Events at Physics |
Events During the Week of November 2nd through November 8th, 2025
Monday, November 3rd, 2025
- Plasma Physics (Physics/ECE/NE 922) Seminar
- Title to be announced
- Time: 12:00 pm - 1:00 pm
- Place: 2241 Chamberlin Hall
- Speaker: Felix Parra-Diaz, PPPL
- Host: Cary Forest
- Preliminary Exam
- Direct measurement of the energy spectrum of a quantum dot artificial molecule
- Time: 12:30 pm - 2:00 pm
- Place: Chamberlin Hall Room 5310
- Speaker: Jack Reily
- Abstract: Quantum dot artificial atoms in semiconducting SiGe/Si/SiGe heterostructures, formed by strong electronic confinement, are promising scalable qubit systems. The single- and few-electron spectra consist of multiple valley-orbital levels, which, in general, modify tunnel couplings and qubit dynamics, which is relevant for qubit encoding and readout. Here, we provide a new method of directly measuring the energy spectrum of an artificial molecule for fixed electron number. By tuning square wave pulses along the delta axis of the artificial molecule, we show how the curvature governed by excited-to-excited, ground-to-excited, and ground-to-ground couplings can all be extracted simultaneously. This new measurement provides the ability to extract many relevant Hamiltonian parameters, which are necessary for the precise operation and control of spin qubits.
- Host: Albrecht Karle
- Theory Seminar (High Energy/Cosmology)
- Probing Dark Matter with Pulsar Timing Arrays and Gravitational Wave Detectors
- Time: 1:00 pm - 2:30 pm
- Place: Chamberlin 5280
- Speaker: Vincent S.H. Lee, UC, Berkeley
- Abstract: Pulsar timing arrays (PTAs) and gravitational wave detectors can serve as valuable tools in the detection of dark matter. Dark matter substructure within the Milky Way Galaxy can induce gravitational pulls on pulsars, leading to observable deviations in pulsar timings. We demonstrate that dark matter models predicting enhanced power on small scales (< pc) are potentially within the reach of future PTA experiments, such as the Square Kilometer Array (SKA). This includes a class of post-inflationary QCD axion models known as axion miniclusters. We also discuss pipelines for searching for dark matter signals in PTA datasets and recent results from the 15-year data release of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), which has reported evidence of a stochastic gravitational background. Furthermore, dark matter particles with a mass on the kilogram scale can also produce signals in laser interferometry-based gravitational wave experiments. We discuss the signals and detection prospects from current and future gravitational wave experiments, such as LIGO and the Einstein Telescope.
- Host: Joshua Foster
Tuesday, November 4th, 2025
- No events scheduled
Wednesday, November 5th, 2025
- Thesis Defense
- Laboratory Investigation of Collisionless Kinetic Effects and 3D Topological Triggers for Magnetic Reconnection
- Time: 11:00 am - 1:00 pm
- Place: Chamberlin 5310
- Speaker: Paul Gradney
- Abstract: Magnetic reconnection is a fundamental plasma process responsible for the rapid conversion of magnetic energy into particle energy. This process occurs ubiquitously in astrophysical, space, and laboratory plasmas, including solar flares, Earth's magnetosphere, and fusion devices. Despite its importance, key questions remain, including how reconnection dynamics are modified in the collisionless kinetic regime, and what mechanisms trigger fast reconnection in three-dimensional (3D) systems as they transition from stable or quasi-stable magnetic configurations to configurations that enable rapid energy release.
This thesis presents experimental studies of collisionless magnetic reconnection using the Terrestrial Reconnection EXperiment (TREX) at the Wisconsin Plasma Physics Laboratory (WiPPL). To minimize collisional effects and access the kinetic regime relevant to space plasmas, a reconnection Drive Cylinder was developed. This system increases both the effective system size and the driving rate of reconnection, enabling TREX to reach a regime where electron dynamics are dominated by kinetic rather than collisional processes. Technical details of the Drive Cylinder design, including its coil configuration and magnetic field shaping, are presented along with measurements of its performance.
Using the 3-coil TREX configuration, the dynamics of 3D reconnection were investigated in a laboratory-generated mini-magnetosphere. Observations demonstrate that the explosive onset of reconnection can be triggered by a bifurcation of the magnetic topological structure. In particular, the rapid approach and crossing of separatrix surfaces led to the bifurcation of magnetic null points, restructuring of separator lines, and a marked increase in the reconnection electric field along the separator. These results provide experimental evidence that geometric reconfiguration of a 3D magnetic topology can initiate the onset of fast reconnection. The findings have broad implications for understanding space weather, astrophysical explosions, and relaxation events in magnetically confined laboratory plasmas. - Host: Jan Egedal
Thursday, November 6th, 2025
- R. G. Herb Condensed Matter Seminar
- Searching for Experimental Signatures of Fractionalization
- Time: 10:00 am
- Place: 5310 Chamberlin
- Speaker: Pietro Bonetti, Harvard
- Abstract: In this talk, I will present some of my recent work which aims at understanding experimental observations in several strongly correlated platforms—cuprates, cold atomic simulators and organic compounds—in terms of exotic fractionalized states exhibiting topological order.
In the cuprates, the pseudogap phase shows multiple signatures of reconstruction of the large Fermi surface into four small hole pockets. In the absence of spontaneous symmetry breaking, this appears to contradict Luttinger’s theorem, which predicts that the volume enclosed by the Fermi surface remains constant. The resolution of this apparent contradiction lies in assuming that the pseudogap phase hosts fractionalized excitations with emergent gauge fields. I will show that these excitations are crucial not only for preserving Luttinger’s theorem but also for reproducing the Fermi surface topology observed in quantum oscillation measurements at low temperatures, where charge density wave order develops and further reconstructs the four hole pockets.
In the second part of the talk, I will discuss a recent experiment realizing a Hubbard model on a Lieb lattice in a cold atomic simulator [Lebrat et al., arXiv:2404.17555 (2024)]. At half-filling, a flat band was observed for zero Hubbard U, and a ferrimagnetic state for nonzero U, in agreement with theoretical predictions. In the presence of strong interactions, a new flat band was reported to emerge at quarter filling without symmetry breaking. Employing parton theories, we consider quantum fluctuations of several magnetic ground states identified using Hartree-Fock theory. We find a state with a flat band at the Fermi level coexisting with fractionalized bosonic spinon excitations carrying Z2 gauge charges. This state represents a striking realization of doping-induced fractionalization.
If time permits, I will briefly review how ultrafast spectroscopic experiments in the cuprates and in the organic compound κ-(BEDT-TTF)2Cu2(CN)3 can be interpreted in terms of fractionalized excitations. - Host: Elio König
- Astronomy Colloquium
- Dynamics and Energetic Transients in Galactic Nuclei
- Time: 3:30 pm - 4:30 pm
- Place: 4421 Sterling Hall
- Speaker: Dr. Itai Linial, Hubble Fellow at NYU
- Abstract: Centers of galaxies host a variety of dynamical processes, owing to the high density of stars and the presence of a central Supermassive Black Hole (SMBH). Orbital relaxation brings stellar objects into close encounters with the SMBH, driving a plethora of energetic phenomena. These include tidal disruption events, stellar collisions, the formation of X-ray binaries, compact object mergers detectable by LIGO, and gravitational wave (GW) inspirals of stellar objects towards the SMBH. In this talk, I will review some of these processes and their observable signatures, with an emphasis on newly discovered classes of repeating flares associated with SMBHs. I will discuss theoretical models, key open questions, and how these phenomena inform our understanding of accretion physics, SMBH growth and evolution, and the connection to their host galaxies. A powerful array of current and upcoming time-domain surveys and instruments will uncover thousands of high-energy sources in galactic nuclei in the coming years. I will highlight how theoretical interpretation of these discoveries can address fundamental questions in astrophysics, and conclude with broader implications for multi-messenger observations in the era of space-based GW detectors such as LISA.
- Host: Nicholas Stone
Friday, November 7th, 2025
- Physics Department Colloquium
- Opportunities and Challenges of the AI Revolution for Scientific Research
- Time: 3:30 pm - 4:30 pm
- Place: Chamberlin 2241
- Speaker: Rick Stevens, University of Chicago
- Host: Uwe Bergmann