## Quantum/Gravity Seminar Series

Except where otherwise noted, seminars are held on Mondays at 11 a.m. via Zoom.

### Spring 2021 Seminars

February 15, 2021

"Towards Transport and Operator Growth at Low Temperature"

Abstract: We now have a variety of tools and solvable models to address energy/charge transport and operator growth at high temperature in quantum lattice models. However, we have much less control over the low temperature regime, where we expect a field theory description. I'll talk about some progress in this direction reported in two recent papers, one on transport with Cris Zanoci (2012.11601) and one on operator growth with Subhayan Sahu (2005.10814). I'll emphasize open questions and opportunities for collaboration.

February 22, 2021

"Lessons from the lattice-continuum correspondence"

Abstract: A year ago, I described plans for a comprehensive study of the lattice-continuum correspondence, an effort to quantitatively understand how continuum QFTs can emerge from large but finite quantum systems. The fundamentals of this correspondence are now established for Abelian QFTs with scalars, fermions, and gauge fields in all dimensions. This has lead to many new insights into old questions, not least of which is a new way to rigorously define many continuum QFTs using their lattice counterparts. I will give a basic overview of this research program and will then describe some of its consequences: a definition of operator product expansions directly on the lattice, new insights into phase structures of compact scalars and Abelian gauge theories, and a generalization of Noether's theorem to discrete symmetries.

March 1, 2021

"Measurement-Induced Phase Transitions at Large N"

Abstract: Measurement-induced phase transitions (MIPT) are a novel class of dynamical quantum many-body phase transitions driven by a competition between unitary scrambling dynamics, which generate many-body entanglement, and local projective measurements, which tend to destroy this entanglement. While these phase transitions are clearly visible in numerical experiments with Clifford circuits, analytical approaches are limited and the nature of the critical point remains unclear. Here we approach this problem using a Brownian circuit model, and obtain a path-integral expression for the order parameter featuring a large-N limit that can be analyzed exactly using steepest-descent methods. For the simplest Brownian circuit model, we find that the transition is described by a $Z_2$ symmetry-breaking transition, reminiscent of the classical Ising model with the measurement rate $p$ playing the role of temperature. In future work we hope to apply this technology to the study of MIPT dynamics in more general models, with a view toward understanding the various universality classes represented by MIPT dynamics.

March 8, 2021

"A Tale of Two Exponentiations in N=8 Supergravity"

Abstract: Four-point gravitational scattering amplitudes can exhibit two types of exponentiation. In momentum space, one-loop infrared divergences exponentiate to give higher-loop amplitudes modulo an IR-finite remainder function. On the other hand, at high energies and small angles, the four-point amplitude can be expressed in impact parameter space as the exponential of an eikonal phase. We explore the interplay of these two types of exponentiation in N=8 supergravity, and show how each can inform us about the structure of the other.

March 15, 2021

March 22, 2021

***Note****:** This seminar will take place at a special time of 2:00 PM.

"Chern-Weil Global Symmetries and How Quantum Gravity Avoids Them"

Abstract: The Swampland program aims to determine the constraints that an EFT must satisfy to be consistent with a UV completion in quantum gravity. One of the most important proposed constraints is the absence of global symmetries, meaning that any global symmetry must be gauged or broken. In this talk, I will discuss a class of generalized global symmetries, which we call “Chern-Weil global symmetries,” that arise ubiquitously in gauge theories. The Noether currents of these Chern-Weil global symmetries are given by wedge products of gauge field strengths and their conservation follows from Bianchi identities, so they are not easy to break. However, exact global symmetries should not be allowed in a consistent theory of quantum gravity, and how quantum gravity avoids them can teach us many things about the physics of UV consistent EFTs. In particular, I will explain how these symmetries are typically gauged or broken in string theory. Interestingly, many familiar phenomena in string theory, such as axions, Chern-Simons terms, worldvolume degrees of freedom on extended objects, and branes ending on or dissolving in other branes, can be interpreted as consequences of the absence of Chern-Weil symmetries in quantum gravity, suggesting that they might be general features of quantum gravity.

March 29, 2021

March 31, 2021

*Please note that this seminar takes place on a Wednesday (Brandeis Monday).

"Sub-system statistics for out-of-equilibrium qubits"

Abstract: Motivated by questions asked in cosmology to understand the origin of structure in the universe, I will introduce a family of qubit systems initialized and evolved according to rules designed to mimic the thermodynamics of the early universe. I will present results on the correlation properties and statistics of all sizes of sub-system, which give preliminary data toward answering the question: What are the minimum ingredients required in order for the variance of the free energy in small sub-systems to increase in time?

April 5, 2021

"Programmable interactions in an array of atomic ensembles"

Abstract: I will present our recent experimental advances in generating highly tunable interactions between neutral atoms which offers new prospects for quantum computation as well as quantum simulation. Our system consists of a 1D array of atomic ensembles coupled to an optical cavity which induces spin-mixing between the atoms. Controlling the spectrum of the drive field enables us to tune the sign as well as the range of these interactions. I will show how we use this technique to implement dynamics in various effective geometries such as frustrated 2D lattices and tree-like geometries where the latter are connected to holographic models of quantum gravity. In the realm of quantum information, these new capabilities pave the way to engineer quantum states with specific spatial entanglement structures for quantum sensing and quantum computation.

April 12, 2021

"Charting the Landscape of 3d CFTs"

Abstract: I will summarize progress at using the conformal bootstrap to solve 3d CFTs. These include the critical 3d Ising model, the O(N) vector models, and the minimal 3d SCFT. Recent results for the O(2) modelresolve a longstanding discrepancy between experiment and and Monte Carlo simulations, while results for the O(3) model prove the instability of Heisenberg magnets to cubic anisotropy.

April 19, 2021

"From Swampland and Weak Gravity to Global Symmetries and Wormholes"

Abstract: After a brief reminder of the Swampland idea in general and the Weak Gravity Conjecture in particular, I will focus on global symmetries. Given that exact global symmetries are forbidden by quantum gravity, it is natural to expect that bounds on the quality of approximate global symmetries exist. So far, holographic arguments have only been provided for the exact case. I will discuss a classification of approximate global symmetries and describe a simple argument, based the Weak Gravity Conjecture, for a quantitative bound on the sub-class of "gauge-derived" global symmetries. This has intriguing relations to wormhole-based arguments, which I will also present. I will end with a brief discussion of the fundamental problems associated with euclidean wormholes and of some recent developments in this context.

April 26, 2021

"Many-body dynamics and quantum advantage with cold atom quantum simulators"

Abstract: The exceptional control available in experiments with cold atoms in optical potentials, and with trapped ions, has opened new opportunities to explore many-body quantum dynamics with time-dependent control. This goes beyond existing many-body systems as we are able to engineer unique features such as genuine long-range interactions, and microscopic control over dissipation. In analogy with the quantum optics of single atoms and photons, we can often work in scenarios where the separations of relevant energy scales allows us to write microscopic models for open quantum systems - with strong inter-particle interactions. I will give an overview of the capabilities of these systems, illustrated with some of our theory group's recent work on coherent dynamics with long-range interactions and controlled dissipative dynamics. I will also address the question of whether quantum simulators already allow us quantitative access to controlled dynamics beyond the computational abilities of existing classical computers, i.e., a practical quantum advantage.

May 3, 2021

"Multi-mouth Traversable Wormholes"

Abstract: Recently, several examples of traversable wormholes supported by well-controlled quantum effects and respecting reasonable energy conditions have been constructed. In this talk, I will describe generalizations of such solutions involving more than two mouths in the same asymptotic region. These wormholes may be traversed between any pair of mouths, are four dimensional, and are asymptotically flat up to the presence of possible magnetic fluxes or cosmic strings that extend to infinity. From a dual field theory point of view, when AdS asymptotics are added to our construction, multiparty entanglement may play an important role in the traversability of the resulting wormhole.

May 10, 2021

"Many-body quantum teleportation via operator spreading in the traversable wormhole protocol"

Abstract: By leveraging shared entanglement between a pair of qubits, one can teleport a quantum state from one particle to another. Recent advances have uncovered an intrinsically many-body generalization of quantum teleportation, with an elegant and surprising connection to gravity. In particular, the teleportation of quantum information relies on many-body dynamics, which originate from strongly-interacting systems that are holographically dual to gravity; from the gravitational perspective, such teleportation can be understood as the transmission of information through a traversable wormhole. In this talk, I will introduce a new mechanism for many-body quantum teleportation -- dubbed peaked-size teleportation. Intriguingly, peaked-size teleportation utilizes precisely the same quantum circuit as traversable wormhole teleportation, yet has a completely distinct microscopic origin: it relies upon the spreading of local operators under generic thermalizing dynamics and not gravitational physics. I will demonstrate peaked-size teleportation across a variety of physical systems, including random unitary circuits and the Sachdev-Ye-Kitaev model (at high temperatures). I will conclude by outlining potential experimental realizations of many-body quantum teleportation, with applications to: (i) characterizing the size distributions of operators in strongly-correlated systems and (ii) distinguishing between generic and intrinsically gravitational scrambling dynamics.

### Fall 2020 Seminars

September 14, 2020

"Maximum Observable Blueshift from Circular Equatorial Kerr Orbiters"

Abstract: The region of spacetime near the event horizon of a black hole can be viewed as a deep potential well at large gravitational redshift relative to distant observers. However, matter orbiting in this region travels at relativistic speeds and can impart a significant Doppler shift to its electromagnetic emission, sometimes resulting in a net observed blueshift at infinity. In this talk we investigate emission produced by isotropic monochromatic emitters on circular equatorial orbits around a Kerr black hole, and obtain simple relations describing how the maximum blueshift encodes black hole spin and inclination. We find that small values of the maximum blueshift yield an excellent probe of inclination, while larger values provide strong constraints on spin or inclination in terms of the other.

September 21, 2020

"Leading order corrections to the quantum extremal surface prescription"

Abstract: We show that a naïve application of the quantum extremal surface (QES) prescription can lead to paradoxical results and must be corrected at leading order. The corrections arise when there is a second QES (with strictly larger generalized entropy at leading order than the minimal QES), together with a large amount of highly incompressible bulk entropy between the two surfaces. We trace the source of the corrections to a failure of the assumptions used in the replica trick derivation of the QES prescription, and show that a more careful derivation correctly computes the corrections. Using tools from one-shot quantum Shannon theory (smooth min- and max-entropies), we generalize these results to a set of refined conditions that determine whether the QES prescription holds. We find similar refinements to the conditions needed for entanglement wedge reconstruction (EWR), and show how EWR can be reinterpreted as the task of one-shot quantum state merging (using zero-bits rather than classical bits), a task gravity is able to achieve optimally efficiently.

September 30, 2020

*Please note that this seminar takes place on a Wednesday (Brandeis Monday)

"The Page Curve for Reflected Entropy"

Abstract: Reflected Entropy is a bipartite correlation measure with a simple geometric holographic dual, the minimal entanglement wedge cross section. This duality further motivates the idea that spacetime emerges from entanglement. Further, it illustrates the richness of the multipartite entanglement structure of holographic systems. A particularly interesting feature that we focus on in this talk is the phase transition between a connected and disconnected entanglement wedge where the reflected entropy jumps discontinuously. We explain how this phase transition is resolved in random tensor networks which serve as toy models for holography. This argument based on the replica trick motivates a general ansatz for the mechanism of the phase transition in AdS/CFT.

October 5, 2020

"Heavy Operators and Hydrodynamic Tails"

Abstract: The late time physics of interacting QFTs at finite temperature is controlled by hydrodynamics. For CFTs this implies that heavy operators -- which are generically expected to create thermal states -- can be studied semiclassically. We show that hydrodynamics universally fixes the OPE coefficients C_{HH'L}, on average, of all neutral light operators with two non-identical heavy ones, as a function of the scaling dimension and spin of the operators. These methods can be straightforwardly extended to CFTs with global symmetries, and generalize recent EFT results on large charge operators away from the case of minimal dimension at fixed charge. I will also revisit certain aspects of late time thermal correlators in QFT and other diffusive systems.

October 12, 2020

"Entropic order parameters for symmetries in QFT"

Abstract: In QFT there is an algebra of operators attached to any spacetime region. Simple degradations of the most harmonious possible relation between algebras and regions are shown to encode generalized symmetries. Nets of algebras with these symmetries allow for a non uniqueness of the algebras that can be assigned to a given region. This non uniqueness suggests a simple geometrical order parameter in terms of a relative entropy. These satisfy a ''certainty relation'' connecting the statistics of the order and disorder parameters for complementary regions. We describe how the lore about phases of theories with generalized symmetries is seen in this new picture.

October 19, 2020

"The Schottky Anomaly of de Sitter Black Holes"

Abstract: Black holes with Λ > 0 (SdS) have fascinating properties that are distinct from the asymptotically flat or AdS cases, starting with the fact that there are two horizons in the spacetime, one black hole and one cosmological. The two horizons have different temperatures and the total gravitational entropy is the sum of the horizon areas. As a result, both the mass M and entropy S are bounded between minimum and maximum values. Intriguingly, there is an extremum in the specific heat ∂M/∂Tb as well as in the curve ∂S/∂Tb, which resemble the Schottky anomaly of a two level system in statistical mechanics. In this talk we investigate classical and quantum mechanical features of SdS thermodynamics that make it resemble the physics of a paramagnet. We start by showing that the Schottky behavior is to be expected for classical fluctuations based on the first laws for SdS black holes. Second, we present calculations of black hole and cosmological particle production in SdS and find that the quantum fluctuations share the behavior of classical ones.

November 2, 2020

"Quantum error correction and large N"

Abstract: In recent years quantum error correction(QEC) has become an important part of AdS/CFT. Unfortunately, there are no field-theoretic arguments about why QEC holds in known holographic systems. The purpose of this talk is to fill this gap by studying the error correcting properties of the fermionic sector of various large N theories. Specifically we examine SU(N) matrix quantum mechanics and 3-rank tensor O(N)^3theories. Both of these theories contain large gauge groups. We argue that gauge singlet states indeed form a quantum error correcting code. Our considerations are based purely on large N analysis and do not appeal to a particular form of Hamiltonian or holography.

November 9, 2020

"Wess-Zumino-Witten terms and the geometry and topology of lattice systems"

Abstract: Wess-Zumino-Witten terms are topologically-nontrivial terms in the effective actions which arise from integrating out short-distance degrees of freedom which couple of slowly-varying scalar fields. They can be viewed as higher-dimensional generalizations of the Berry connection. This viewpoint suggests that WZW terms encode the non-trivial topology of the space of massive field theories and should arise also in the context of gapped lattice systems. I will explain how to compute WZW terms for a family of gapped lattice systems in d spatial dimensions. The answer suggests an important role for coarse geometry as introduced by J. Roe.

November 16, 2020

November 23, 2020

"Constraints on Tree Level Gravitational Scattering"

Abstract: Motiviated by a combination of the Chaos bound and AdS/CFT, we conjecture that all classical theories that give rise to S matrices that grow faster than s^2 in the Regge limit are unphysical. We then present a complete and exhaustive classification of all kinematically allowed exchange and contact S matrices (assuming that the vertices that generate the contact diagrams are polynomial in momenta and that no more than a finite number of intermediate particles are exchanged) and demonstrate that the only classical S matrix obeying these constraints and also the conjectured bound on Regge scattering in six or lower spacetime dimensions is the Einstein S matrix.

November 30, 2020

"Towards Emergent Gravity in Approximate Quantum Error Correction Codes"

Abstract: It is known that the AdS/CFT correspondence is related to approximate quantum error correction codes. However, the exact manner in which gravity can arise in such codes remains largely unexplored. Here we construct an approximate quantum error correction code which can be represented as a holographic tensor network. In the "noiseless" limit, it admits a local log-depth decoding circuit and reproduces certain properties of holography, such as the Ryu-Takayanagi formula and subregion duality, much like other known holographic codes. However, the code becomes approximate when "coherent noise" is injected, allowing it to capture features analogous to those of gravity, such as back-reaction, subspace-dependence, and approximate bulk locality. I will explain how these features are manifested in the tensor network.