Quantum and Gravitational Theory Group

Quantum/Gravity Seminar Series

Unless otherwise noted, seminars take place at 11:10 am on Fridays in Abelson 333.

Fall 2023 Seminars

Group Meet-and-Greet

August 30, 2023

**Note, this meeting takes place on Wednesday**
Greg Bentsen, Brandeis

September 22, 2023

Title:  (Almost) Good Quantum Codes from Long Wormholes

Abstract: Quantum error correction is critical for quantum computation on realistic noisy devices, and the search for quantum error-correcting codes with desirable properties is an essential ongoing effort in quantum information science. A recent exciting development is the construction of `good’ quantum low-density parity-check codes, whose rate and distance are both proportional to the total number of physical qubits. The existence of these codes raises a natural question: can we find quantum many-body systems where codes of this type naturally appear? In this talk, I will discuss families of approximate quantum error-correcting codes that arise as nearly degenerate ground states in the SYK model and its low-rank generalizations. These codes turn out to have constant rate and a distance that scales as N^c as N goes to infinity. For SYK, we find c=1/2, while for the low-rank models, we find that c can be tuned arbitrarily close to 1, e.g. c=.99, while preserving a non-vanishing rate. Hence, certain of these codes are `almost good.’ They are also roughly analogous to LDPC codes in that the terms in the Hamiltonian are all low-weight. Finally, I will discuss a holographic model of these codes in which the large code distance is a consequence of the emergence of a long wormhole geometry in a simple model of quantum gravity.

Indranil Halder, Harvard

September 29, 2023

Title: Fractional winding condensates, replica trick and stringy microstates

Abstract: I will discuss the role of various winding condensates in bosonic string theory on AdS₃ at non-zero temperatures. First, we will prove that a marginal deformation of free theory on the cylinder consisting of a winding condensate on the space circle of winding ±1 is equivalent to the usual sigma model description of the worldsheet CFT of empty AdS₃ in terms of SL(2,R) WZW model. This equivalence is a strong weak duality in the sense of α’. Then we will use this description for the BTZ blackhole and calculate the thermal partition function from the worldsheet. Along the way, I will explain two conceptual subtitles involved in the calculation and resolve them by formulating a version of the replica trick on the worldsheet in terms of a factional winding condensate. It will be shown that the entropy of BTZ blackhole can be obtained from the one-point function of the non-local ‘area’ operator on the worldsheet. In the final part of the talk, we will focus on deformation consisting of two winding condensates in space and time circles. Near Hagedorn temperature such a deformation would be interpreted as the analog of the Horowitz-Polchinski-like solution. We will present detailed results on a likely new CFT corresponding to a stringy `small’ blackhole at the Hawking-Page temperature.

Brian Swingle, Brandeis

October 6, 2023

Title: A simple (bosonic) model of quantum gravity?

Abstract: I'll discuss ongoing work with Mike Winer exploring the physics of all-to-all quantum p-spin models. The 2-body versions of these models are typically glassy at low temperature, but the q-body generalizations, which can be viewed as bosonic analogs of SYK, might have interesting gravitational dynamics at low energies.

Adam Levine, MIT

October 13, 2023

Title: One shot holography
Abstract: In this talk, I will discuss recent work in which we define a generally covariant max- and min-entanglement wedge of a boundary region B. I will conjecture how these regions pertain to bulk reconstruction. We will prove that the min- and max-entanglement wedges obey various properties necessary for this conjecture, such as nesting, inclusion of the causal wedge, and a reduction to the usual quantum extremal surface prescription in the appropriate special cases. This requires us to reformulate the standard quantum extremal surface prescription in yet another new manner. These proofs furthermore all rely on one-shot versions of the (restricted) quantum focusing conjecture (QFC) that we conjecture to hold. I will discuss what this one-shot QFC might imply in the non-gravitational limit.
No Seminar

October 20, 2023

No Seminar

October 27, 2023

Roberto Emparan, Barcelona U. and ICREA, Barcelona

October 30, 2023

**Note the change of day and location. This seminar will take place on Monday at 11:10 in Abelson 229**

Title: Holographic duals of evaporating black holes

Abstract: I will discuss how the dynamical evaporation of a black hole through quantum Hawking emission admits a dual description in terms of classical gravitational evolution in Anti-de Sitter space. The idea was first proposed two decades ago, but it remained underdeveloped and poorly understood due to conceptual subtleties and technical difficulties. Recently, in collaboration with Luna, Suzuki, Tomasevic and Way, we have applied novel methods to study the dual dynamical evaporation of a variety of black holes interacting with colder radiation baths. We have also found the dual of the collapse of holographic radiation to form a black hole on the brane.

Guglielmo Grimaldi, Brandeis

November 3, 2023

Title: chasing the minimax squirrel

Abstract: I will talk about recent progress in (i) proving the cooperating theorem for minimax time-sheets for some explicit configurations of increasing level of complexity and in (ii) searching for counterexamples to either the cooperating theorem or to the higher inequalities in time-dependent configurations in the context of 3d gravity. A recurring theme is how global properties of AdS often come to save the day, so I will discuss some of these tools.

Cynthia Keeler, Arizona State University

November 6, 2023

**Note the special day and time. This seminar will take place at 1:30pm in Abelson 333**

Title: “Entropies for Stabilizer states and Beyond”
Abstract: “We will examine two different graphical approaches to studying the entropies of stabilizer states. First, we introduce the notion of a “reachability graph”, which shows how stabilizer states are connected to each other via Clifford gates. We then introduce the double coset structure behind “contracted graphs”, where vertices represent entropy vectors instead of individual states. And last, we discuss a new directed graph protocol whose max flows compute the entropies of a broader class of quantum states (which could include all stabilizer states).”
Ning Bao, Northeastern

November 10, 2023

(AI generated) Title: "Accelerating Insights: Algorithmic Speedup in Unveiling the Holographic Entropy Cone"

(AI generated) Abstract: In this enlightening talk, Dr. Ning Bao takes us on a journey into the cutting-edge realm of quantum gravity and holography. One of the central enigmas in this field is the "Holographic Entropy Cone," a complex structure that encodes critical information about the fundamental nature of spacetime and quantum entanglement. Dr. Bao will present groundbreaking research that leverages algorithmic speedup techniques to efficiently probe and determine the elusive Holographic Entropy Cone. This innovative approach promises to unravel the mysteries of quantum gravity, shedding new light on the interplay between black holes, quantum field theory, and the holographic principle. Join us in this exploration of algorithmic acceleration as we aim to decipher the secrets hidden within the Holographic Entropy Cone, pushing the boundaries of our understanding of the universe.

Diandian Wang, Harvard

November 17, 2023

Title: Chaos from pole skipping and shockwaves

Abstract: Pole skipping is an interesting phenomenon that happens in chaotic quantum many body systems, and it can be used to determine the Lyapunov exponent and the butterfly velocity, both important quantifiers of the out-of-time correlator (OTOC). I will talk about a systematic way of deriving pole-skipping conditions for general holographic CFTs dual to classical bulk theories and how to use this framework to derive a few interesting statements including: (1) theories with higher spins generally violate the chaos bound; (2) the butterfly velocity calculated using pole skipping agrees with that calculated using shockwaves for arbitrary higher-derivative gravity coupled to ordinary matter; (3) shockwaves are related to a special type of quasinormal modes. If there is time, I will briefly mention how fermionic fields and gauge fields can be systematically incorporated into the formalism. 
Valérie Bettaque, Brandeis

December 8, 2023

Title: Majorana Clifford Algebras & Stabilizer States

Abstract: In this talk I will present the current state of a mathematical framework used to describe operator strings of Majorana fermions and the associated Clifford algebras in terms of binary vectors and matrices respectively. Special focus will be on the subgroups of those algebras that preserve the commutation relations and fermion parity of the Majorana strings, as they represent sub-ensembles of unitary Clifford operators with additional “local” structure. I will then discuss the implications of this orthogonal structure (which does not arise naturally in the case of Pauli strings) for the construction of Majorana stabilizer states and random circuits, and how this might eventually lead to a class of efficient quantum error correcting codes.

Fabricio Lozano, IFEG and Universidad Nacional de Cordoba

December 15, 2023

Spring 2024 Seminars

January 11, 2024

January 18, 2024

January 25, 2024

Scott Collier, MIT

February 1, 2024

February 8, 2024

February 15, 2024

February Break -- No Seminar

February 22, 2024

Yikun Jiang, Northeastern

February 29, 2024

Yizhi You, Northeastern

March 7, 2024

March 14, 2024

Fabian Ruehle, Northeastern

March 21, 2024

March Break -- No Seminar

March 28, 2024

April 4, 2024

No Seminar

April 11, 2024

April 18, 2024