An Introduction to Open Quantum Systems

• Yukinao Akamatsu

Room: Lecture Hall Open quantum systems provide a framework to describe quantum systems interacting with their environments. The theory has been developed mainly in quantum optics and has recently found applications in various areas of physics, including condensed-matter physics, quantum information, high-energy physics, and cosmology. In this lecture, I will give an introduction to the theory of open quantum systems, focusing on the Lindblad master equation. After introducing its basic formulation, I will discuss general properties of Lindblad dynamics, such as the Liouvillian spectrum and stochastic unraveling. In the final part, I will show an application of the open quantum system approach to quarkonium evolution in relativistic heavy-ion collisions.

Open quantum systems for observers

• Sarah Shandera

Room: Lecture Hall Observers, unlike experimentalists, have no control over the autonomously evolving quantum systems they study. They infer dynamics and states from partial observations, limited to spatial subsystems, finite time windows, accessible charges, and finite measurement resolution. This talk will discuss features revealed by analytic and numerical examples of closed systems described as ensembles of their subsystems, with dynamics described by open-system propagators. Restricting to systems with a symmetry, we contrast the statistical properties that emerge from Hamiltonian dynamics and random circuits with constrained circuits that achieve non-equilibrium steady states. These ensembles provide a foundation for better understanding effective theories and inference for observers, especially in out-of-equilibrium systems.

Tensor catalysis and resummation of the decoherence of primordial scalar fluctuations

• Cliff Burgess

Room: Lecture Hall

Branching structure in the late time universe

• Fumiya Sano

Room: Lecture Hall Admitting the inflationary scenario in the primordial universe, the cosmological structure today originated from quantum fluctuations during cosmic inflation. Since the dynamics of curvature perturbations after inflation is well described by classical equations of motion, there should have been a quantum-to-classical transition in the history of the universe. This classicalization process has been mainly discussed through decoherence of the cosmological perturbations due to the limitation in observable degrees of freedom. However, the decoherence may not be sufficient to completely explain the classicalization. We would like to sharpen the discussion based on quantum Darwinism. This formulation demonstrates the "measurement" process of the cosmological perturbations by radiations and matters, resulting in branch structure of the universe and an effective coherent state of cosmological perturbations.

Quantum correlations in interacting primordial inhomogeneities

• Amaury Micheli

Room: Lecture Hall The statistical properties of the CMB anisotropies, reflecting the curvature inhomogeneities in the early Universe, are very well accounted for by assuming that they emerged from amplified vacuum fluctuations. Being the result of a genuine quantum process, it is natural to wonder which properties of these primordial inhomogeneities are quantum, and which, if any, persisted until their observations despite interactions that decohered, i.e. classicalised, them. I will review the latest progress on these questions, emphasising the quantum information approaches.

Boosting Theoretical Physics with Agentic AI

• Yi Wang

Room: Lecture Hall Over the past few months, AI and AI agents have fundamentally shifted research methodologies in theoretical physics. This talk is divided into two parts. First, we provide an overview and real-world examples of AI integration in daily research, ranging from "vibe physics" via chat interfaces to systematic agent-assisted-research using Codex and Claude Code. Second, we introduce our custom-built AI framework for the theoretical physics workflow, showcasing how it automates and enhances literature reviews, idea discovery, mathematical calculations, and result checking.

Schwinger–Keldysh methods in open quantum systems

• Andrew Tolley (Imperial, London)

Room: Lecture Hall

A Closed-Time-Path look at neutrino dynamics in the early universe

• Yannis Georis (Kavli IPMU)

Room: Lecture Hall The effective number of neutrinos, Neff, is a key parameter in the standard Lambda-CDM cosmology, which captures how many light degrees of freedom are in the early universe plasma before Big Bang Nucleosynthesis. It is therefore a crucial probe of light and feebly interacting hidden sectors. In this talk, I will review how one can describe the evolution of neutrinos in the early universe using techniques from nonequilibrium quantum field theory. Working in this framework, I will then discuss recent works studying whether higher-order QED corrections can significantly impact the theoretical prediction for Neff in the Standard Model.

Schwinger-Keldysh Path Integral for Gauge Theories

• Andrew Tolley

Room: Lecture Hall Schwinger’s Closed Time Path formalism is the basis of modern treatments of cosmological field theories, hydrodynamics and open quantum systems. Its application to gauge theories at finite temperature is well studied, relying on KMS boundary conditions and complex-time contours. By contrast the discussion of gauge theories such as Yang-Mills out of equilibrium has been less well developed, in large part due to a lack of development of how to treat gauge issues and Faddeev-Popov-DeWitt ghosts on the CTP. I will show how to construct the CTP in the BRST formalism, where a single diagonal copy of BRST symmetry survives, and how to implement the boundary conditions for ghosts for arbitrary initial physical states. As an illustration I will discuss how Hard-thermal-loop EFTs can be viewed as open quantum systems, and how to construct an open EFT for a gauge theory in a Higgs phase

Bottom-up open EFT for non-Abelian gauge theory with dynamical color environment

• Kanji Nishii

Room: Lecture Hall This talk presents a bottom-up open effective field theory (EFT) for non-Abelian gauge theories within the Schwinger-Keldysh formalism. Instead of integrating out the environment completely and starting from a nonlocal influence functional, we retain slow environmental response variables and construct a local system-environment EFT. The environmental sector, described by dynamical color-frame variables, Stückelberg-like fields, and color-current degrees of freedom, encodes interactions and dissipation between the system and the environment. This provides a gauge-covariant Markovian embedding of nonlocal and non-Markovian color response. Integrating out the retained variables with retarded boundary conditions yields nonlocal dissipative kernels and stochastic sources in the reduced system theory. This talk is based on joint work with Yoshihiko Abe, arXiv:2605.22822.

Open EFT for Cosmology and Gravity

• Toshifumi Noumi

Room: Lecture Hall I will begin by reviewing the motivation for developing open-system effective field theories for cosmology and gravity. I will then discuss how dynamical gravity can be consistently incorporated into the Schwinger–Keldysh EFT framework for systems with local dissipation, emphasizing the role of collective modes that encode fluctuations in the environmental sector. Finally, I will discuss possible applications of this framework.

Quantum fields in de Sitter: The stochastic Schwinger effect

• Lucas Vicente García-Consuegra

Room: Lecture Hall De Sitter spacetime describes both the earliest and latest stages of our Universe, but its lack of time-translation symmetry means that many of the standard tools of quantum field theory do not directly apply. In this talk, I will introduce the in-in formalism as a framework for studying quantum fields in this setting and use it to investigate a stochastic version of the Schwinger effect. Focusing on massless QED in a fixed de Sitter background, relevant to Standard Model matter before electroweak symmetry breaking, I exploit conformal flatness to relate the problem to flat-space QED and study particle production induced by a de Sitter-invariant stochastic electromagnetic field. Along the way, I will discuss the physical picture behind the mechanism, the challenges that arise in curved spacetime, and some open questions for extending the approach beyond the massless and exactly de Sitter limits.

Many-body states under measurements: from quantum entanglement to magic

• Yuto Ashida

Room: Lecture Hall Recent advances in programmable quantum systems have made it possible to monitor and control many-body states at the single-quantum level. In this talk, I shall discuss how quantum measurements open new frontiers at the interface of many-body physics and quantum information. I will first introduce how measurement backaction leads to intriguing many-body phenomena in open systems, such as measurement-induced entanglement transitions and criticality. I will then turn to many-body quantum resources, focusing on quantum magic, as a resource for universal quantum computation. By formulating stabilizer Rényi entropy as a participation entropy in a doubled Hilbert space, I will explain how many-body magic can be viewed as a property of quantum states under Bell measurements. This perspective enables a boundary conformal field theory analysis of universal magic scaling in one-dimensional critical states, including systems with periodic boundaries, open boundaries, and topological defects. I will also highlight how universal subleading terms are governed by boundary entropies, scaling dimensions, and defect fusion rules.

Quantum Information and the Physical Environment

• Jason Pollack

Room: Lecture Hall Quantum information theorists, many-body physicists, and cosmologists often have very different intuitions about open systems. In particular, the first group usually consider all possible quantum physical evolutions compatible with unitarity, while the latter groups usually prefer to derive open-system behavior by integrating out a specific, often infinite, environment sector. I'll give a couple of examples in which the latter groups can benefit by passing to simpler "environment-agnostic" descriptions, and then finish by discussing how the quantum information treatment of Markovianity can be drastically simplified by making physical assumptions about the environment.

Quantum Metrology for High-Energy Physics

• Hajime Fukuda (Kavli IPMU)

Room: Lecture Hall Quantum sensing provides powerful advantages over classical measurement techniques for detecting extremely weak signals, such as those induced by dark matter. By exploiting genuinely quantum resources, including entanglement and superposition, quantum sensors can achieve enhanced sensitivity and precision. In this talk, I will review basic applications of quantum sensing to dark matter searches, and discuss our recent work on noise suppression in such experiments, including approaches based on quantum error correction.

Impossibility of detecting the quantum nature of gravitational waves

• Takahiro Tanaka

Room: Lecture Hall We will summarise how it is difficult to detect the quantum nature, especially squeezed nature, of gravitational waves.

News from the Pulsar Timing Array Frontier

• Kai Schmitz (Kavli IPMU, University of Tokyo)

Room: Lecture Hall I will review the 2023 pulsar timing array (PTA) evidence for a stochastic gravitational-wave background at nanohertz frequencies and discuss how PTA observations allow us to search for new particle and gravitational physics, including: (1) physics beyond the Standard Model in the early Universe, (2) dark matter in our galaxy and in the vicinity of supermassive black-hole binaries, and (3) scenarios of modified gravity. Along the way, I will present new gravitational-wave templates for certain signal hypotheses and recommend best practices for the analysis of future PTA data sets.

Gravitational-wave lensing beyond rays: a disordered-system approach

• Ripalta Amoruso

Room: Lecture Hall The work describes gravitational wave propagation through a stochastic distribution of weak gravitational lenses beyond the geometric optics limit. The lens distribution is modeled as a static random background field and we formulate the problem in the language of quenched disorder, treating the disorder averaged densitymatrix as the fundamental object from which observables are computed. Using the Schwinger Keldysh formalism, a path-integral representation of the averaged density matrix is constructed and derived explicitly in a perturbatively form for a suitable class of couplings. The result naturally separates into a quadratic exponential term, which governs the suppression of phase sensitive contributions in the averaged description, and a purely oscillatory contribution, which modifies coherent propagation through a disorder-induced correction to the propagation kernel. This provides a unified description of interference, diffraction, and statistical fluctuations of the lens distribution within a single framework. We also identify the physical scales controlling the onset of coherence loss and illustrate the formalism in the case of Gaussian wave packets. More generally, the derivation applies to any system described by the same class of actions, making the framework relevant beyond gravitational wave lensing to wave propagation in disordered media. Insights on future directions will be discussed.

The Gravity of Tinkering with Einstein

• Cliff Burgess

Room: Lecture Hall

Quest for the Graviton

• Jiro Soda

Room: Lecture Hall The quest for the graviton—the hypothetical quantum of gravity—remains one of the ultimate frontiers in modern physics. In this talk, I will first introduce Freeman Dyson’s seminal work on graviton detection. Although Dyson famously reached a negative conclusion, arguing that detecting a single graviton is practically impossible, a closer look at his arguments reveals invaluable hints for modern research. Building on these insights, I will review several recent proposals for graviton detection and discuss potential future directions. In this context, I will highlight our own related work, demonstrating how contemporary quantum technologies might open new pathways toward witnessing the quantum nature of gravity.

A Field-Theoretical Perspective on Hydrodynamics

• Masaru Hongo

Room: Lecture Hall Hydrodynamics provides a universal description of the long-wavelength and low-frequency real-time dynamics of many-body systems near local thermal equilibrium. While its traditional formulation is based on conservation laws, thermodynamics, and a derivative expansion of constitutive relations, recent developments have revealed a unified field-theoretical framework underlying both deterministic and fluctuating hydrodynamics. In these lectures, I will first briefly review the phenomenological foundations of hydrodynamics and then discuss the quantum statistical mechanical formulation based on local Gibbs ensembles, with an emphasis on the imaginary-time (Matsubara) formalism for systems in local thermal equilibrium. I will then turn to fluctuating hydrodynamics and introduce the Schwinger–Keldysh formalism as a top-down effective field-theory framework that naturally incorporates dissipation, thermal fluctuations, and the symmetry principles governing nonequilibrium dynamics.

Towards building effective field theory for non-hydrodynamic modes

• Yi Yin

Room: Lecture Hall Understanding thermalization in quantum field theory stems largely from understanding properties of non-hydrodynamic excitations. These nonhydrodynamic excitations are known to differ structurally between weakly and strongly coupled theories. In this talk, I demonstrate that a large class of non-hydrodynamic excitations can be understood as a consequence of different realizations of a symmetry principle. As a main example, I show how to apply emergent shift symmetry to construct a far-from-equilibrium generalization of the hydrodynamic effective action. This shift-symmetry-based approach naturally includes the effects of excitations and stochastic fluctuations outside the hydrodynamic regime. Furthermore, I will present the application of non-Abelian algebra in describing spin in the context of quark-gluon plasma.

Frame invariant analysis of first order hydrodynamics as IR effective theory

• Atsuhisa Ota

Room: Lecture Hall In this talk we discuss the mode structure of general U(1)-charged first-order relativistic hydrodynamics, formulated within an effective field theory for dissipative fluids in flat Minkowski spacetime. Although first order relativistic hydrodynamics is known to be illposed as a system of partial differential equations, we argue that this conclusion is potentially misleading because hydrodynamics is not a fundamental theory. We derive the most general quadratic action for hydrodynamic modes, including stochastic noise, and analyze the resulting dispersion relations within a controlled gradient expansion. We then show that frame-invariant combinations of hydrodynamic transport coefficients fix the first-order dispersion relations in the low-energy limit, making the mode analysis manifestly independent of the choice of hydrodynamic frame. Assuming local Kubo-Martin-Schwinger (KMS) symmetry and unitarity of the underlying UV theory, we find that first-order hydrodynamics is stable provided the enthalpy density is positive.

Superfluid Transport Theory in Open Quantum Systems

• Hongchao Li

Room: Lecture Hall Superfluidity encompasses a variety of remarkable physical phenomena, including zero viscosity in transport, nucleation of quantized vortices, and nonclassical rotational inertia. Recent advances in ultracold-atom experimental techniques have enabled unprecedented control over superfluids in quantum systems, such as bosonic superfluidity in interacting Bose-Einstein condensate (BEC) and fermionic superfluidity in Bardeen-Cooper-Schrieffer (BCS) superconductivity. However, inelastic atomic collisions and molecular chemical reactions inevitably induce particle loss, rendering these systems intrinsically open. One of the fundamental questions is the behavior and properties of dissipative superfluidity in open quantum systems. In this talk, I will introduce our recent works on a comprehensive superfluid transport theory for open quantum many-body systems. In the first part, I will introduce our work on bosonic superfluid theory for a molecular BEC subject to uniform two-body loss. By employing the Schwinger-Keldysh formalism, we reveal that dissipation acts as an effective repulsive interaction that suppresses density fluctuations and generates superfluidity even in the absence of interaction. Furthermore, we also show that the two-body loss can enhance the stability of a molecular BEC against collapse. In the second part, I will introduce our work on fermionic superfluid transport theory. By using the Schwinger-Keldysh formalism, we show the Ward-Takahashi identity in open quantum systems for a Lindbladian dynamics possessing weak U(1) symmetry. We demonstrate that gauge invariance follows directly from the weak U(1) symmetry. This framework enables the calculation of superfluid density in the presence of dissipation. Finally, we derive the low-energy excitation spectrum for dissipative BCS superconducting systems and show that two-body loss induces a diffusive propagation in the low-energy mode.

Open Systems, Black Holes, and Holography

• Shivam K. Sharma

Room: Lecture Hall Open quantum systems provide the natural setting for describing fluctuations, dissipation, and non-equilibrium dynamics. While these phenomena are well understood for weakly coupled baths, strongly coupled baths remain difficult to treat using conventional methods. In this talk, I will discuss how recent developments in real-time holography (a real-time formulation of the AdS/CFT correspondence) provide a new approach to this problem. After briefly introducing the Schwinger-Keldysh ("in-in") formalism and its holographic counterpart, I will explain how holography enables the derivation of open EFTs for systems coupled to strongly interacting baths. In particular, I will show how black holes encode dissipation and fluctuations, leading naturally to local open EFTs. Finally, building on this picture, I will discuss how black hole dynamics provide a holographic derivation of the fluctuation-dissipation relations.

Exact solutions to gauge theories and generalized symmetries

• Hitoshi Murayama (Kavli IPMU)

Room: Lecture Hall I discuss exact solutions to two- and four-dimensional gauge theories and their generalized symmetries.

Organizing Non-Equilibrium Photon Dynamics from Symmetry Principles

• Genki Yoshimura

Room: Lecture Hall Symmetry provides a unifying framework for understanding physical phenomena. Recent developments have broadened this framework to include higher-form symmetry. From this viewpoint, photons can be understood as Nambu–Goldstone modes associated with spontaneously broken higher-form symmetries. So far, however, higher-form symmetry has been discussed mainly in zero-temperature and equilibrium settings, and its consequences for dissipative real-time dynamics remain less well understood. In this talk, we develop an effective field theory for non-equilibrium photon dynamics based on higher-form symmetry and the Schwinger–Keldysh formalism. The theory describes dissipative photon dynamics in insulating media and incorporates the fluctuation–dissipation relation and the second law of thermodynamics as consequences of the underlying symmetry structure, rather than as phenomenological inputs.

Introduction to relational frames in quantum gravity

• Luca Marchetti (Kavli IPMU)

Room: Lecture Hall Internal quantum reference frames provide a general framework for handling symmetries in quantum theory, with applications ranging from quantum gravity and gauge theories to quantum information and foundational physics. I will first introduce the formalism in simple mechanical systems, before turning to classical gravity. There, I will motivate the need for internal, dynamical frames in background-independent theories to define relationally local gauge-invariant observables, and show how this framework leads to a relational update of general covariance: frame covariance. I will then move to non-perturbative quantum gravity, showing how quantum reference frames can be used to define a manifestly gauge-invariant relational path integral, which is also invariant under transformations between quantum reference frames. It therefore provides a perspective-neutral description of quantum gravitational physics. I will also discuss the associated relational effective actions. Although effective actions are, in general, not frame-covariant off shell, the on-shell physics they encode is. Finally, I will present several physical consequences of this framework, including the fuzziness of frame-changed local correlators, the non-trivial interplay between quantum-reference-frame transformations and time evolution, and the frame-dependence properties of ground sectors and Hartle-Hawking prescriptions. I will conclude by outlining future directions, with particular emphasis on a relational notion of the renormalization group flow.

Time-like Entanglement and Holography

• Tadashi Takayanagi

Room: Lecture Hall The holographic principle provides a powerful framework to study quantum gravity by establishing a connection between bulk geometry and boundary quantum information. While holographic entanglement entropy serves as a standard tool to map algebraic properties in quantum information to geometric properties in gravity, this only cares about the space-like correlations. To explore time-like correlations, we argue that we need to extend it, which is called time-like entanglement entropy. We will provide both field theoretic and holographic calculations of this quantity. We will also discuss our recent application of this idea to traversable AdS wormholes and dS/CFT Correspondence.

Holographic AME states in black hole interiors

• Kotaro Tamaoka

Room: Lecture Hall Understanding the black hole interior remains one of the central challenges in quantum gravity and holography. In this talk, we will argue that a special extremal slice inside an AdS black hole is dual to an absolutely maximally entangled (AME) state. This claim is supported by showing that the holographic n-th Rényi entropies are independent of n for arbitrary bipartitions of the corresponding subsystems, indicating a flat entanglement spectrum. Our result provides a holographic realization of an AME state in an infinite-volume system, with the effective local bond dimension set by the density of black hole entropy. In particular, this construction offers concrete support from the gravity side for the emergence of random structures and infinite-dimensional Hilbert spaces in recent non-isometric holographic codes. We will also comment on possible connections to non-Hermitian quantum systems.

Application of Quantum Computation to High Energy Physics

• Masazumi Honda

Room: Lecture Hall In my talk, I will overview application of quantum computation to numerical approaches in high energy physics. I will start with basics of quantum computation and then introduce how to put quantum field theories on quantum computers. Then I will introduce previous works on quantum simulations of quantum field theories and discuss our recent work on application to non-equilibrium physics of gauge theories. I will also mention challenges of this topic and possible applications to cosmology.

An Efficient Framework for Simulating 3+1-Dimensional Lattice Gauge Theories with Fermions

• Arnab Pradhan

Room: Lecture Hall Hamiltonian lattice gauge theories provide access to important physics that remains difficult to study using conventional Euclidean methods. Quantum computers are naturally suited to implementing such formulations. With near-term quantum hardware in mind, we present an efficient framework for incorporating fermionic matter into 3+1-dimensional lattice gauge theories. The key idea is to combine trivalent lattice geometries with staggered fermions, reducing the local Hilbert-space dimension while preserving gauge invariance and the essential physics of the underlying theory. This construction offers a scalable route toward quantum simulations of non-Abelian gauge theories with fermions in 3+1 dimensions.

The EFT Anatomy of Thermal Correlation Functions

• Patrick Angus Hager

Room: Lecture Hall Thermal correlation functions depend on widely separated scales: the temperature, particle masses, and the external kinematics. In vacuum, the method of regions organises such expansions and tells us which modes an effective theory must contain. At finite temperature the picture is incomplete. Beyond the familiar hard and soft contributions, thermal correlators contain terms exponentially suppressed in m/T, precisely the terms which are not captured by a naive expansion by regions. I will introduce the method of regions on simple examples, then apply it to thermal correlators and present recent progress towards a systematic classification of their regions. The emphasis is on the exponentially suppressed contributions: where they originate and how they can be extracted.

Cutting Rules for In-In Correlators & Cosmo Collider Signals

• Kyohei Mukaida

Room: Lecture Hall Equal-time expectation values, rather than asymptotic scattering amplitudes, are the natural observables in many time-dependent quantum systems. In this talk, I will present cutting rules for such in-in correlators in the Schwinger–Keldysh formalism, which reorganize perturbative contributions in a way that makes their causal structure transparent. We will then discuss cosmological correlators and cosmological-collider signals as a concrete example.

The Entangled State from a cosmological Euclidean Wormhole

• Wei-Chen Lin

Room: Lecture Hall We use the Euclidean path-integral method to approximate the wavefunction of the universe, focusing on a scenario in which a single Euclidean wormhole instanton dominates the path integral. This solution, which connects two Lorentzian spacetimes, provides an approximation to the emergence of classical spacetime in quantum cosmology. Beyond the background level, perturbations about this instanton are treated quantum mechanically in both the Euclidean and Lorentzian regimes. We show that the initial wavefunction for these perturbations can be fully determined at the free-field level, exhibiting an entangled structure between the two Lorentzian spacetimes. In a fully symmetric setting, we further argue that these perturbations can be understood as a generalization of the thermofield-double-state interpretation of the Unruh effect. Interestingly, we find that this generalization uniquely selects the vacuum state in the Lorentzian spacetimes.