Colloquia This Term

Single-particle control and detection of strongly correlated quantum many-particle systems has enabled a wide variety of applications including quantum simulation of condensed matter systems and quantum computing demonstrations. Exact numerical simulations on classical computers are intractable for most quantum many-body systems beyond a few particles. Quantum simulators can answer questions by implementing the Hamiltonian of interest, while digital quantum computers realize universal quantum computing using a set of gates.

In the first part of the talk, I will give an introduction into the field of quantum simulation using ultracold atoms with focus on quantum gas microscopy techniques which allow us to probe many-body systems at the single-particle level. Over the past years, we developed a platform to study geometrically frustrated Hubbard systems and reveal their quantum correlations. We prepared fermionic atomic Mott insulators on a triangular lattice, detected them with single-site resolution and measured spin-spin correlations. Currently we are working on upgrades to the experiment which enable the study of kinetic magnetism and exotic quantum phases.

The second part of the talk will cover the progress of our new experiment with ytterbium atoms in optical tweezers. I will discuss the advantages and disadvantages of the tweezer platform and present our plans to realize large entangled states.

We acknowledge funding by NSF (CAREER award PHY-2047275), ONR (DURIP award N00014-22-1-2681), AFOSR (award FA9550-23-1-0166), the Thomas F. and Kate Miller Jeffress Memorial Trust and the Jefferson Trust.

Rapid cooling or heating of a physical system can lead to unusual thermal relaxation phenomena. A prime example of anomalous thermal relaxation is the Mpemba effect. The phenomenon occurs when a system prepared at a hot temperature overtakes an identical system prepared at a warm temperature and equilibrates faster to the cold environment. A similar effect exists in heating. Comparing two identical physical systems in their equilibration, we would expect that the system with a smaller mismatch between its and the environment’s temperature will thermalize faster – yet it is not always the case. I will present theoretical results on the Mpemba effect in over-damped Langevin dynamics and Markov jump processes. I will link the Mpemba effect’s occurrence with the physical systems’ properties and dynamics. In particular, I will derive the necessary conditions for the Mpemba effect in the small diffusion limit of one-dimensional over-damped Langevin dynamics on a double-well potential. Our results show the strong Mpemba effect occurs when the probability of being in a well at initial and bath temperature match, which agrees with experiments. I also derive the conditions for the weak Mpemba effect and express the conditions for both effects in terms of mean first passage time. Next, I will provide analytical results and insights on when the Mpemba effect happens in Markov jump processes as a function of the dynamics. Markov jump processes that obey detailed balance (microscopic reversibility) relax to equilibrium. However, the detailed balance only determines the ratio of the backward and forward rates, not their magnitudes. The magnitudes specify the dynamics. I will introduce a control parameter to vary the dynamics and show when we see the effect as a function of the dynamics. Lastly, I will explore the connections between the Mpemba effect and optimal transport. This material is based upon work supported by the National Science Foundation under Grant No. DMR-1944539.

Precise and reliable measurements of neutron star radii are essential to our understanding of cold, catalyzed matter beyond nuclear saturation density.  Recently, NASA's Neutron Star Interior Composition Explorer (NICER) satellite has provided high-quality data sets that have yielded measurements of the mass (M=1.44+-0.15 Msun) and radius (R=13+1.2-1.0 km) of the 206 Hz pulsar PSR J0030+0451, and of the radius (R=13.7+2.6-1.5 km) of the M=2.08+-0.07 Msun, 346 Hz pulsar PSR J0740+6620.  I will discuss our group's work on these pulsars and will in particular discuss the assumptions that have gone into our analyses, to help in the assessment of our results.  I will also discuss the implications of our results, combined with other observations including of gravitational waves, for the properties of the dense matter in the cores of neutron stars.

Dark matter makes up 85% of the matter in our Universe, but we have yet to learn its identity.  While most experimental searches focus on Weakly Interacting Massive Particles (WIMPs) with masses above the proton (about 1 GeV/c^2), many natural dark-matter candidates have masses below the proton and are invisible in traditional WIMP searches.  In this talk, I will discuss the search for dark matter with masses between about 500 keV/c^2 to 1 GeV/c^2 (“sub-GeV dark matter”), which has seen tremendous progress in the last few years.  I will describe several direct-detection strategies, and discuss how to search for dark matter interactions with electrons and nuclei in various target materials, such as noble liquids and semiconductors.  I will in particular highlight SENSEI, a funded experiment that will uses new ultra-low-threshold silicon CCD detectors (“Skipper CCDs”) capable of detecting even single electrons.  I will describe the latest results from SENSEI, and how we expect to probe orders of magnitude of novel dark matter parameter space in the next few years.  

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In the absence of a detailed first principles based understanding of condensed matter phenomena a scaling approach is many times a good starting point. Although it is also true that scaling in physical properties and laws can also be a consequence or flow out of more fundamental theories.  In this talk I will examine two rather different physical systems, one metallic and the other primarily insulating, where scaling ideas have helped us to understand our measurements.  In the heavy fermion (metallic) systems I will review our work on scaling properties observed in their magnetic, ultrasonic and thermal response.  In a quantum spin liquid system (insulating) I will describe recent work where scaling ideas have led to a confirmation of a Dirac Quantum Spin liquid state.

Earlier this summer, the pulsar timing array community announced strong evidence for the presence of a stochastic background of nanoHertz frequency gravitational waves. This has been the primary goal of the community for the past two decades, and it took thousands of hours of telescope time, over 500,000 pulse arrival times from ~70 millisecond pulsars, and a highly sophisticated and very computationally demanding analysis effort to accomplish. While we can't yet say for certain what is causing the gravitational waves, our best guess is a population of slowly merging super-massive black hole binaries throughout the universe. But it is possible that the signal also heralds new physics. So what does it all mean and what are we expecting next? And what other cool things can we do with all of this high-precision pulsar data?

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High energy (> TeV) neutrinos are unique messengers to the distant, high-energy universe. As chargeless and weakly interacting particles, neutrinos arrive from cosmic distances, giving us insights to the nature of astrophysical accelerators like black holes and gamma ray bursts. In this talk, I will discuss the ongoing work of the IceCube Neutrino Observatory to detect and study extraterrestrial neutrinos. I will review how the IceCube detector, which is a cubic kilometer instrument buried deep at the South Pole, detects high energy neutrinos. I will then discuss the latest physics results of the detector, including efforts to measure and characterize the high energy neutrino flux and to find neutrino sources.