- CMSA EVENT: CMSA Swampland Seminar: IIB Explored – Dualities, Bordisms, and the Swampland
Speaker: Markus Dierigl – LMU Munich
17 Oxford Street, Cambridge, MA 02138 USA
In this talk I will discuss the application of the Cobordism Conjecture to type IIB supergravity with non-trivial duality bundle. Calculating the relevant bordism groups we find that they are highly non-trivial and would predict the presence of various global symmetries in the underlying theory. Since quantum gravity theories do not allow for global symmetries, we discuss which defects need to be included to break them completely. Interestingly, we find many backgrounds that are well-known in the F-theory literature, such as [p,q]-7-branes, non-Higgsable clusters, as well as S-folds and their generalizations to higher codimensions. Further including worldsheet reflections, predicts the existence of a new non-supersymmetric 7-brane with interesting properties and applications, which I will discuss in more detail.
- HARVARD-MIT ALGEBRAIC GEOMETRY SEMINAR: Harvard-MIT Algebraic Geometry: On Quot schemes of rank zero quotients over a curve
Speaker: Alina Marian – Northeastern
I will discuss various aspects of the geometry of Quot schemes of torsion quotients on a smooth projective curve. I will describe in particular results on the cohomology of tautological vector bundles over the Quot scheme, which mirror a parallel picture in the geometry of the Hilbert scheme of points over a surface. The subject has an interesting combinatorial and computational flavor. The talk is partly based on joint work with D. Oprea and S. Sam.
- CMSA EVENT: CMSA Topological Quantum Matter: Controlling Quantum Matter with Quantum Cavity Fields
Speaker: Vasil Rokaj – Harvard University
20 Garden Street, Cambridge, MA 02138
Cavity modification of material properties and phenomena is a novel research field motivated by the advances in strong light-matter interactions~. For condensed matter systems it has been demonstrated experimentally that the transport properties of 2D materials can be modified via coupling to vacuum fields~[2,3]. While in polaritonic chemistry it has been shown that ground state chemical properties can be controlled with cavity fields~. In the first part of my talk, I will present how the quantized cavity field can alter the conduction properties of a condensed matter system by focusing on the paradigmatic Sommerfeld model of the free electron gas~. The exact analytic solution of the Sommerfeld model in the cavity will be presented as well as its fundamental properties. Then, in the second part of the talk, I will focus on a many-particle system of cold ions in a harmonic trap coupled to the cavity field. I will show how this system couples collectively to the cavity and that hybrid states between light and matter, known as polaritons, emerge. The formation of polaritons leads to the modification of the properties of the cold ions and enhances the localization of the many-body wave function~. Connections to experiments will be discussed as well.
 F. Garcia-Vidal, C. Ciuti, T. W. Ebbesen, Science, 373, 178 (2021)
 G. L. Paravicini-Bagliani et al., Nat. Phys. 15, 186-190 (2019)
 F. Appugliese et al., Science 375 (6584), 1030-1034 (2022)
 T. W. Ebbesen, Acc. Chem. Res. 49, 11, 2403–2412 (2016)
 V. Rokaj, M. Ruggenthaler, F. G. Eich, A. Rubio, Phys. Rev. Research 4, 013012 (2022)
 V. Rokaj, S.I. Mistakidis, H.R. Sadeghpour, arXiv:2207.03436 (2022)
For more information on how to join, please see: https://cmsa.fas.harvard.edu/event_category/topological-quantum-matter-seminar/
- CMSA EVENT: CMSA New Technologies: How do Transformers reason? First principles via automata, semigroups, and circuits
Speaker: Cyril Zhang – Microsoft Research
20 Garden Street, Cambridge, MA 02138
The current “Transformer era” of deep learning is marked by the emergence of combinatorial and algorithmic reasoning capabilities in large sequence models, leading to dramatic advances in natural language understanding, program synthesis, and theorem proving. What is the nature of these models’ internal representations (i.e. how do they represent the states and computational steps of the algorithms they execute)? How can we understand and mitigate their weaknesses, given that they resist interpretation? In this work, we present some insights (and many further mysteries) through the lens of automata and their algebraic structure.
Specifically, we investigate the apparent mismatch between recurrent models of computation (automata & Turing machines) and Transformers (which are typically shallow and non-recurrent). Using tools from circuit complexity and semigroup theory, we characterize shortcut solutions, whereby a shallow Transformer with only o(T) layers can exactly replicate T computational steps of an automaton. We show that Transformers can efficiently represent these shortcuts in theory; furthermore, in synthetic experiments, standard training successfully finds these shortcuts. We demonstrate that shortcuts can lead to statistical brittleness, and discuss mitigations.
Joint work with Bingbin Liu, Jordan Ash, Surbhi Goel, and Akshay Krishnamurthy.
This seminar will be held in person and on Zoom. For more information on how to join, please see: https://live-hu-cmsa-222.pantheonsite.io/event_category/new-technologies-in-mathematics-seminar-series/