2025 Simons Collaboration on Ultra Quantum Matter Annual Meeting

Xie Chen
California Institute of Technology

Are all Phase Transitions Symmetry Breaking Transitions?

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

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.
 

Michael Hermele
University of Colorado Boulder

Navigating the Fracton Landscape with P-Modularity

There are now many examples of gapped fracton models, which are defined by the presence of restricted-mobility excitations above the quantum ground state. However, because the theory of fracton orders has been challenging to develop and remains in its early stages, the complex landscape of examples is far from being mapped out. In this talk, following a survey of what is known about fracton matter, Hermele will describe new progress in this direction by introducing the p-modular fracton orders. These are a relatively simple yet still rich class of quantum orders, for which a substantial amount of general theory can be developed starting from the definition. Michael Hermele will describe this structure and associated new phase invariants, and discuss progress toward the characterization and classification of p-modular fracton orders.
 

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

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

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

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

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.

Participation in the meeting falls into the following four categories. An individual’s participation category is communicated via their letter of invitation.

Group A – PIs and Speakers

Economy Class: For flights that are three hours or less to your destination, the maximum allowable class of service is Economy class.
Premium Economy Class: For flights where the total air travel time (excluding connection time) is more than three hours and less than seven hours per segment to your destination, the maximum allowable class of service is premium economy.
Business Class: When traveling internationally (or to Hawaii/Alaska) travelers are permitted to travel in Business Class on those segments that are seven hours or more. If the routing is over budget, a premium economy or mixed-class ticket will be booked.

Group B – Funded Participants

The foundation will arrange and pay for round-trip air or train travel to the conference as well as hotel accommodations and reimbursement of local expenses. Economy-class airfare will be booked for all flights.

Group C – Unfunded Participants

Individuals in Group C will not receive financial support, but are encouraged to enjoy all conference-hosted meals.

Group D – Remote Participants

Individuals in Group D will participate in the meeting remotely.

Air and Rail

For funded individuals, the foundation will arrange and pay for round-trip travel from their home city to the conference.

All travel and hotel arrangements must be booked through the Simons Foundation’s preferred travel agency.

Travel Deviations

The following travel specifications are considered deviations and will only be accommodated if the cost is less than or equal to the amount the Simons Foundation would pay for a standard round-trip ticket from your home city to the conference city:

• Preferred airline
• Preferred travel class
• Specific flights/flight times
• Travel dates outside those associated with the conference
• Arriving or departing from an airport other than your home city or conference city airports, i.e. multi-segment or triangle trips.

All deviations must be reviewed and approved by the Simons Foundation and, if the cost is in excess of what would normally be paid, a reimbursement quote must be obtained through the foundation’s travel agency before proceeding to booking and paying for travel out of pocket. All reimbursements for travel booked directly will be paid after the conclusion of the meeting.

Changes After Ticketing

All costs related to changes made to ticketed travel are to be paid for by the participant and are not reimbursable. Please contact the foundation’s travel agency for further assistance.

Personal & Rental Cars

Personal car and rental trips over 250 miles each way require prior approval from the Simons Foundation via email.

Rental cars must be pre-approved by the Simons Foundation.

The James NoMad Hotel offers valet parking. Please note there are no in-and-out privileges when using the hotel’s garage, therefore it is encouraged that participants walk or take public transportation to the Simons Foundation.

Hotel

Funded individuals who require hotel accommodations are hosted by the foundation for a maximum of three nights at The James NoMad Hotel, arriving one day before the meeting and departing one day after the meeting.

Any additional nights are at the attendee’s own expense. To arrange accommodations, please register at the link included in your invitation.

The James NoMad Hotel
22 E 29th St
New York, NY 10016
(between 28th and 29th Streets)
https://www.jameshotels.com/new-york-nomad/

For driving directions to The James NoMad, please click here.

Overview

In-person participants will be reimbursed for meals and local expenses including ground transportation. Expenses should be submitted through the foundation’s online expense reimbursement platform after the meeting’s conclusion.

Expenses accrued as a result of meetings not directly related to the Simons Foundation-hosted meeting (a meeting held at another institution, for example) will not be reimbursed by the Simons Foundation and should be paid by other sources.

Below are key reimbursement takeaways; a full policy will be provided with the final logistics email circulated approximately 2 weeks prior to the meeting’s start.

Meals

The daily meal limit is $125; itemized receipts are required for expenses over $24 USD. The foundation DOES NOT provide a meal per diem and only reimburses actual meal expenses up the following amounts.

• Breakfast $20
• Lunch $30
• Dinner $75

Allowable Meal Expenses

• Meals taken on travel days (when you traveled by air or train).
• Meals not provided on a meeting day, dinner on Friday for example.
• Group dinners consisting of fellow meeting participants paid by a single person will be reimbursed up to $75 per person and the amount will count towards each individual’s $125 daily meal limit.

Unallowable Meal Expenses

• Meals taken outside those provided by the foundation (breakfast, lunch, breaks and/or dinner).
• Meals taken on days not associated with Simons Foundation-coordinated events.
• Minibar expenses.
• Meal expenses for a non-foundation guest.

• Ubers, Lyfts, taxis, etc., taken to and from restaurants in Manhattan.

  • Accommodations will be made for those with mobility restrictions.

Ground Transportation

Expenses for ground transportation will be reimbursed for travel days (i.e. traveling to/from the airport or train station) as well as subway and bus fares while in Manhattan are reimbursable.

Transportation to/from satellite meetings are not reimbursable.

Attendance

In-person participants and speakers are expected to attend all meeting days. Participants receiving hotel and travel support wishing to arrive on meeting days which conclude at 2:00 PM will be asked to attend remotely.

Entry & Building Access

Upon arrival, guests will be required to show their photo ID to enter the Simons Foundation and Flatiron Institute buildings. After checking-in at the meeting reception desk, guests will be able to show their meeting name badge to re-enter the building. If you forget your name badge, you will need to provide your photo ID.

The Simons Foundation and Flatiron Institute buildings are not considered “open campuses” and meeting participants will only have access to the spaces in which the meeting will take place. All other areas are off limits without prior approval.

If you require a private space to conduct a phone call or remote meeting, please contact your meeting manager at least 48-hours ahead of time so that they may book a space for you within the foundation’s room reservation system.

Guests & Children

Meeting participants are required to give 24 hour advance notice of any guests meeting them at the Simons Foundation either before or after the meeting. Outside guests are discouraged from joining meeting activities, including meals.

With the exception of Simons Foundation and Flatiron Institute staff, ad hoc meeting participants who did not receive a meeting invitation directly from the Simons Foundation are not permitted.

Children under the age of 18 are not permitted to attend meetings at the Simons Foundation. Furthermore, the Simons Foundation does not provide childcare facilities or support of any kind. Special accommodations will be made for nursing parents.

Meeting & Policy Questions

Meghan Fazzi
Senior Manager, Events & Administration, MPS
[email protected]

Travel & Hotel Support

FCM Travel Meetings & Events
[email protected]
Hours: M-F, 8:30 AM-5:00 PM ET
+1-877-300-7108