# Ultra Quantum Matter Virtual Seminars

### Spring 2021 Virtual Seminar Series at 2:00 pm- 2:45pm EDT, Tuesdays.

Date     Speaker Title
Feb. 9

Ruben Verresen (Harvard)

Prediction of Toric Code Topological Order from Rydberg Blockade.Video Recording

Feb 16 No seminar
Feb. 23 Matthew Lapa (Chicago) Stability of (some) gapped phases to long-range interactions
Mar. 2 Torsten Zache (Innsbruck) Quantum Variational Entanglement Hamiltonian Learning
Mar. 30 Umang Mehta (Chicago) From higher rank symmetry to volume preserving diffeomorphism
Apr. 6 Kantaro Ohmori (SCGP) Parametrized invertible QFTs from massive fermions
Apr. 13 Srinivas Raghu (Stanford) Lattice duality web and manifestation of hidden symmetries
Apr. 20 Dominic Williamson (Stanford) Type-I nonabelian fractal spin liquids
Apr. 27 Nathan Seiberg (IAS) Standard and exotic systems, from the lattice, to the continuum, and back
May 4 Natalie Paquette (IAS) A candidate EFT for ν=5/2
May 11 Amir Raz (UT Austin) Subsystem conformal symmetries

February 9:

Speaker: Ruben Verresen (Harvard)

Title: Prediction of Toric Code Topological Order from Rydberg Blockade

Abstract: The physical realization of Z_2 topological order as encountered in the paradigmatic toric code has proven to be an elusive goal. In this talk, I will show that this phase of matter can be created in a two-dimensional array of strongly interacting Rydberg atoms on a ruby lattice. The Rydberg blockade on this lattice effectively gives rise to a diluted dimer model on the kagome lattice, for which we observe a Z_2 spin liquid using the numerical density matrix renormalization group method. One can directly access the topological loop operators of this model, which can be measured experimentally using a dynamic protocol, providing a smoking gun'' experimental signature of the topological phase. Time permitting, I will show how to trap an emergent anyon and realize different topological boundary conditions, and more broadly discuss the implications for exploring fault-tolerant quantum memories.

This talk is based on work with Mikhail Lukin and Ashvin Vishwanath ( https://arxiv.org/abs/2011.12310 ).

February 23:

Speaker: Matthew Lapa (Chicago)

Title: Stability of (some) gapped phases to long-range interactions

Abstract: Much of our intuition about gapped phases comes from special exactly solvable models. These models often exhibit interesting and useful properties such as exact ground state degeneracy and exotic quasiparticles. Since we can never engineer any model exactly in the lab, it is important to understand whether these properties are stable to different kinds of perturbations. For short-range perturbations containing finite-range or exponentially decaying interactions, powerful and general stability results are known. On the other hand, very little is known about the effects of perturbations containing long-range (e.g., power-law) interactions. In this talk I will present a rigorous result on the stability of certain gapped phases to long-range interactions. This result applies, for example, to the Kitaev p-wave wire model perturbed by power-law density-density interactions with an exponent greater than 1. In this case, the result implies that the splitting between the model’s two nearly degenerate ground states decays exponentially with the system size, as is known to be true for short-range perturbations. This talk is based on forthcoming work with Michael Levin.

March 2:

Speaker: Torsten Zache (Innsbruck)

Title: Quantum Variational Entanglement Hamiltonian Learning

Abstract:  The capabilities of quantum simulation platforms, such as ultracold atoms in optical lattices, Rydberg tweezer arrays and trapped ions, already allow to realise a variety of model Hamiltonians and probe them with local control, e.g. single-site single-shot readout with quantum gas microscopy. However, the development of efficient and scalable protocols to measure the entanglement properties of quantum many-body systems - as encoded in the Entanglement Hamiltonian (EH) and its eigenvalues, the Entanglement spectrum (ES) - remains an outstanding challenge.

In this talk, I will present ongoing work to learn the EH in quantum simulations using a variational quantum-classical hybrid algorithm that is based on the capability to realise deformed system Hamiltonians experimentally. Our protocol is enabled by the approximate quasi-locality of the EH, which is motivated by the Bisognano-Wichmann theorem of quantum field theory. For the example of a Fermi-Hubbard model on a ladder geometry our results demonstrate the experimental feasibility of the proposed algorithm. I will further discuss the application of our protocol to topological states, where it will allow to probe the Li-Haldane conjecture that the low-lying ES is related to the conformal field theory of edge excitations with existing quantum simulators.

March 30:

Speaker: Umang Mehta (Chicago)

Title: From higher rank symmetry to volume preserving diffeomorphism

Abstract:  We show that the traceless dipole symmetry in 2+1D and 3+1D can be expanded into a non-abelian symmetry of volume-preserving diffeomorphisms. Constructing a background gauge multiplet to probe the matter sector, we show that the charge density obeys the Girvin-MacDonald-Platzman algebra.  We demonstrate that various well-studied theories, including composite fermions, crystals on the lowest Landau level, and ferromagnets can be obtained from the requirement of volume-preserving diffeomorphism invariance, and interpret the fractonic behaviour of the excitations as a consequence of this symmetry.

April 6:

Speaker: Kantaro Ohmori (SCGP)

Title: Parametrized invertible QFTs from massive fermions

Abstract:  It has become a central theme that SPT phases are classified by a suitable generalized cohomology theory. Recently, this statement is generalized to a family of invertible phases. In the talk I will talk about the family coming from the fermions, and in particular how to determine the class of the family directly from the fermion partition function, generalizing the notion of the eta-invariant.

April 13:

Speaker: Srinivas Raghu (Stanford)

Title: Lattice duality web and manifestation of hidden symmetries

Abstract:  I will discuss 3 aspects of the duality web in 3 spacetime dimensions using euclidean lattice constructions.  1) A boson-fermion duality, 2) modular transformations, and 3) how certain hidden symmetries are manifest in the lattice formulation.

April 20:

Speaker: Dominic Williamson (Stanford)

Title: Type-I nonabelian fractal spin liquids

Abstract:  A lot of progress has been made on understanding fracton phases of matter in recent years. However, the construction and understanding of models with nonabelian fractal operators remains limited. In this talk I will present a generalization of Yoshida's type-I fractal spin liquid models that support nonabelian fractons. The construction proceeds by identifying a fractal subsystem symmetry of a stack of two dimensional topological orders. The domain walls of this symmetry are composite "p-fractal" excitations made up of abelian anyons in the two dimensional layers. Gauging the symmetry induces condensation of these p-fractal excitations and produces the nonabelian fractal spin liquid model. Our construction can be applied to promote any anyon that braids nontrivially with an abelian boson into a fracton, consequently the resulting models host a wide variety of nonabelian fractons including those with noninteger quantum dimension (squared).

April 27:

Speaker: Nathan Seiberg (IAS)

Title: Standard and exotic systems, from the lattice, to the continuum, and back

Abstract:  We are interested in models with subsystem global symmetries; e.g., the XY-plaquette model and models of fractons.  Such models challenge a continuum field theory description.  Our previous analysis of their continuum limits exhibited emergent subsystem global symmetries (with ‘t Hooft anomalies) and new dualities. These continuum theories are non-standard and subtle because one needs to account for discontinuous field configurations.  In order to understand better these systems, we deform the original lattice models to new ones. The new lattice models are closer to the continuum theories and they exhibit many of the emergent global symmetries (and their anomalies) and the new dualities already at the level of the lattice.  Also, these new lattice models provide a clear and rigorous formulation of the continuum theories and their singularities, thus confirming our earlier analysis.

Fall 2020 Virtual Seminar Series at 2:00 pm- 2:45pm. Fridays.

Date Speaker Title
Oct. 23

Shu-Heng Shao (IAS)

Fractons: Going Beyond Standard QFT

Oct 30 Bowen Shi (UCSD) Entanglement bootstrap approach for gapped domain walls
Nov 6 Luca Iliesiu (Stanford) The statistical mechanics of near-extremal black holes
Nov 13 Zhu-Xi Luo (KITP) Moiré magnetism in van der Waals heterostructures
Nov 20 T. Senthil (MIT) Strange metals as ersatz Fermi liquids: emergent symmetries, anomalies, and experimental tests
Dec 4 John McGreevy (UCSD) Three-manifolds and entanglement
Dec 11 Haoyu Sun (UT Austin) Ising/gravity duality: handlebodies & nonhandlebodies

October 23:

Speaker: Shu-Heng Shao (IAS)

Title: Fractons: Going Beyond Standard QFT

Abstract: We present a framework of exotic continuum QFT to describe various fracton models at long distances.  These include the X-cube model, an anisotropic model, and the checkerboard model.  Our nonstandard QFTs capture the universal peculiarities of these lattice models, and reveal new relations between them.

October 30:

Speaker: Bowen Shi (UCSD)

Title: Entanglement bootstrap approach for gapped domain walls

Abstract: Recent progress has pointed to the possibility of deriving the emergent physical laws of anyon in two dimensional topologically ordered systems from an assumption on the ground state entanglement entropy - an approach we refer to as entanglement bootstrap. We develop a theory of gapped domain wall between topologically ordered systems by generalizing this approach. We find a new type of superselection sector - referred to as the parton sector - that subdivides the known superselection sectors localized on gapped domain walls. Moreover, we introduce and study the properties of composite superselection sectors that are made out of the parton sectors. We explain a systematic method to define these sectors, their fusion spaces, and their fusion rules. We define an analog of topological entanglement entropy for gapped domain walls and derive its exact expression.

November 6:

Speaker: Luca Iliesiu (Stanford)

Title: The statistical mechanics of near-extremal black holes

Abstract: In recent years, properties of the SYK model have been extensively used to study various features of low-temperature black holes, called near-extremal. In this talk, I will explain how this relation can be used to solve a long-lasting puzzle in the thermodynamics of near-extremal black holes, concerning the scale of the energy gap between the zero temperature, extremal, state and the lightest near-extremal state. Using this relation, we will be able to point out important differences between black holes that do not preserve super-symmetry and are relevant in AdS/CMT, and black holes that do preserve super-symmetry and are typically used in string theory constructions.

November 13:

Speaker:  Zhu-Xi Luo (KITP)

Title: Moiré magnetism in van der Waals heterostructures

Abstract: We introduce a general framework to study moiré structures of two-dimensional Van der Waals magnets using continuum field theory. The formalism eliminates quasiperiodicity and allows a full understanding of magnetic structures and their excitations. In particular, we analyze in detail: (1) Twisted homobilayers of Néel antiferromagnets on the honeycomb lattice. A rich phase diagram with non-collinear twisted phases is obtained, and spin waves are further calculated. (2) Untwisted heterobilayers of ferromagnet and antiferromagnet, with a Dzyaloshinskii-Moriya interaction in the ferromagnetic layer. Combining perturbative and strong-coupling analyses with Landau-Lifshitz-Gilbert simulations, various moiré-periodic commensurate phases are found, among which is an interesting skyrmion lattice phase wherein each moiré unit cell hosts one skyrmion. The commensurate-incommensurate transition is further discussed.

November 20:

Speaker:  T. Senthil (MIT)

Title: Strange metals as ersatz Fermi liquids: emergent symmetries, anomalies, and experimental tests

Abstract:  Despite decades of work, strange metals that defy Fermi liquid theory challenge our understanding. In this talk (based on work with Dominic Else and Ryan Thorngren) I will discuss properties of  such  phases of matter in systems with a global U(1) symmetry and  discrete translation symmetries.    I will show that  the low-energy theory must have a very large emergent symmetry group -- larger than any compact Lie group. I will  revisit standard Fermi liquid theory from a modern point of view of characterizing its emergent symmetry and its associated anomaly.  Many, if not all,  non-Fermi liquids will have the same emergent symmetry group/anomaly as a Fermi liquid (even though they could have very different dynamics). Such phases - dubbed ersatz Fermi liquids -  share some (but not all) universal properties with Fermi liquids. I will discuss the implications for understanding the strange metal physics observed in experiments in some systems. Combined with a few experimental observations, I will show that these general model-independent considerations lead to concrete predictions about a class of strange metals. The most striking of these is a divergent susceptibility of an observable that has the same symmetries as the loop current order parameter.

December 4:

Speaker:  John McGreevy (UCSD)

Title: Three-manifolds and entanglement

Abstract:  The entanglement bootstrap is a program to understand liquid topological orders from a few well-motivated axioms about the structure of their entanglement.  In this talk, based on work in progress with Bowen Shi and Jin-Long Huang, I'll describe some consequences of this line of thought for 3d topological orders and 3-dimensional topology. In particular, the bootstrap axioms imply non-degeneracy of braiding between several pairs of excitation types, as well as associated Verlinde formulae.  In the important example of 3d quantum double models, these braiding data are related to some fun finite group invariants, and give independent proofs of some facts about finite groups.  More generally, the entanglement bootstrap implies a pairing between 3-manifolds with boundary.  I'll end with some discussion of fusion spaces of the excitations and our understanding of their relation to known invariants of knots, links and spatial graphs.

December 11:

Speaker:  Haoyu Sun (UT Austin)

Title: Ising/gravity duality: handlebodies & nonhandlebodies

Abstract:  After reviewing the duality between Ising CFT and 3d pure gravity for handlebodies of any genus, I will present new results for nonhandlebodies. We organize the gravitational path integral (for a fixed conformal class on the asymptotic boundary) as a double sum including a summation over topology, in addition to the usual summation over hyperbolic geometry. We use rational Dehn surgery to organize the topology sum. One simple example of nonhandlebody is a twisted I-bundle over a Riemann surface, which we study in detail. We argue that in their presence, the gravitational partition function still matches that of the Ising CFT up to a finite multiplicative constant. This is a collaboration with Chao-Ming Jian, Andreas W.W. Ludwig, Zhu-Xi Luo, and Zhenghan Wang.