question. I will first provide an overview of planet formation and describe how planetesimals are an integral step in the planet formation process. I will then discuss the progress my group has made in understanding planetesimal formation with theoretical and computational models. In particular, by using computational fluid and particle dynamics, run on large-scale supercomputing facilities, we are working towards understanding under what conditions and in what locations planetesimals can or cannot form in circumstellar disks. I will conclude with a set of future goals to connect what we have learned so far to the larger picture of planet formation.**Host:**Ke Zhang### Friday, February 23rd, 2024

**Physics Department Colloquium****Symmetry in Quantum Field Theory****Time:**3:30 pm - 5:00 pm**Place:**Chamberlin 2241**Speaker:**Clay Córdova, University of Chicago**Abstract:**Symmetry is a powerful mathematical tool for organizing physical phenomena and anchors our understanding of the laws of nature. Over the past century, the theory of symmetry has evolved in parallel with quantum physics and has become fundamentally intertwined with quantum field theory and topology. Recently, new generalized concepts of symmetry have been developed and rekindled a longstanding hope for a classification of phases of field theories based on patterns of symmetry realization in the vacuum. These novel symmetries may also play a role in addressing long-standing mysteries of particle physics, such as hierarchies in the masses and couplings of elementary particles. I will survey these developments and applications.**Host:**Gary Shiu