Quantum and Gravitational Theory Group

September 11, 2025

Title: Entanglement Bootstrap and conformal field theory

Abstract: Theoretical physicist John McGreevy (UCSD) will introduce the Entanglement Bootstrap, a program to extract and understand the universal information characterizing a phase of matter starting from the entanglement structure of a piece of a single representative state. This universal information is usually packaged in the form of a quantum field theory; the program therefore provides a surprising new perspective on quantum field theory. McGreevy will discuss what we can learn about gapped topological phases and their associated topological field theories, and about quantum critical points and their associated conformal field theories.

September 25, 2025

Title: Particle-Soliton Degeneracy in 2D QCD from Spontaneously Broken Non-invertible Symmetry.

Abstract: Quantum chromodynamics in two spacetime dimensions admits a finite non-invertible symmetry described mathematically by a fusion category. This symmetry is spontaneously broken at long distances, leading to distinct vacua. When the theory has a mass gap, the spectrum is therefore characterized by particle excitations above a single vacuum and soliton sectors interpolating between vacua. In this talk, I will explain how to use the theory of 2D topological cosets, anyon condensation of 3D TQFTs, and the representation theory of fusion categories to obtain exact results about this spectrum, exhibiting the allowed multiplets. Often, particles and solitons are in the same representation and therefore must have equal masses. Furthermore, the fusion category symmetry frequently implies the existence of certain stable states in the spectrum. The resulting degeneracies are encoded in quiver diagrams where nodes are vacua and arrows are excited states.

October 9, 2025

Title: Operational Symmetry of Entanglement and Its Implications

Abstract: In this talk, I will explore the conditions under which quantum operations preserve environment-assisted invariance (envariance), a symmetry of entanglement originally introduced by Wojciech H. Zurek. While envariance has typically been analyzed in the context of local unitary operations, I extend the discussion to encompass non-unitary local operations. I show that in order to preserve envariance, these operations must have Kraus representations that exhibit a direct-sum structure, effectively establishing decoherence-free subspaces. One important implication of this result is that environment-assisted shortcuts to adiabaticity are not achievable via non-unitary operations. Furthermore, we demonstrate that coupling the boundary conformal field theories (CFTs) to external baths breaks the static condition of the eternal black hole in the AdS/CFT correspondence.

Reference: A. Sone, A. Touil, K. Maeda, P. Cappellaro and S. Deffner, New J. Phys. 27 064509 (2025)

October 16, 2025

Title: 2D Gauge Theories on the Hamiltonian Lattice

Abstract: Lately there has been renewed interest in two-dimensional gauge theories, stemming from their applicability as toy models of QCD along with rapidly advancing techniques for simulating them both numerically and experimentally. I will start by discussing the U(1) theory coupled to a fermion in two dimensions, and important lessons from its ABJ anomaly that can be used to dramatically improve the accuracy of results obtained from the Hamiltonian lattice. These methods enable very precise calculations outside of the perturbatively accessible regime. I will then discuss recent progress on non-abelian theories coupled to an adjoint Majorana fermion. In this case, all the mixed 't Hooft anomalies are naturally realized in a Hamiltonian lattice formulation, and using infinite tensor networks, we obtain numerical results that far exceed the capabilities of earlier methods.

October 23, 2025

Title: Average-case quantum complexity from glassiness

Abstract: In the classical setting, glassiness characterizes many natural problems (e.g., random k-SAT) and underlies average-case hardness by obstructing a family of "stable" classical algorithms (e.g., constant-time Langevin dynamics). In this work, we develop analogous quantum results. Our techniques, based on quantum optimal transport, differ significantly from classical probabilistic approaches due to the sign problem in the absence of a known eigenbasis. We show that quantum glassiness obstructs stable quantum algorithms, including constant-time Lindbladian dynamics and shallow variational algorithms. Using the replica trick, we also find that random 3-local Pauli Hamiltonians are quantumly hard, and we give numerical bounds on the temperature of the glass transition. We also give strong analytical evidence, by studying the 1RSB saddle point equations with sharpened log-Sobolev inequalities, that random k-local Hamiltonians are quantumly easy for sufficiently large constant k. These results support the glass phase diagram previously studied by Swingle and Winer, and they significantly improve upon both upper and lower bounds on quantum Gibbs sampling algorithms for non-commuting Hamiltonian ensembles. (Talk based on arXiv:2510.08497.)

October 30, 2025

Title: Emergent Mixed States for Baby Universes and Black Holes

Abstract: I will discuss the behavior of sequences of states in the large N limit of AdS/CFT duality in cases in which the bulk duals involve baby universes or black holes. Such sequences generally fail to converge as pure states. Under suitable conditions, such as diverging coarse-grained entropy, they can converge to mixed states for the large N algebra, as in the case of black holes. For Euclidean preparations that produce baby universes, the sequences do not converge, due to wormhole contributions, and so these states cannot admit large N limits. Nevertheless, appropriate averaging over N can lead to convergence to a mixed state. The associated algebras have nontrivial commutants, which can possibly be interpreted as operators in the baby universe.

November 6, 2025

November 13, 2025

November 20, 2025

December 4, 2025