CM Theory Seminar Recordings

Valentin Crepel (University of Toronto)

Building a Quantum Simulator from Stacks and Twists

Van der Waals heterostructures — stacks of two-dimensional materials held together by weak, non-chemical bonding forces — have triggered immense interest in recent years because their layer-by-layer assembly enables combining materials with vastly different properties and tuning their environment with external fields. This vast combinatorial landscape for creating compounds with tailored properties has fueled the vision of using these materials as programmable quantum simulators.

Yet, the curse of dimensionality quickly sets in: identifying which heterostructure is best suited to realize a particular quantum phase has remained a formidable challenge. A quantum simulator not only requires tunable degrees of freedom but crucially needs a guiding design principle to reach specific goals — and, once reached, sufficient experimental accessibility to probe it.

Here, I will describe how recent works have brought us closer to this design principle. After reviewing the architecture of van der Waals stacks and showcasing experimental probes that offer unique access to their physics, I will show how the motion and interactions of electrons within these heterostructures can be efficiently predicted. If time permits, I will briefly explore how these systems might transition from analog quantum simulators to digital quantum computers through methods such as inhomogeneous gating.

Ilya Gruzberg (Ohio State University)

Quantum Hall Transitions on Random Networks and Exact Results from Quantum Gravity

The paradigmatic Chalker-Coddington network model for the integer quantum Hall transition (QHT) was generalized to random networks. Numerical studies of the random networks showed that critical exponents of the integer QHT are modified by the geometric randomness. It was conjectured that the changes are similar to the ones induced by random geometry (two-dimensional quantum gravity) at critical points of conventional statistical mechanics models (Ising, Potts, O(N), etc.) and described by the so called Knizhnik-Polyakov-Zamolodchikov (KPZ) scaling relation. Here we investigate these issues for the spin QHT which can be mapped to classical percolation. The mapping works even in the presence of geometric disorder, and we solve the spin QHT on random networks as percolation on certain random graphs using methods of discrete quantum gravity (matrix models and loop equations). We confirm that the KPZ scaling works in this case and determine various exact critical exponents for the spin QHT on random networks. We also discuss how our findings are related to the (violation of the) Harris criterion and the Chase-Chase-Fisher-Spencer inequality.

Ilya Esterlis (University of Wisconsin Madison)

Quantum Dynamics in Two-dimensional Electron Crystals

Electron crystals — states of matter in which itinerant electrons spontaneously crystallize — feature prominently in the phase diagrams of two-dimensional materials. At experimentally relevant electron densities, these crystals are highly quantum: significant overlap between electron wave functions leads to frequent tunneling between lattice sites, giving rise to rich quantum dynamics. I will present new theoretical results on the consequences of these quantum effects in both monolayer and bilayer electron crystals. In monolayers, increasing electron density can drive a transition to a metallic charge density wave state, which can be viewed as a quantum crystal with a finite density of itinerant ground-state defects. In bilayers, interlayer Coulomb interactions stabilize new lattice geometries, where ring-exchange processes give rise to a variety of magnetic ground states, and metallic density wave states emerge under a small density imbalance between layers.

Clay Córdova (The University of Chicago)

Representation Theory of Solitons

Solitons in two-dimensional quantum field theory exhibit patterns of degeneracies and associated selection rules on scattering amplitudes. We develop a representation theory that captures these intriguing features of solitons. This representation theory is based on an algebra defined in terms of the non-invertible symmetry, a fusion category, and its action on boundary conditions. We present a straightforward method for analyzing these representations in terms of quiver diagrams where nodes are vacua and arrows are solitons and provide examples demonstrating how the representation theory reproduces known degeneracies and implies new ones in 2d QCD. Our analysis provides the general framework for analyzing non-invertible symmetry on manifolds with boundary and applies both to the case of boundaries at infinity, relevant to particle physics, and boundaries at finite distance, relevant in conformal field theory or condensed matter systems.

Raquel Queiroz (Columbia University)

Quantum Geometry: How to Picture Bound Electrons in Periodic Lattices

The concept of quantum geometry has been at the forefront of condensed matter physics, starting from how quantized Berry curvature leads to quantized Hall conductivity, anomalous velocities in Dirac metals, or other topological responses in a growing list of so-called topological materials. Recently, the real part of the quantum geometric tensor - the quantum metric - has also been suggested to play an important role, both in response and in the tendency for materials to assume correlated ground states at low temperatures. In this talk, I will give a local picture of quantum geometry to create an intuition about what it is and when it is essential, relating it to how bonds are formed in infinite lattices.

Rahul Nandkishore (University of Colorado, Boulder)

Quantum Games and Many-body Physics on Quantum Hardware

The advent of quantum hardware provides a new playground for many body physics. Nonlocal quantum games provide a new approach to diagnose and harness the correlations inherent in phases of matter, on quantum hardware. I will introduce the notion of many body quantum games, Then, I will introduce a class of quantum games that allow the robust identification and harnessing of topological (and fracton) order on quantum hardware. I will discuss some interesting open directions for this field.

Patrick A. Lee (Massachusetts Institute of Technology and California Institute of Technology)

Strongly Driven Superconductors: What the Data tell us about Cuprates and Organicx

In the past decade, Cavalleri's group has reported "superconducting-like" behavior up to several times Tc in HiTc cuprates, the organic superconductors and K3C60. I will review some of the data and focus on the first two systems. In collaboration with M. Michael and E. Demler, I undertook a new analysis of the YBCO data. The goal is to find the minimal set of assumptions which can explain the observations. Unlike earlier discussions, we find that intense drive does not enhance the in-plane pair correlation or the inter-bilayer correlation. However, in order to explain the data, short range order within the plane and correlation between members within the bi-layer are required in the equilibrium state. The implication is that pairing correlation survives up to the pseudogap scale. Therefore the pseudogap is a pairing gap. For the organic superconductor, I will also discuss a proposal with Zhehao Dai that the observed enhanced gap under drive is an induced Mott gap rather than a pairing gap.

Liang Fu (Massachusetts Institute of Technology)

Mott, Chern and Wigner Insulators in Semiconductor Heterostructures

The advent of moire superlattices in van der Waals heterostructures opens a new venue for exploring quantum phases of matter with unprecedented tunability. I will describe various kinds of quantum phases at integer fillings in semiconductor bilayer systems at zero magnetic field, including Mott and Chern insulators at the filling of one charge per unit cell, as well as Wigner solids at higher integer fillings.

**There were technical issues with the video recording of this seminar, so only audio is available.**

Dan Stamper-Kurn (University of California, Berkeley)

Many-body Physics of Atoms in Optical Lattices and Optical Cavities

Ultracold atomic gases allow us to create a variety of many-body quantum systems, which we might regard as synthetic quantum materials. In some of these systems, we encounter many of the properties pertinent to condensed-matter systems. In that vein, I will present experiments on atoms in optical lattices where we test a scaling hypothesis that relates phase transitions occurring in different lattice configurations and examine transport and equilibration of itinerant particles in geometrically frustrated bands. I will also share some results obtained from experiments on atoms placed within optical cavities, where they interact strongly with an optical field. This field can serve both as a precise probe, e.g. allowing us to examine non-equilibrium thermodynamics of a mesoscopic system, and also as a dynamical component within a hybridized atom + matter quantum system. I hope to spark some discussion where we can generate some new ideas on applications of quantum simulation, on feedback-controlled quantum matter, and on phase transitions in open quantum systems.

Erez Berg (Weizmann Institute)

What does the Wiedemann-Franz Law Tell Us About Non-Fermi Liquids?

The Wiedemann-Franz (WF) law, stating that the Lorenz ratio L=κ/Tσ between the electronic thermal and electrical conductivities in a metal approaches a universal constant L_0 at low temperatures, is often taken to be a signature of fermionic Landau quasi-particles. In contrast, we show that various models of weakly disordered non-Fermi liquids, where the fermionic quasi-particles are either marginally defined or ill-defined, also obey the WF law at T→0. Instead, we argue that the behavior of the leading correction to the WF law at low temperature distinguishes different types of strange metals. In particular, in a solvable model of a marginal Fermi liquid, we find that the leading low-temperature deviation of L-L_0 scales as T, in contrast to a Fermi liquid where it is proportional to T^2. Moreover, by invoking a quantum Boltzmann equation approach, we demonstrate that this behavior is generic in a class of marginal- and non-Fermi liquids characterized by a weakly momentum-dependent inelastic scattering rate.

CM Theory Seminar Recordings

2026