Scientific Series

Many cosmological scenarios beyond the Standard Model lead to the formation of a network of cosmic strings. In this talk, I will review how these models lead in a natural way to the production of a stochastic gravitational wave background and how this signal could account for the recently reported results from the NANOGrav collaboration. Finally, we will explain how future observations could allow us to confirm this interpretation of the NANOGrav data.

In this talk I introduce nonlocal (infinite derivative) field theories. First of all, I discuss how and which principles of quantum field theory are affected when higher-order derivative operators are taken into account in a Lagrangian. In particular, I focus on the issue of unitarity and on how to make higher-derivative theories healthy by means of non-polynomial differential operators. I extend the treatment to the gravity sector and consider nonlocal theories whose graviton propagators are ghost-free, and explore the possibility of regularizing singularities. Next, I discuss some recent progress in proving perturbative unitarity for a very general class of nonlocal field theories. Finally, I will make some remarks on nonlocality and quantum gravity.

We will discuss quantum singular value transformation (QSVT), a simple unifying framework for quantum linear algebra algorithms developed by Gilyén, Low, Su, and Wiebe. QSVT is often applied to try to achieve quantum speedups for machine learning problems. We will see the typical structure of such an application, the barriers to achieving super-polynomial quantum speedup, and the state of the literature that's attempting to bypass these barriers. Along the way, we'll also see an interesting connection between quantum linear algebra and classical sampling and sketching algorithms(explored in the form of "quantum-inspired" classical algorithms).

The study of spectral statistics is of importance due to its universality and utility as a robust diagnostic of quantum chaos. For closed many-body quantum chaotic systems, I will present two results: (i) a quantum-classical mapping that connects the Thouless time, which characterizes the onset of RMT of the spectral form factor (SFF); and the spectral gap of a dual classical stochastic system; (ii) a set of Lyapunov exponents which characterize the spectral statistics in the thermodynamic limit. For open quantum systems with complex spectra, I will propose and analyze a generalized SFF, and show that dissipative quantum chaotic systems display a “dip-ramp-plateau” behaviour with a quadratic ramp.

When a generic isolated quantum many-body system is driven out of equilibrium, its local properties are eventually described by the thermal ensemble. This picture can be intuitively explained by saying that, in the thermodynamic limit, the system acts as a bath for its own local subsystems. Despite the undeniable success of this paradigm, for interacting systems most of the evidence in support of it comes from numerical computations in relatively small systems, and there are very few exact results. In the talk, I will present an exact solution for the thermalization dynamics in the "Rule 54" cellular automaton, which can be considered the simplest interacting integrable model. After introducing the model and its tensor-network formulation, I will present the main tool of my analysis: the space-like formulation of the dynamics. Namely, I will recast the time-evolution of finite subsystems in terms of a transfer matrix in space and construct its fixed-points. I will conclude by showing two examples of physical applications: dynamics of local observables and entanglement growth. The talk is based on a recent series of papers: arXiv:2012.12256, arXiv:2104.04511, and arXiv:2104.04513.

Dark matter constitutes 85% of the matter content in the Universe, but its physical nature remains unknown and requires new physics to explain. I will review the status of the recent early-universe and late-universe searches for the identity of dark matter, summarizing the best current limits on scattering between dark matter and baryons, and discussing cosmological limits on the mass of thermal-relic dark matter. I will highlight the interplay between the thermal history of the universe and the formation of structure as complementary probes of dark matter physics, using the example of the 21-cm signal from the Cosmic Dawn. Finally, I will discuss the prospects for unveiling the physics of dark matter in the coming decade.

Kitaev quantum spin liquids (QSLs) are exotic states of matter that are predicted to host Majorana fermions and gauge flux excitations. However, so far all known Kitaev QSL candidates are known to have appreciable non-Kitaev interactions that pushes these systems far from the QSL regime. Co based magnets have been proposed to be perhaps a more ideal platform for realizing Kitaev QSLs. In this talk I will show evidence for a Kitaev interactions in both the quasi-one-dimensional ferromagnet CoNb2O6 as well as the hexagonal magnet BaCo2(AsO4). Although it is usually believed to be the best material realization of a 1D Ising chain, by combining terahertz spectroscopy and calculations we have shown that CoNb2O6 is well described by a model with bond-dependent interactions. We call this model the ‘twisted Kitaev chain’, as these interactions are similar to those of the honeycomb Kitaev spin liquid. The ferromagnetic ground state of CoNb2O6 arises from the compromise between two axes. Owing to this frustration, even at zero field domain walls have quantum motion, which is described by the celebrated Su–Schrieffer–Heeger model of polyacetylene and shows rich behavior as a function of field. Most recently, we have shown also that the honeycomb cobalt-based Kitaev QSL candidate, BaCo2(AsO4)2, has dominant Kitaev interactions. Due to only small non-Kitaev terms a magnetic continuum consistent with Majorana fermions and the existence of a Kitaev QSL can be induced by a small out-of-plane-magnetic field. Our results demonstrate BaCo2(AsO4)2 as a far more ideal version of Kitaev QSL compared with other candidates.

C. M Morris et al. "Hierarchy of bound states in the one-dimensional ferromagnetic Ising chain CoNb2O6 investigated by high-resolution time-domain terahertz spectroscopy.” Phys. Rev. Lett. 112.13 137403 (2014).

C.M. Morris et al. “Duality and domain wall dynamics in a twisted Kitaev chain”, Nature Physics

volume 17, pages 832–836 (2021).

X. Zhang, et al., "In- and out-of-plane field induced quantum spin-liquid states in a more ideal Kitaev material: BaCo2(AsO4)”, https://arxiv.org/abs/2106.13418

Zoom Link: https://pitp.zoom.us/meeting/register/tJcqc-ihqzMvHdW-YBm7mYd_XP9Amhypv5vO

This talk is about how to think about probabilistic reasoning and its use in physics.  It has become commonplace, in the literature on the foundations of probability, to note that the word “probability” has been used in at least two distinct senses: an objective, physical sense (often called “objective chance”), thought to be characteristic of physical situations, independent of considerations of knowledge and ignorance, and an epistemic sense, having to do with gradations of belief of agents with limited information about the world.  I will argue that in order to do justice to the use of probabilistic concepts in physics, we should go beyond this familiar dichotomy, and make use of a third concept, which I call “epistemic chance,” which combines epistemic and physical considerations.

Factorization homology is a local-to-global invariant which "integrates" disk algebras in symmetric monoidal higher categories over manifolds. In this talk I will discuss how to compute categorical factorization homology on oriented surfaces with principal D-bundles, for D a finite group, in terms of categories of modules over algebras defined in purely combinatorial terms. This is an extension of the work of Ben-Zvi, Brochier and Jordan to D-decorated surfaces. The main example for us comes from an action of Dynkin diagram automorphisms on representation categories of quantum groups associated to a reductive group G. We will see that in this case factorization homology gives rise to a quantization of character varieties which are twisted by the group of outer automorphisms of G.

This talk is based on joint work with L. Müller.

Zoom Link: https://pitp.zoom.us/j/93950433494?pwd=WXI2VE9IdnRweEh5RmZsZ21BV1BQQT09

The problem of time is often discussed as an obstacle in the canonical quantisation of gravitational systems: general covariance means there is no preferred time parameter with respect to which evolution could be defined. We can instead characterise dynamics in relational terms by defining one degree of freedom to play the role of an internal clock for the other variables; this leads to a multiple choice problem of which variable should play the role of clock. I will review recent results obtained in a quantum cosmological model with three dynamical degrees of freedom: a volume or scale factor variable for the geometry, a massless scalar matter field, and a perfect fluid. Each of these variables can be used as a clock for the other two. We obtain three different theories which, if we require them to have unitary time evolution with respect to the given clock, make very different statements about the fate of the Universe. Only one resolves the classical singularity, and only one leads to a quantum recollapse of the Universe at large volume. Nonclassical behaviour arises whenever a classical solution terminates in finite time so that reflecting boundary conditions are needed to make the theory unitary. We discuss general implications for a canonical quantisation of gravitational systems.

Zoom Link: https://pitp.zoom.us/j/95556457739?pwd=S0dZUVYwTTBOaFNpNGcrc2ladHZ5QT09

In this talk, I will present how gravitational-wave (GW) measurements can be utilized to search for ultralight bosons and primordial black holes (PBHs), which can be the keys to some of the unsolved questions about our Universe. In the first half, I will discuss the current constraints on excluding the mass of ultralight bosons around ~10^-13 eV, based on the mass-spin measurements of BBHs from the second GW catalog (GWTC-2). These constraints are driven by the phenomenology called “superradiance”, which extracts the angular momentum of a rapidly spinning black hole to form a slowly spinning black hole-boson cloud system. In the second half, I will illustrate how the distance measurements of very high-redshift BBHs made by the next-generation GW detectors may provide cleaner hints or constraints for the existence of PBHs in the future.

Zoom Link: https://pitp.zoom.us/j/95012157554?pwd=UWdBZ1FOTSt2dVlBaS81MjRLbVBvUT09

Classical machine learning (ML) provides a potentially powerful approach to solving challenging quantum many-body problems in physics and chemistry. However, the advantages of ML over more traditional methods have not been firmly established. In this work, we prove that classical ML algorithms can efficiently predict ground state properties of gapped Hamiltonians in finite spatial dimensions, after learning from data obtained by measuring other Hamiltonians in the same quantum phase of matter. In contrast, under widely accepted complexity theory assumptions, classical algorithms that do not learn from data cannot achieve the same guarantee. We also prove that classical ML algorithms can efficiently classify a wide range of quantum phases of matter. Our arguments are based on the concept of a classical shadow, a succinct classical description of a many-body quantum state that can be constructed in feasible quantum experiments and be used to predict many properties of the state. Extensive numerical experiments corroborate our theoretical results in a variety of scenarios, including Rydberg atom systems, 2D random Heisenberg models, symmetry-protected topological phases, and topologically ordered phases.