Alex Atanasov

atanasov@g.harvard.edu








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About Me

I'm a PhD student in theoretical physics at Harvard. I am interested in using a variety of approaches to gain insight into computation and generalization in neural systems. My work is funded by the US Department of Defense through the NDSEG Fellowship and by the Fannie and John Hertz Foundation.

I received my B.S. in physics and M.S. in mathematics from Yale in 2018. There, I performed research under David Poland, studying conformal field theory in dimensions three and four using the bootstrap program. Conformal field theory is a powerful idea that allows us to gain insight into nonlinear, strongly-interacting systems at their phase transitions. I also had the opportunity to learn from Philsang Yoo about the connections between quantum field theory and the Langlands program.

I am excited by the intersection of machine learning and computational neuroscience. I first worked under John Murray as an undergrad, modeling the dynamics of working memory in recurrent neural networks (RNNs) and for Google, developing convolutional neural network (CNN) models for computer vision and face recognition for the internet of things.

My past collaborations with Dr. Erik Schnetter at the Perimeter Institute for Theoretical Physics, focused on finding ways around the curse of dimensionality in Einstein's field equations. You can learn more about this work from our arXiv paper and implementation on Github.

This website serves as a repository for my research and personal projects. If you are interested in any of these projects, feel free to reach out.




Academic Papers and Publications

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PsychRNN: An Accessible and Flexible Python Package for Training Recurrent Neural Network Models on Cognitive Tasks

A publication resulting from my past undergraduate work in John Murray's Lab. I worked on this from 2016-2018 in collaboration with David Brandfonbrener, Daniel Ehrlich, and Jasmine Stone. The goal of this package is to give a platform for computational neuroscience researchers to more easily construct neural network architectures with biological constraints. The focus is on RNN-type architectures relevant for the modeling of working memory.

(2021) eNeuro [Journal Link] [bioRxiv] [Code Repository]

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Conformal Block Expansion in Celestial Conformal Field Theory

Work with Andy Strominger and collaborators. Flat space amplitudes can be recast as correlation function on a ``celestial sphere'' at null infinity. These correlation functions are constrained to satisfy conformal covariance coming from the Lorentz group in flat spacetime. The corresponding 4-point correlation functions for a 2-to-2 scattering process can be expanded in terms of conformal blocks. This expansion captures the spectral data that is exchanged from the celestial perspective. For scalar external particles with massive scalar exchange, we find a new class of light-ray operators that are exchanged, and construct their corresponding waveforms in flat space.

(2021) In Submission [arXiv] [IAS Talk]

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(2,2) Scattering and the Celestial Torus

Work with Andy Strominger and collaborators on understanding the asymptotic structure of spacetime in mixed 2,2 signature, which we call "Klein Space", and its relationship to scattering and the celestial holography program. We provide the resolution of null infinity in 2,2 signature as a torus fibration over an interval. The corresponding "celestial torus" is conjectured to hold a conformal field theory containing information on scattering processes. We derive a new basis of scattering states motivated by the symmetries of this torus, and relate this to the conventional Mellin basis used in the existing literature.

(2021) In Submission [arXiv] [Princeton Talk]

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Bootstrapping the Minimal 3D Superconformal Field Theory

A paper written in collaboration with Aaron Hillman and my undergraduate physics advisor, Professor David Poland, published in the Journal of High Energy Physics (JHEP). We apply the conformal bootstrap program to study a space of interesting conformal field theories (CFTs). In the past, the conformal bootstrap has had great success in pinning down the critical exponents of the famously impenetrable 3D Ising model to the highest precision ever achieved.

Here, we present a bootstrap study of the space of 3D CFTs sharing the same parity symmetry and relevant operator structure as the 3D Ising model. This space is diagrammed and elaborated upon in this beautifully-written Quanta article. We note a new kink that lies near the conjectured scaling dimensions of the 3D N = 1 Ising model: the Ising model's supersymmetric cousin. A noted prior work conjectures that this supersymmetric theory may in fact be emergent at the boundary of topological superconductors. By combining a scan over three-point function coefficients together with an analytic formula derived form supersymmetry that constrains the three-point correlators, we obtain tight bounds for the scaling dimensions and operator spectra of this model. Our results lead us to conjecture a surprising exact value for the 3-point coefficient ratio of this CFT. This then gives a conjectured exact analytic value for the scaling dimensions of the first two relevant scalars.

(2018) Journal of High Energy Physics [Journal Link] [arXiv] [Code Repository]

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Magnetic Monopoles, `t Hooft Lines, and the Geometric Langlands Correspondence

My undergraduate thesis in mathematics under my advisor, Professor Philsang Yoo. Provides an overview of the Geometric Langlands correspondence from the point of view of physics. This thesis is based on the material I learned from my advisor's two seminar courses on the subject, a variety of independent reading, and by attending a conference on the same topic at the Perimeter Institute.

(2018) Yale Senior Thesis [PDF] [Presentation Slides]

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Sparse Grid Discretizations based on a Discontinuous Galerkin Method

A paper written in collaboration with Dr. Erik Schnetter at the Perimeter Institute for theoretical physics, on using the discontinuous Galerkin method together with sparse grids to evolve scalar wave equations in higher dimensional spacetimes. We developed a julia package, GalerkinSparseGrids.jl for the implementation of this theory, and present its results for 5+1 and 6+1-dimensional wave evolutions in the paper. This yields a significant computational step forward in the capacity to solve hyperbolic partial differential equations in higher dimensional space, and we hope to extend this work to Einstein's equations in an anisotropic spacetime.

(2017) [arXiv] [PDF] [Code Repository]

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Simple, accurate electrostatics-based formulas for calculating detachment energies of fullerenes

A paper written in collaboration with Dr. James Ellenbogen at the MITRE Corporation, published in Physical Review A, on deriving a simple analytic formulas via electrostatics-based methods to accurately calculate the values of electron affinities, ionization potentials, and capacitances of icosahedral carbon fullerenes. The aforementioned analytic formulae yield significant insight into both the physics of electron detachment in the fullerenes and shed light on prior investigations of the relationship between sizes of the fullerenes and their molecular capacitances.

(2017) Physical Review A [Journal Link] [PDF]

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Graded Lie Algebras, Supersymmetry, and Applications

Final term paper on supersymmetry for Francesco Iachello's class on Group Theory, Lie Algebras, and their applications to physics. Many of the examples are taken from the material he provided me (c.f. the references section).

(Fall 2015) [PDF]




Lectures and Lecture Notes

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Solutions to "String Theory in a Nutshell"

In my first two years of grad school I wanted to get as much reading out of the way as possible. Alongside several books on quantum field theory, I did a rather intensive reading of E Kiritsis' "String Theory in a Nutshell". Most problems are done, save for various topics in compactification and running of effective couplings, as well as the new edition's great chapter on applied holography. Perhaps I will one day finish these remaining problems. If you find errors and want me to make corrections, please do reach out over email. Also, if you have solutions to problems I haven't done, please also reach out. I'd love to add them in, of course crediting you.

(2019-2020) [PDF]

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Aspects of Open String Field Theory

Final project and presentation for Prof. Xi Yin's second semester course on string theory. Here I discuss string field theory, a "second quantized" formalism for dealing with off-shell processes in string theory. This is more involved than the commonly taught "first quantized" on-shell treatment of strings and superstrings. At least for open strings, there is a remarkably simple formulation in terms of Witten's "cubic" open string field theory (OSFT). In this final paper I give an exposition to this theory, and detail how the standard formalism can be used to recover the Veneziano amplitude, following Giddings. Then, I review Sen's remarkable paper demonstrating how the simple technique of level-truncation in OSFT can "see" the infamous bosonic string tachyon's potential and equate its minimum with the condensation of the D-brane.

(Spring 2020) [PDF]

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The Kondo Effect, Conformal Field Theory, and Quantum Dots

Final project and presentation for Prof. Ashvin Vishwanath's course on topics in quantum matter. Ken Wilson won the Nobel Prize in 1982 for applying his renormalization group (RG) technique to understand the low-temperature "strongly-interacting" regime of a the Kondo effect in a metal with magnetic impurity. From the modern perspective of studying field theory as a flow from one RG fixed point to another, we study a simple 1+1D version of the Kondo effect, and show how both the noninteracting and strongly-coupled fixed points can be understood in terms of WZW conformal field theories. We use this as a jumping-off point to think about how this formalism could shed new light into recent developments in understanding the Kondo effect in quantum dots.

(Fall 2018) [PDF]

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Holographic Duality and Applications

My set of handwritten notes for Prof. Hong Liu's Lectures on (MIT course 8.871) on the AdS/CFT correspondence. We begin with a motivation for holography from the thermodynamics of black holes demonstrating that their entropy must be proportional to horizon area. Then, after introducing the machinery from matrix field theory and string theory, we show how the large-N limit of matrix QFTs can be recast into an equivalent string theory-as in Maldacena's seminal paper. From here, we introduce the relevant concepts on both sides of the duality: anti-de Sitter space and conformal field theory, and apply this to understand phenomena such as quark confinement and entanglement entropy.

(Fall 2018) [PDF] [Chapter 1: Black Holes and the Holographic Principle] [Chapter 2: Matrices and Strings] [Chapter 3: Holographic Duality]

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The Path Integral, Wilson Lines, and Disorder Operators

My lectures for Prof. Philsang Yoo's second-semester graduate seminar on the Langlands program. I define the ideas of quantum field theory for an audience of mathematicians. From here, I define the Wilson and `t Hooft line defect operators and elaborate on their relationship to the geometric Satake symmetries acting on both sides of the Langlands correspondence.

(Spring 2018) [Lecture 1] [Lecture 2]

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3D Monopoles and the Equations of Bogomolny and Nahm

My final presentation for Prof. Philsang Yoo's first graduate seminar on the ideas of the geometric Langlands program. This is introductory paper introduces the ideas of gauge theory for a mathematical audience, and then uses those ideas to define SU(n) monopoles in 3D in terms of translation invariant solutions to the anti-self-dual instanton equations in 4D (see prior lecture notes on instantons and the ADHM construction). This study of magnetic monopoles on 3D space serves to motivate the ideas behind the Nahm Transform, which will be explored in greater depth next semester for my senior thesis.

(Fall 2017) [PDF]

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3D Conformal Field Theory and the Ising Model

A set of lectures given both as a final presentation for my research in CFT over the spring of 2017, as well as a related talk given in the Fall as part of the Yale graduate seminar in representation theory, as an attempt to introduce the ideas of higher-dimensional CFT to a more mathematical audience with some background in the ideas of the Virasoro algebra for the 2D case.

(Spring, Fall 2017) [Spring Talk] [Fall Talk]

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2D Conformal Field Theory and the Ising Model

These lecture notes are from the final presentation given in Professor David Poland's graduate seminar in conformal field theory and its applications. The notes are based on the results from a paper by Belavin, Polyakov, and Zamolodchikov on the applications of 2D conformal field to various critical systems.

(Fall 2016) [PDF]

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Talks on the ADHM Construction, Hilbert Schemes, and the Heisenberg Algebra

The hardest, most rewarding math class I ever took during my time at Yale was Igor Frenkel's graduate representation theory seminar. I prepared a series of three one-hour talks, discussing how the ADHM construction gives rise to a moduli space of solutions that can be compactified and resolved. In particular, although U(1) instantons are known not to exist in physics, the resolved compactification of the moduli space of U(1) instantons is nonempty, and in fact corresponds exactly to the Hilbert scheme of points on the complex affine plane. I discussed their homology rings and show that the algebraic structure is in fact intimately related to the Heisenberg algebra. This geometric realization is studied, in broad terms, by recognizing Hopf algebra structure both in the Heisenberg algebra and in the homology rings of the Hilbert schemes.

(Fall 2016) [Review notes on Fiber Bundles] [Talk 1] [Talk 2]

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Galois Representations, Cuspidal Eigenforms, and Maass Forms at 1/4

Notes written for a final lecture in Math 868 Spectral Geometry. A brief introduction to algebraic number theory, and then a focus on the correspondence between two dimensional representations of the absolute Galois group and Hecke/Maass eigenforms on the upper half plane.

(Spring 2016) [PDF]

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6j symbols and the Tetrahedron

Notes written for a final lecture in Math 501 Representation Theory. Exposition of the 6j symbols and the remarkable fact that geometric information about an associated tetrahedron volume gives asympototically correct estimates for the symbols themselves.

(Spring 2016) [PDF]




Mentoring

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Dylan Raphael - RSI

I mentored Dylan over the summer of 2019, as part of the Research Science Institute (RSI) program at MIT. I worked him through a directed reading course in undergraduate quantum mechanics and multilinear algebra, culminating in the introduction of tensor networks as a tool to understand interacting, many-body quantum systems. He developed a computational approach in the Julia programming language to study the phase transition of the tricritical quantum Ising model. Dylan is now an undergrad at MIT 🎉

(Summer 2019) [Final Presentation]

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Perimeter Institute ISSYP Mentorship

Lecture that I gave to the high school students participating in the Perimeter Institute's International Summer School for Young Physicists (ISSYP). Introduction of manifolds and vector fields to students with calculus background. Covered linear algebra topics such as direct sums and tensor products of vector spaces and co/contravariance. The end goal was to motivate the intuition behind the idea of a Lie Algebra/Lie Derivative in terms of vector flows.

(Summer 2016) [Lecture]




Books

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Complex Analysis: In Dialogue

A book I wrote back in high school on complex analysis. It was initially a set of notes written as a result of self-study over the Summer of 2013. Gradually, it grew into a larger pedagogical work. The dialogue of the book is in the style of Johann Joseph Fux's work Gradus ad Parnassum, a formal conversation between teacher and student at length on a large subject. You can see the type of fun I had as a 17 year-old, but its still fun to look back and re-read parts. Despite it being a teenager's first effort at textbook-writing, the book definitely has some charming parts.

(Fall 2013) [PDF]

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Complex Analysis: Appendix of Color Plots

Accompanying color plots for students and readers to be able to visualize some of the complex functions mentioned in the above work. The plots were made in Java using elementary classes in complex-number arithmetic and graphics. You can find it on Amazon here.

(Fall 2013) [PDF]