Physics Department Colloquium

Spin glasses are canonical examples of complex matter and form a basis for describing artificial neural networks.  Repeatable control over microscopic degrees of freedom might open a new window into their structure and dynamics.  I will present how we achieved this at the atomic level using a quantum-optical system comprised of ultracold gases of atoms coupled via photons resonating within multimode cavities.

Abstract: Effective field theories (EFTs) form the basis of our most successful theories of matter, both in particle physics and in condensed matter physics. But the structure of EFTs poses a challenge to many standard philosophical accounts of theory structure and content. In particular, the inability to cast EFTs in terms of exact mathematical objects defined at all scales suggests that philosophical accounts of theory interpretation ought to be modified to deal with approximate, scale-relative ontologies.

Abstract: Low energy collisions between molecules in the atmosphere lead to about 50% of the particles that act as the seeds for cloud droplets. Many of these molecules, and many of the other particles, are the result of human activity. Therefore cloud droplet concentrations have increased over the industrial period. The increase has led to a poorly quantified cooling effect on Earth that has offset perhaps a third of historical warming from greenhouse gases. The CLOUD experiment at CERN is a laboratory facility for the study of atmospheric particle formation.

Abstract: The mineralogy of our planet is a fingerprint of history—a durable archive of the physical and chemical conditions that have evolved over 4.5 billion years. Minerals record temperatures and pressures, redox states and fluid compositions, preserving evidence that spans the earliest violent collisions of solar-system formation to human activities that occurred only yesterday. Yet Earth’s mineral story reaches far deeper in time, extending back to the very origins of the elements themselves.

Synchronization—the spontaneous emergence of phase coherence among interacting oscillators—is a ubiquitous phenomenon in classical systems, from pendulum clocks to biological rhythms. In quantum systems, however, coherence is fragile, and environmental noise is usually viewed as its primary adversary. This colloquium explores a counterintuitive regime in which noise itself becomes a resource, driving rather than destroying coherent behavior.

Earth's clouds are critical for weather and climate. Cloud formation in earth and other planetary atmospheres is a deceptively simple physical process of condensation. And yet clouds are very challenging to understand and predict due to the interplay of clouds with their environment. Cloud physics spans 12 orders of magnitude in space from the micro-scale of cloud drops to the planetary scale of the general circulation and a similar order of magnitude in time from fractions of a second of cloud drop collisions to centennial climate time scales.

In 2017-2018 liquid crystal research groups working independently in the UK and Japan, exploring two distinct families of rod-shaped organic molecules, each reported an unknown nematic-like liquid crystal phases in their materials. In 2020 we showed that the unknown phase in the UK compound, RM734, was a ferroelectric nematic: a 3D liquid phase with a fluid spontaneous polarization field, P. This was a notable event in LC science because ferroelectricity was put forth in the 1910’s, by Peter Debye and Max Born, as a possible stabilizing mechanism for the nematic phase.

Moiré superlattices formed from transition metal dichalcogenide (TMDC) heterostructures have emerged as a compelling platform for exploring quantum many-body physics. These systems are viewed as a solid-state counterpart to ultracold atomic gases in optical lattices for quantum simulation. A central open question concerns the coherence and dynamics of quantum phases arising from photoexcited moiré excitons, especially under dissipative conditions.

Abstract: A new zoo consisting of dozens of heavy subatomic particles that contain more than three quarks and antiquarks have been discovered beginning in 2003.  Although they must be described by the fundamental quantum field theory QCD, the pattern of these exotic heavy hadrons remained unexplained for more than 20 years.  I will present a simple proposal for the pattern based on the Born-Oppenheimer approximation for QCD.  There are simple calculations in lattice QCD that would corroborate the pattern.

The year 2024 was a breakthrough year towards the development of a nuclear optical clock, with three experiments reporting success in the laser spectroscopy of the lowest nuclear excited state of 229-Th. The highest accuracy was achieved at JILA via direct frequency comb spectroscopy of this, previously elusive, nuclear state. This success is the result of several decades of effort to precisely determine the transition energy and a first step towards nuclear precision spectroscopy and the development of a nuclear frequency standard of extremely high accuracy.