Talks

Quantum many-body scars (QMBS) have emerged as a captivating anomaly within the landscape of quantum physics, challenging the conventional expectations of the eigenstate thermalization hypothesis (ETH). According to ETH, an isolated quantum system is expected to evolve toward thermal equilibrium, with local observables equilibrating to values predicted by statistical mechanics, independent of the initial state of the system. However, QMBS present a remarkable exception by exhibiting resistance to thermalization, thus maintaining quantum information for unexpectedly long durations.

This colloquium will delve into the intriguing realm of QMBS, highlighting their pivotal role in advancing our understanding of quantum thermalization and their potential applications in quantum dynamics and technology. The discussion will cover recent theoretical and experimental progress in identifying systems that display these scars, focusing on their properties and the mechanisms by which they arise.

A specific area of interest is the construction of QMBS states emerging from Einstein-Podolsky-Rosen (EPR) states in bilayer systems, where each layer is maximally entangled. We will explore applications of this framework in quantum dimer models, examining various features of the bilayer model that contribute to the emergence of these states. Furthermore, if time allows, the talk will extend to systems of itinerant bosons, demonstrating how an infinite tower of many-body scar states can manifest in bilayer Bose-Hubbard models with charge conservation. We will discuss the implications of these findings in the context of recent experimental advancements, considering how these theoretical constructs relate to physically realizable systems in laboratory settings.

Part I: OGRePy: Object-Oriented General Relativity in Python   I will present a detailed introduction to my new Python package, OGRePy: (O)bject-Oriented (G)eneral (Re)lativity for (Py)thon, a port of my popular Mathematica package OGRe, which is used by many researchers in general relativity and related areas. I will demonstrate the package's usage and features, including its ability to manipulate tensors of arbitrary rank using an intuitive interface, and calculate arbitrary tensor formulas involving any combination of addition, multiplication, trace, contraction, and partial and covariant derivatives - while automatically figuring out the proper index configuration and coordinate system to use for each tensor, eliminating user error.   Part II: Time Travel Paradoxes and Entangled Timelines   If time travel is possible, it seems that it would inevitably lead to paradoxes, indicating an internal inconsistency in our current theories of nature. Can these paradoxes be resolved by new laws of physics, or perhaps even existing ones? I will first review the different types of time travel paradoxes and their proposed resolutions. Then I will present the results of my 3 recent papers (1911.11590, 2110.02448, 2303.07635) discussing different aspects of time travel paradoxes from the perspectives of both general relativity and quantum mechanics. I will argue that generic time travel paradoxes can only be resolved using the concept of parallel timelines, and suggest possibilities for how such timelines may manifest themselves.

Lorentzian path integrals exhibit profoundly different properties from Euclidean ones due to the oscillatory integrand which weighs different configurations through interference. Key troubles encountered in Euclidean quantum gravity are the conformal factor problem of Euclidean quantum GR and divergences due to spike configurations in Euclidean quantum Regge calculus. The first part of this talk will focus on how these troubles are resolved in Lorentzian quantum Regge calculus. I will emphasize the unambiguous choice of contour for the integral over the conformal mode in a saddle-point expansion and furthermore show that bulk-length expectation values are finite for spike and spine configurations away from the classical regime. The second part of this talk will focus on properties of Lorentzian path integrals beyond GR. I will illustrate that higher-derivative and non-local actions can be expected to suppress spacetime configurations with curvature singularities. Finally, I will revisit the long-standing question of global symmetries in quantum gravity by providing examples for non-local actions designed to suppress global-symmetry-violating black-hole configurations in the Lorentzian path integral.