2025 Simons Collaboration on Ultra Quantum Matter Annual Meeting

The 6th annual meeting of the Simons Collaboration on Ultra-Quantum Matter (UQM) was held January 21–22, 2025, with over 110 in person participants. In addition to the speakers, the PIs and postdoctoral fellows of the UQM Collaboration, and other students and postdocs, the meeting was attended by nearly 25 other faculty members working on various aspects of UQM.

The meeting included three talks that introduced new conceptual and computational tools, which were then used to offer fresh insights into longstanding problems. These included the opening talk by Xie Chen that utilized the ‘sandwich’ approach to symmetry breaking and topology. This approach efficiently reinterprets dualities between different phases and transitions, in terms of boundary states of bulk topological orders. By changing the boundary terminations of a slab of 2D toric code, the famous Kramers-Wannier duality was recovered. This method was then extended to relate various other problems in 1+1 and 2+1 dimensions. The following talk was given by John McGreevy, who discussed a modern renormalization group approach to the problem of electron crystallization, which could have applications for the recently discovered crystalline phases in 2D electronic systems. The talk by Max Metlitski outlined a semiclassical approach to impurities and boundaries coupled to critical fluctuations, which was then utilized to extract the full scaling functions of the classic Kondo impurity problem, in both the under- and over-screened cases. He also described extensions to line defects at the three-dimensional Ising transition which was solved using the bootstrap approach.

The first day also featured two talks on twisted bilayer graphene (TBG). Patrick Ledwith’s theory talk presented the first analytical calculation of the electron spectral function of TBG in an intermediate-temperature ‘fluctuating-moment’ regime. He explained the unusual spectral features that emerged from his calculations using a picture of nonlocal moments. This was followed by an experimental talk by Shahal Ilani, who reported remarkable progress on the quantum twisting microscope, especially of momentum resolved spectral features in magic angle TBG.

The second day began with an insightful talk by Nathan Seiberg, who pointed out the subtle interplay between background gauge charge and translation symmetry in quantum systems in various dimensions. Connections were made between the famous ABJ anomaly and the issue of translation symmetry in continuum ferromagnets.

The final two talks focused on achieving coherent and entangled many body states in a nonequilibrium setting, rather than relying on the traditional approach of cooling into a many-body ground state. Monika Schleier-Smith described experiments in her lab that utilized the photons in an optical resonator to engineer interactions between well-separated atoms. She showed how these systems could be used to naturally engineer interesting models that would be challenging with other setups with local interactions. These included toy models of gravity and entangled states which give a quantum boost to sensing. The meeting concluded with Matthew Fisher’s stimulating talk on the conditions that allow ultra-quantum matter to persist, despite facing both incoherent and coherent dynamics, through the process of measurement.

Xie Chen
California Institute of Technology

Are all Phase Transitions Symmetry Breaking Transitions?
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Xie Chen will discuss a way to think about transitions between gapped phases using the topological holography framework such that various non-Landau transitions can be reinterpreted as symmetry breaking transitions.
 

Matthew Fisher
University of California Santa Barbara

Quantum Dynamics of the 1d Repetition Code
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Fault tolerant error correction thresholds for quantum codes are traditionally obtained via mappings to classical statistical mechanics models. For example, the 1d repetition code subject to bit-flip noise and faulty measurements, which has a dynamical Ising Z2 symmetry, is mapped to the classical 2d random bond Ising model. Here, we revisit the 1d repetition code and develop an exact “stabilizer expansion” of the full time evolving density matrix that gives a dual representation of the classical 2d random bond Ising model. However, with generic Z2 respecting dynamics the stabilizer expansion breaks down and a full quantum description is required. The resulting steady state of the quantum dynamics has three possible phases which can be characterized by the spontaneous breaking of “strong” and “weak” Ising symmetries. Classicality follows if the strong, but not weak, symmetry is spontaneously broken — recovering the 2d classical random bond Ising model. If neither symmetry is broken one has a non-trivial mixed state density matrix that describes a “quantum paramagnet.” And with both strong and weak symmetry breaking the steady state retains all encoded information of the quantum code.
 

Shahal Ilani
Weizmann Institute of Science

The Interacting Energy Bands of Magic Angle Twisted Bilayer Graphene Revealed with the Quantum Twisting Microscope

One of the core mysteries of magic-angle twisted bilayer graphene (MATBG) lies in understanding the nature of its interacting energy bands. While MATBG has shown topological phenomena, explained by topological Chern bands in momentum space, its electronic behavior also displayed localized characteristics, hinting at a real-space picture. This dichotomy has led to various theoretical models, including the topological heavy fermion model and the Mott semimetal framework, each attempting to reconcile how these contrasting features emerge within the flat bands of MATBG.

Until now, no tool has been capable of imaging these energy bands at low temperatures and with high enough energy and momentum resolution to resolve these puzzles. Recently, we developed the Quantum Twisting Microscope (QTM), which utilizes momentum-resolved tunneling at a twisting van der Waals interface to directly map the energy bands of quantum materials. So far, however, our measurements of electronic bands have been at room temperature.

In this talk, Shahal Ilani will present the first cryogenic measurements of the MATBG bands which reveal its interacting energy bands.
 

Patrick Ledwith
Harvard University

Nonlocal Moments and Mott Semimetal in the Chern Bands of Twisted Bilayer Graphene
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Twisted bilayer graphene (TBG) has elements in common with two paradigmatic examples of strongly correlated physics: quantum Hall physics and Hubbard physics. On the one hand, TBG hosts flat topological Landau-level-like bands which exhibits quantum anomalous Hall effects. On the other hand, these bands have concentrated charge density and show signs of extensive entropy resembling local moments. The combination of these features leads to a question: can decoupled moments emerge in an isolated topological band, despite the lack of exponentially localized Wannier states? In this work, we answer the question affirmatively by proposing a minimal model for these bands in TBG that combines topology and charge concentration at the AA sites, leading to analytic wavefunctions that closely approximate those of the BM model with realistic parameters. Importantly, charge concentration also leads to Berry curvature concentration at Γ, generating a small parameter “s” that yields analytic tractability. We show that, rather surprisingly, the model hosts nearly decoupled flavor moments without any extra degrees of freedom. These moments are non-local due to topology-enforced power-law tails, yet have parametrically small overlap. We further develop a diagrammatic expansion in which the self energy can be computed exactly to leading order in s^2 in the fluctuating moment regime. At charge neutrality, we find a “Mott semimetal,” with large flavor entropy and a Mott gap everywhere in the BZ except for the vicinity of the Γ point. Away from neutrality, the Mott semimetal gaps out in a spectrally imbalanced manner, with one Mott band having zero Zk at the Γ point. The model accurately reproduces results from finite temperature thermodynamic measurements, leads to new experimental predictions, and resolves the problem of the emergence of Hubbard physics in isolated topological bands.
 

John McGreevy
University of California San Diego

Death of a Fermi Liquid by Freezing
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John McGreevy will describe a direct transition between a liquid metal and a solid. The critical point has a Fermi surface as well as a Bose surface, a sphere in momentum space of gapless bosonic excitations. We can find a fixed point of the renormalization group governing such a non-Fermi liquid using an expansion in the codimension of both the Fermi and Bose surfaces. McGreevy will discuss some possibilities for the nature of the solid phase.

This talk is based on work with Tarun Grover.
 

Max Metlitski
Massachusetts Institute of Technology

New Approaches to the Kondo Problem in Fermi & Bose Systems
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The Kondo model of a spin impurity in a Fermi gas is an example of strong coupling physics that has for decades provided fruitful ground for the development of new ideas and methods from the renormalization group to Bethe ansatz. In this talk, Max Metlitski present a new analytical renormalization group approach to the Kondo model that quantitatively captures the full weak to strong coupling crossover in the regime of large impurity spin. In the second part of the talk, Metlitski will introduce a new numerical conformal bootstrap method for quantum impurities (line defects) in 2+1D conformal field theories and illustrate it with an application to the 2+1D Ising model.
 

Monika Schleier-Smith
Stanford University

Atoms Interlinked by Light: Programmable Nonlocal Interactions for Quantum Simulation
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The connectivity of interactions in a quantum simulator plays a crucial role in dictating which problems can efficiently be mapped onto the hardware. While local interactions are well suited to simulating a wide range of problems in condensed-matter physics, adding nonlocal connectivity opens the door to new applications, from simulating models of quantum gravity to manipulating topologically encoded quantum information. Monika Schleier-Smith will report on experiments in which we achieve programmable nonlocal connectivity within an array of atom clouds trapped in an optical resonator, letting photons mediate interactions between distant atoms. We have harnessed this toolbox to simulate a toy model of holographic duality and to interferometrically probe topological edge states. To illustrate the capacity for accessing demonstrably entangled states, we have further demonstrated a versatile protocol for preparing graph states enabling a quantum enhancement in multiparameter sensing.
 

Nathan Seiberg
Institute for Advanced Study

Anomalous Continuous Translations
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Nathan Seiberg will discuss a large class of non-relativistic continuum field theories where the Euclidean symmetry of the classical theory is violated in the quantum theory by an Adler-Bell-Jackiw-like anomaly. In particular, the continuous translation symmetry of the classical theory is broken in the quantum theory to a discrete symmetry. Furthermore, that discrete symmetry is extended by an internal symmetry. Seiberg will show that in some cases, the anomalous continuous translation symmetry is resurrected as an exact noninvertible continuous translation symmetry. Seiberg will also discuss the relation between this phenomenon and underlying lattice models.