Scientific Series

No. Obviously not. It's a daft question. But, buried underneath
this daft question is an extremely interesting one: is it possible to
simulate the known laws of physics on a computer? Remarkably, there is a
mathematical theorem, due to Nielsen and Ninomiya, that says the answer is
no. I'll explain this theorem, the underlying reasons for it, and some
recent work attempting to circumvent it.

I will review recent progress in application of separation of variables method.

In particular I will review the construction for the integrable spin chains with gl(N) symmetry. 

By finding, for the first time, the matrix elements of the SoV measure explicitly I will show how to compute various correlation functions and wave function overlaps in a simple determinant form. 

General philosophy of application of these methods to the problems related to AdS/CFT, N=4 SYM etc. will be discussed too. 

With much of the cosmological information in the primary CMB having already been mined, the next decade of CMB observations will revolve around the secondary CMB lensing effect, which will touch nearly all aspects of observation in some way. At the same time, the increasingly low noise levels of these future observations will render existing "quadratic estimator" methods for analyzing CMB lensing obsolete. This leaves us in an exciting place where new methods need to be developed to fully take advantage of the upcoming generation of CMB data just on our doorstep. I will describe my work developing such new lensing analysis tool, made possible by Bayesian methods, modern statistical techniques, and borrowing ideas from machine learning. I will present the recent first-ever application of such methods to data (from the South Pole Telescope; https://arxiv.org/abs/2012.01709) and discuss prospects for this analysis in the future with regards to not just lensing but also primordial B modes, reionization, and extragalactic foreground fields.

In this talk I will present a general framework to distinguish different classes of charge insulators based on whether or not they insulate or conduct higher multipole moments (dipole, quadrupole, etc.). This formalism applies to generic many-body systems that support multipolar conservation laws. Applications of this work provide a key link between recently discovered higher order topological phases and fracton phases of matter.

A series of recent works has shown that placing communication channels in a coherent superposition of alternative configurations can boost their ability to transmit information. Instances of this phenomenon are the advantages arising from the use of communication devices in a superposition of alternative causal orders, and those arising from the transmission of information along a superposition of alternative trajectories. The relation among these advantages has been the subject of recent debate, with some authors claiming that the advantages of the superposition of orders could be reproduced, and even surpassed, by other forms of superpositions. To shed light on this debate, we develop a general framework of resource theories of communication. In this framework, the resources are communication devices, and the allowed operations are (a) the placement of communication devices between the communicating parties, and (b) the connection of communication devices with local devices in the parties' laboratories. The allowed operations are required to satisfy the minimal condition that they do not enable communication independently of the devices representing the initial resources. The resource-theoretic analysis reveals that the aforementioned criticisms on the superposition of causal orders were based on an uneven comparison between different types of quantum superpositions, exhibiting different operational features.

Ref. https://iopscience.iop.org/article/10.1088/1367-2630/ab8ef7

A central challenge in quantum many-body physics is a characterization of properties of `natural' quantum states, such as the ground states and Gibbs states of a local hamiltonian. The area-law conjecture, which postulates a remarkably simple structure of entanglement in gapped ground states, has resisted a resolution based on information-theoretic methods. We discuss how the right set of insights may come, quite unexpectedly, from polynomial approximations to boolean functions. Towards this, we describe a 2D sub-volume law for frustration-free locally-gapped ground states and highlight a pathway that could lead to an area law. Similar polynomial approximations have consequences for entanglement in Gibbs states and lead to the first provably linear time algorithm to simulate Gibbs states in 1D. Next, we consider the task of learning a Hamiltonian from a Gibbs state, where many-body entanglement obstructs rigorous algorithms. Here, we find that the effects of entanglement can again be controlled using tools from computer science, namely, strong convexity and sufficient statistics. 

Euclidean quantum gravity approaches have a long history but suffer from a number of severe issues.  This gives a strong motivation to develop Lorentzian approaches. Spin foams constitute an important such approach, which incorporate a rigorously derived discrete area spectrum.  I will explain how this discrete area spectrum is connected to the appearance of an anomaly, which explains the significance of the Barbero-Immirzi parameter and forces an extension of the quantum configuration space, to also include torsion degrees of freedom. This can be understood as a defining characteristic of the spin foam approach, and provides a pathway to an (experimental) falsification.

All these features are captured in the recently constructive effective spin foam model, which is much more amenable to numerical calculations than previous models.  I will present numerical results that a) show that spin foams do impose the correct equations of motion b) highlight the influence of the anomaly and c) underline the difference to Euclidean quantum gravity.  I will close with an outlook on the features that can be studied with a truly Lorentzian model, e.g. topology change.

Models for black hole formation from stellar evolution predict the existence of a pair-instability supernova mass gap in the range ~50 to ~120 solar masses. The binary black holes of LIGO-Virgo's first two observing runs supported this prediction, showing evidence for a dearth of component black hole masses above 45 solar masses. Meanwhile, among the 30+ new observations from the third observing run, there are several black holes that appear to sit above the 45 solar mass limit. I will discuss how these unexpectedly massive black holes fit into our understanding of the binary black hole population. The data are consistent with several scenarios, including a mass distribution that evolves with redshift and the possibility that the most massive binary black hole, GW190521, straddles the mass gap, containing an intermediate-mass black hole heavier than 120 solar masses.

In this talk, I will review the recent advances in the understanding of intriguing infinite-dimensional symmetries that emerge at the boundary of asymptotically flat spacetime and in the vicinity of black hole horizons. These symmetries play an important role in the description of many phenomena such as gravitational memory effects and scattering amplitudes, and have paved the way to a holographic description of gravity in asymptotically flat spacetimes in terms of correlators on the celestial sphere.

We study the real-time dynamics of a small bubble of "false vacuum'' in a quantum spin chain near criticality, where the low-energy physics is described by a relativistic (1+1)-dimensional quantum field theory. Such a bubble can be thought of as a confined kink-antikink pair (a meson). We carefully construct bubbles so that particle production does not occur until the walls collide. To achieve this in the presence of strong correlations, we extend a Matrix Product State (MPS) ansatz for quasiparticle wavepackets [Van Damme et al., arXiv:1907.02474 (2019)] to the case of confined, topological quasiparticles. By choosing the wavepacket width and the bubble size appropriately, we avoid strong lattice effects and observe relativistic kink-antikink collisions. We use the MPS quasiparticle ansatz to identify scattering outcomes: In the Ising model, with transverse and longitudinal fields, we do not observe particle production despite nonintegrability (supporting recent numerical observations of nonthermalizing mesonic states). With additional interactions, we see production of confined and unconfined particle pairs. Although we simulated these low-energy, few-particle events with moderate resources, we observe significant growth of entanglement with energy and with the number of collisions, suggesting that increasing either will ultimately exhaust our methods. Quantum devices, in contrast, are not limited by entanglement production, and promise to allow us to go far beyond classical methods. We anticipate that kink-antikink scattering in 1+1 dimensions will be an instructive benchmark problem for relatively near-term quantum devices.

The talk will focus on the spectrum of near-extremal black holes in gravity and near-BPS black holes in supergravity. For concreteness, we will study cases in asymptotically four-dimensional flat space and three-dimensional Anti-de Sitter. This will be done by analyzing quantum effects near the horizon captured by an emergent Jackiw-Teitelboim mode at low temperatures. This will allow us to systematically study questions such as the extremal degeneracy and the size of the gap in the black hole spectrum, which can be compared to some string theory constructions.

Recent years new concepts of symmetries have been developed such as higher form symmetries, and categorical symmetries. The higher form symmetries can be either explicit in a Hamiltonian, or inexplicit as a dual of an ordinary symmetry. The behavior of higher form symmetries are easy to evaluate in phases with gaps. But at quantum criticalities their behaviors are more nontrivial. We evaluate the behaviors of higher form symmetries (either explicit or inexplicit) at various quantum critical points, and demonstrate that for many quantum critical points a universal logarithmic contribution arises, which is analogous to the quantum entanglement entropy. This logarithmic contribution is related to the universal conductance at the quantum critical points, and in some cases can be computed exactly using duality between CFTs developed in last few years. We also evaluate the behavior of categorical symmetries for more exotic cases with subsystem symmetries.