Welcome to the home page of the online seminars on **t-, T- & đ-dependence in Quantum Field Theory**! (tTmuQFTseminar.org)

This is a joint initiative of the *Gravity, Quantum Fields and Information*Â groupÂ at the **Albert Einstein Institute** in Potsdam (Michal Heller, Johannes Knaute, Viktor Svensson), **CERN** (Urs Wiedemann, Wilke van der Schee, Aleksas Mazeliauskas), the **University of Barcelona** (David Mateos, Jorge Casalderrey Solana), **Utrecht University** (Umut GĂŒrsoy), **University of Regensburg** (Andreas SchĂ€fer), **University of Helsinki** (Aleksi Vuorinen), **University of Oviedo** (Carlos Hoyos Badajoz), **Brookhaven National Laboratory** (Raju Venugopalan) and **Bielefeld University** (SĂ¶ren Schlichting).

Our aim is to provide an online platform for researchers working on this topic all around the globe toÂ present theirÂ work from anywhere they like! (Office, home, restaurant, airport, or even the beach.)Â We hope this seminar series can make a small contribution towardsÂ cuttingÂ down costs, unnecessary travel, and carbon emissions.

**How does it work?**

A link to the virtual seminar room for each talk is sent out to participating groups via our mailing list. Anyone with the link can tune-in remotely to the live stream, ask questions, and participate in discussion.Â

In addition, the talks are typically recorded and posted on ourÂ YouTube channel:Â https://www.youtube.com/c/GravityQuantumFieldsandInformationAEI, in case you miss the live stream, and/or want to revisit the talk.

If you are interested in being added to the mailing list to receive information (including the link to the virtual seminar room) please subscribe here: https://lists.aei.mpg.de/mailman/listinfo/tTmuQFTseminarlist

**Contact:Â **

tTmuQFTseminar@aei.mpg.de

**Group seminars**

The full spectrum of our other joint group seminars is available under the following links:

www.QGIseminar.org (Quantum Gravity & Information)

www.heptnseminar.org (Tensor Networks in High Energy Physics)

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## Upcoming Seminars

TBA

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## Past Seminars

#### 9. Markus Heyl

**When:** Thursday, July 9 @ 15:30 CET (Berlin-time)

**Title:** Quantum many-body dynamics in two dimensions with artificial neural networks

**Abstract:** The efficient numerical simulation of nonequilibrium real-time evolution in isolated quantum matter constitutes a key challenge for current computational methods. This holds in particular in the regime of two spatial dimensions, whose experimental exploration is currently pursued with strong efforts in quantum simulators. In this work we present a versatile and efficient machine learning inspired approach based on a recently introduced artificial neural network encoding of quantum many-body wave functions. We identify and resolve some key challenges for the simulation of time evolution, which previously imposed significant limitations on the accurate description of large systems and long-time dynamics. As a concrete example, we study the dynamics of the paradigmatic two-dimensional transverse field Ising model, as recently also realized experimentally in systems of Rydberg atoms. Calculating the nonequilibrium real-time evolution across a broad range of parameters, we, for instance, observe collapse and revival oscillations of ferromagnetic order and demonstrate that the reached time scales are comparable to or exceed the capabilities of state-of-the-art tensor network methods.

**Link:** https://youtu.be/2YIPg-BbfIA

#### 7. Yi Yin

**When:** Thursday, June 11 @ 15:30 CET (Berlin-time)

**Title:** Quark-gluon plasma, the QCD critical point and Hydro+

**Abstract:** Heavy-ion collisions at RHIC and LHC produce quark-gluon plasma (QGP) whose bulk evolution is well described by relativistic hydrodynamics. However, to further explore the properties of QGP and the phase diagram of QCD, one typically has to go beyond this hydrodynamic paradigm. In this talk, I will present the development of a general dynamic framework which couples slow non-hydro. modes with hydro. modes, namely Hydro+. I will demosntrate the application of Hydro+ to describe off-equilibrium dynamics near the conjectured QCD critical point. If time permits, I will brieflyÂ report my recent attempt to explore the dynamic properties of QGP in the non-hydrodynamic yet non-perturbative regime.

**Link:** https://youtu.be/5pd9i_ZB14U

#### 6. & 8. Uwe-Jens Wiese

**When:** Thursday, May 28 @ 15:30 CET (Berlin-time)

**Title:** Asymptotically Free Quantum Fields from Dimensional Reduction of Discrete Variables

**Abstract:** Quantum Chromodynamics (QCD) is the (3+1)-d asymptotically free relativistic SU(3) gauge theory that is formulated in terms of fundamental quark and gluon fields. CP(N-1) models in (1+1)-d have a global SU(N) symmetry and share many features with QCD. They are also asymptotically free, have a non-perturbatively generated mass gap, and non-trivial theta vacuum states. CP(N-1) models can be regularized unconventionally by using discrete SU(N) quantum spins forming a (2+1)-d spin ladder that consists of n transversely coupled quantum spin chains. The (1+1)-d asymptotically free CP(N-1) fields then emerge from dimensional reduction when n is increased. Even n leads to the vacuum angle theta=0, while odd n leads to theta=pi. In a similar way, gluon fields emerge naturally from the dimensional reduction of (4+1)-d quantum links, which are discrete gauge variables that generalize quantum spins. In this formulation, quarks arise as domain wall fermions. In contrast to the usual quantum fields, quantum spins and quantum links realize asymptotically free field theories with finite-dimensional local Hilbert spaces. This is advantageous in the context of quantum simulation experiments. Both CP(N-1) models and QCD can be quantum simulated with ultra-cold alkaline-earth atoms in optical super-lattices.Â When CP(N-1) models are studied at non-zero chemical potential, non-trivial condensed matter physics arises in these quantum field theories. In particular, there are Bose-Einstein condensates, with and without ferromagnetism.

**Link:** *Part 1:* https://youtu.be/tKNpl5PC8lUÂ Â *Part 2:* https://youtu.be/L83AouG_s1Q

#### 5. Raju Venugopalan

**When:** Thursday, April 23 @ 15:30 CET (Berlin-time)

**Title:** World-lines, the Regge limit, and a hybrid approach to quantum computation

**Abstract:** The world-line approach to quantum field theory provides useful semi-classical intuition into real-time problems. We will discuss here the application of this formalism to construct a hybrid quantum-classical approach to computing structure functions in the Regge limit. We will then outline a scaleable single particle digitization strategy to implement such hybrid approaches to scattering amplitudes.

**Link:** https://youtu.be/dHFbtq9GGQw

#### 4. David Mateos

**When:** Thursday, April 2 @ 15:30 CET (Berlin-time)

**Title:** Real-time Dynamics of Plasma Balls

**Abstract: **Plasma balls are droplets of deconfined plasma surrounded by a confining vacuum. We present the first holographic simulation of their real-time evolution via the dynamics of localized, finite-energy black holes in the five-dimensional AdS soliton background. The dual gauge theory is four-dimensional, N=4 super Yang-Mills compactified on a circle with supersymmetry-breaking boundary conditions. We consider horizonless initial data sourced by a massless scalar field. Prompt scalar field collapse then produces an excited black hole at the bottom of the geometry together with gravitational and scalar radiation. The radiation disperses to infinity in the non-compact directions and corresponds to particle production in the dual gauge theory. The black hole evolves towards the dual of an equilibrium plasma ball on a time scale longer than naively expected. This feature is a direct consequence of confinement and is caused by long-lived, periodic disturbances bouncing between the bottom of the AdS soliton and the AdS boundary. [arXiv:2001.05476]

**Link:** https://youtu.be/kp0wLNTJQxo

#### 3. Simone Montangero

**When:** Friday, February 14 @ 15:30 CET (Berlin-time)

**Title:** Tensor network methods applied to high energy physics problems

**Abstract:** We briefly introduce tensor network methods, a classical numerical approach that promises to become a powerful tool to support future quantum simulations and computations, providing guidance, benchmarking and verification of the quantum computation and simulation results. We review some of the latest achievements we obtained: the gauge-invariant formulation of tensor networks and their application to abelian and non-abelian, one- and two-dimensional lattice gauge theories in regimes where Monte Carlo methods efficiency is hindered by the sign problem. Finally, we present the application of tensor network machine learning techniques to the event classification of LHCb simulated data.

**Link:** https://youtu.be/vrZHkyDvYhI

#### 2. Paul Chesler

**When:** Thursday, December 12 @ 15:30 CET (Berlin-time)

**Title:** Searching for quark matter cores with binary neutron star inspirals

**Abstract:** I will discuss the feasibility of detecting quark matter cores in merging neutron stars with ground-based gravitational-wave detectors. I will argue that the existence of quark matter cores can be confirmed at the 70% confidence level with as few as several tens of detections. Likewise, with such a sample, some models of quark matter cores can be excluded with high confidence.

**Link:** https://youtu.be/Un5-FCmJmGI

#### 1. Paul Romatschke (Opening seminar of the series)

**When:** Thursday, November 14 @ 15:30 CET (Berlin-time)

**Title:** Pure CFT Thermodynamics and Fractional Degrees of Freedom

**Abstract:** "Pure" CFTs are CFTs that have vanishing trace of the energy-momentum tensor for all values of the coupling. A famous example for a pure CFT is N=4 SYM in 3+1 dimensions, which has the property that its entropy density at infinite coupling is exactly 3/4 of the Stefan-Boltzmann limit (the entropy at intermediate coupling is not known exactly, nor is monotonicity in the coupling established). In this talk I will present other examples of pure CFTs in 2+1 dimensions which can be solved exactly in the large N limit for all values of the coupling. It is found that for these pure CFTs the entropy density is monotonically decreasing as a function of the coupling. Furthermore, I show that for a large class of CFTs (not only pure CFTs), the strong/weak ratio for the entropy density is a simple fraction bounded from below by 4/5. I entertain the hypothesis for CFTs at infinite coupling, the degrees of freedom contributing to the entropy density are fractional.

**Link:** https://youtu.be/QJX10vHFxm8

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