Format results






A new tangential structure for type IIA string theory
Matthew Yu University of Oxford

From nonlocality transitivity to resource marginal problems and back
YeongCherng Liang National Cheng Kung University


Symmetry enforced entanglement in maximally mixed states
Subhayan Sahu Perimeter Institute for Theoretical Physics



Upper Bound on Thermal Gravitational Wave Backgrounds from Hidden Sectors
Juraj Klaric University of Amsterdam

Celestial Holography from Euclidean AdS space.
We will explore the connection between Celestial and Euclidean Antide Sitter (EAdS) holography in the massive scalar case. Specifically, exploiting the socalled hyperbolic foliation of Minkowski spacetime, we will show that each contribution to massive Celestial correlators can be reformulated as a linear combination of contributions to corresponding massive Witten correlators in EAdS. This result will be demonstrated explicitly both for contact diagrams and for the fourpoint particle exchange diagram, and it extends to all orders in perturbation theory by leveraging the bootstrapping properties of the Celestial CFT (CCFT). Within this framework, the KantorovicLebedev transform plays a central role, which will be introduced at the end of the talk. This transform will allow us to make broader considerations regarding nonperturbative properties of a CCFT.

Categories of line defects and cohomological Hall algebras
BPS line defects in 4d N=2 supersymmetric QFT are described by a monoidal category with a list of desired properties. For example, the Grothendieck group of this category is supposed to coincide with quantization of functions on Coulomb branch of the theory compactified on a circle. Based on an observation, that at a given vacuum the spectrum of PBS particles can be quipped with an algebra structure – cohomological Hall algebra of the corresponding BPS quiver – we propose a category generated by certain bimodules over this algebra that possesses expected properties of the category of lines. Based on a joint work with Davide Gaiotto and Wei Li.

Partitions in quantum theory
Augustin Vanrietvelde Télécom Paris
The standard perspective on subsystems in quantum theory is a bottomup, compositional one: one starts with individual "small" systems, viewed as primary, and composes them together to form larger systems. The topdown, decompositional perspective goes the other way, starting with a "large" system and asking what it means to partition it into smaller parts. In this talk, I will 1/ argue that the adoption of the topdown perspective is the key to progress in several current areas of foundational research; and 2/ present an integrated mathematical framework for partitions into three or more subsystems, using subC* algebras. Concerning the first item, I will explain how the topdown perspective becomes crucial whenever the way in which a quantum system is partitioned into smaller subsystems is not unique, but might depend on the physical situation at hand. I will display how that precise feature lies at the heart of a flurry of current hot foundational topics, such as quantum causal models, Wigner's friend scenarios, superselection rules, quantum reference frames, and debates over the implementability of the quantum switch. Concerning the second item, I will argue that partitions in (finitedimensional) quantum theory can be naturally pinned down using subC* algebras. Building on simple illustrative examples, I will discuss the oftenoverlooked existence of nonfactor C*algebras, and how it leads to numerous subtleties  in particular a generic failure of local tomography. I will introduce a sound framework for quantum partitions that overcomes these challenges; it is the first topdown framework that allows to consider three or more subsystems. Finally, as a display of this framework's technical power, I will briefly present how its application to quantum causal modelling unlocked the proof that all 1D quantum cellular automata admit causal decompositions.
(This is joint work with Octave Mestoudjian and Pablo Arrighi. This talk is complementary to my Causalworlds 2024 presentation, which will focus on the issue of causal decompositions.)

Think Emergent  what if we abandon the fundamentality of (quantum) gravity?
Angela Chen University of Tokyo
In this talk, I will present a published and an ongoing work in the direction of emergent gravity. The first is what I dubbed as the generalized Unruh effect, a mapping from arbitrary states in the Fock space of positive Minkowski (Kruskal) modes to Rindler (Schwarzschild) modes obtained by Bogoliubov transformation. The special case of vacuum state  thermal bath mapping has been well known in the textbooks, the original Unruh effect. I will discuss the interesting physical implications of the generalized Unruh effect on the black hole information paradox. In the second part, I will give a novel conjecture of the dark energy when considering the spacetime as a 4d volumeconserved fluid. Einstein's equation with an alwayspositive metric term (Lambda, but not necessarily constant) can be interpreted as the differential version of this assumed conservation of 4d volume at linear order. The rich phenomenology of this theory will be discussed, including its solution to the anthropic problem of our currently darkenergydominated universe, and highredshift overevolved astrophysical objects that have been recently popping up in the JWST survey.

Binary neutron star mergers: from numerical relativity simulations to electromagnetic observables
Eduardo Gutierrez Penn State University
In this talk, I will provide an overview of neutron star (NS) mergers, highlighting the insights gained through numerical relativity simulations. I will mainly focus on the role of the cocoon shock breakout emission as a key early electromagnetic counterpart of NS mergers, with special relevance to events like GW170817. I will explore how the properties of the merger ejecta and the nature of the central engine influence the resulting emission. Additionally, I will present recent advancements in the development of our new general relativistic magnetohydrodynamics (GRMHD) code GRAthena++, and share ongoing research efforts on the evolution of magnetic fields in the postmerger remnant.

A new tangential structure for type IIA string theory
Matthew Yu University of Oxford
The GreenSchwarz anomaly cancellation condition says that the target space of heterotic string theory must come with a string structure for the theory to be consistent. In this talk we discuss a new tangential structure called string^h, first introduced by Devalapurkar, as a spin^c analogue of string. We will show that the spectrum of string^h has the notable property that it orients tmf_1(n), just like how the spectrum of string orients tmf, by the work of AndoHopkinsRezk. Finally we will show that the anomaly of the partition function of type IIA, studied by DiaconescuMooreWitten induces a string^h structure on the target space of type IIA, in parallel to the GreenSchwarz anomaly for heterotic string theory, and discuss applications for anomaly cancellation. This is joint work in progress with Arun Debray.

From nonlocality transitivity to resource marginal problems and back
YeongCherng Liang National Cheng Kung University
In this talk, I shall recall the nonlocality transitivity problem, which concerns the possibility of inferring the Bellnonlocality of certain marginals in a multipartite scenario based on other given marginals. Then, I explain how considering this problem has led to a more general class of problems known as resource marginal problems (RMPs). More precisely, RMPs concern the possibility of having a resourcefree target subsystem compatible with a given collection of marginal density matrices. We briefly discuss how a resource theory for a collection of marginal density matrices naturally arises from any given RMP and present some general features of such a theory. After that, we focus on a special case of RMPs known as the entanglement transitivity problems and explain how our progress on this problem has led to progress in the original nonlocality transitivity problem.

Quantum Geometry of the Light Cone
Wolfgang Wieland University of ErlangenNuremberg
PIRSA:24080003Abstract: In relativity, the geometry of the light cones determines the causal structure of spacetime. Under the influence of gravity, the light cones bend and curve. A previously expanding light cone can fall back into itself. In this way, the causal structure becomes a dynamical aspect of spacetime. How do we understand this link between gravity, geometry and causality at the quantum level? Is there a quantum light cone geometry? In my talk, I will argue that the answer to this problem is crucial for making progress in quantum gravity. It is, in fact, a problem that is shared among different approaches, from holography, to celestial amplitudes and loop quantum gravity. In my presentation, I report on three new results on this frontier. First, I provide a nonperturbative characterization of impulsive gravitational null initial data for tetradic gravity on a light cone. Second, the description is taken to the quantum level. Third, an immediate physical implication is found: in the model, the Planck luminosity separates the eigenvalues of the radiated power. Below the Planck power, the spectrum of the radiated power is discrete. Above the Planck power, the spectrum is continuous and the resulting physical states contain caustics that can spoil the semiclassical limit. The talk is based on arXiv:2402.12578, arXiv:2401.17491, arXiv:2104.05803.

Symmetry enforced entanglement in maximally mixed states
Subhayan Sahu Perimeter Institute for Theoretical Physics
Entanglement in quantum manybody systems is typically fragile to interactions with the environment. Generic unital quantum channels, for example, have the maximally mixed state with no entanglement as their unique steady state. However, we find that for a unital quantum channel that is `strongly symmetric', i.e. it preserves a global onsite symmetry, the maximally mixed steady state in certain symmetry sectors can be highly entangled. For a given symmetry, we analyze the entanglement and correlations of the maximally mixed state in the invariant sector (MMIS), and show that the entanglement of formation and distillation are exactly computable and equal for any bipartition. For all Abelian symmetries, the MMIS is separable, and for all nonAbelian symmetries, the MMIS is entangled. Remarkably, for nonAbelian continuous symmetries described by compact semisimple Lie groups (e.g. SU(2)), the bipartite entanglement of formation for the MMIS scales logarithmically ∼logN with the number of qudits N.

Causality Protection in Effective Field Theory
I discuss how quantum mechanical effects prevent violations of causality in low energy processes, even when fasterthanlight propagation is possible. At the classical level, fasterthanlight propagation can be used to build "timemachine" configurations that violate causality. However, lowenergy quantum excitations propagating on these backgrounds lead to divergent backreaction through loop effects. These divergences fully probe the UV dynamics of the system, making it impossible to prepare and describe causality violating configurations in the regime of validity of effective field theory. In light of these results, I conclude that effective field theories with negative Wilson coefficients or Galileonsymmetric interactions are not in tension with causality, despite leading to fasterthanlight propagation.

Quantum spatiotemporal correlations and Bayes' rule
While quantum correlations between two spacelikeseparated systems are fully encoded by the bipartite density operator associated with the joint system, what operator encodes quantum correlations across space and time? I will describe the general theory of such "quantum states over time" as well as a canonical example that encodes the expectation values of certain observables measured sequentially in time. The latter extends the theory of pseudodensity matrices to arbitrary dimensions, not necessarily restricted to multiqubit systems. In addition, quantum states over time admit a natural proposal for a generalpurpose quantum Bayes' rule. Our results specialize to many wellstudied examples, such as the stateupdate rule, the twostate vector formalism and weak values, and the Petz recovery map. This talk is based on joint work with James Fullwood and the two papers: arXiv: 2212.08088 [quantph] and 2405.17555 [quantph].

Upper Bound on Thermal Gravitational Wave Backgrounds from Hidden Sectors
Juraj Klaric University of Amsterdam
Hot viscous plasmas unavoidably emit a gravitational wave background, similar to electromagnetic black body radiation. In this talk we will discuss the contribution from hidden particles to the diffuse background emitted by the primordial plasma in the early universe. While this contribution can easily dominate over that from Standard Model particles, both are capped by a generic upper bound that makes them difficult to detect with interferometers in the foreseeable future. We will illustrate our results on the examples of axionlike particles and heavy neutral leptons. We will also discuss how this bound affects the previous estimates of gravitational wave backgrounds from particle decays out of thermal equilibrium.
