Maissam Barkeshli Profile picture
Associate Professor of Physics at University of Maryland, College Park and Joint Quantum Institute. Previously at Berkeley, MIT, Stanford, Microsoft Station Q
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Aug 21, 2022 7 tweets 2 min read
1/The Nobel Laureate Philip Anderson once wrote,

"It is only slightly overstating the case to say that physics is the study of symmetry."

But what is symmetry? Amazingly, results over the past decade are revising our basic understanding of this question 2/These developments have come from disparate fields of physics -- condensed matter, high energy, quantum information theory -- and pure math. Together they weave a new picture of symmetry, showing us that what we understood over the past century is just the tip of the iceberg.
Jul 31, 2021 11 tweets 3 min read
1/ TIME CRYSTALS. The very name evokes a sense of awe and mystery. Everyone loves crystals, and no one understands time. Most popular accounts are misleading at best. Here is a brief discussion summarizing my take. Feel free to add anything I missed. 2/ The primary phenomenon is spontaneous symmetry breaking in time: Apply periodic drive, and the system responds at half the frequency. This is an OLD and WELL-KNOWN phenomenon in dynamical systems. Sec. 8 of arxiv.org/abs/1910.10745 and nature.com/articles/s4156… discuss examples
Dec 24, 2020 7 tweets 2 min read
1/ The 1982 experimental discovery of the fractional quantum Hall effect dramatically changed our understanding of the nature of quantum matter. Eventually it was understood that FQH states are examples of symmetry-enriched topological phases of matter (SETs). 2/ Depending on the symmetry of the system, there can be distinct topological invariants that characterize the quantum state. In 2014, we developed a mathematical framework to systematically characterize SETs using G-crossed braided tensor categories: arxiv.org/abs/1410.4540
Jul 14, 2020 10 tweets 2 min read
1/ The study of topological quantum matter has been one of the most popular and exciting developments in physics in the last 15 years, although there has been a steady stream of amazing developments for the last 50 years. 2/ These ideas contain two distinct intellectual threads. There is topological band theory, and then there is the theory of topological phases of matter. The distinction is often blurred. But they are not the same.
Jul 6, 2020 51 tweets 6 min read
An absolutely incredible, highly interconnected web of ideas connecting some of the most important discoveries of late twentieth century physics and mathematics. This is an extremely abridged, biased history (1970-2010) with many truly ground-breaking works still not mentioned: 1971: Wegner reformulates the Ising model as a Z/2 gauge theory and discovers phase transitions without local order parameters (beginning of understanding of topological order).