The BIG meetings aim to bring together all actors working in the field of gravity and cosmology in Barcelona, from observations to theory. 

These series of meetings has the purpose to set an informal stage for the exchange of ideas and to encourage joint discussions on relevant topics in the field.  It also provides the opportunity for people working in Barcelona to get informed on the lines of research of the other institutes.

Organizers

Diego Blas (IFAE), Jacopo Fumagalli (ICCUB), Jaume Garriga (ICCUB), Sascha Husa (ICE-CSIC), Oriol Pujolas (IFAE), Carlos Sopuerta (ICE-CSIC).

Contact
 Jacopo Fumagalli: jfumagalli@fqa.ub.edu
 Jaume Garriga: jgarriga@fqa.ub.edu

Abstract: Understanding the computational complexity of quantum states is a central challenge in quantum many-body physics. In qubit systems, fermionic Gaussian states can be efficiently simulated on classical computers and thus provide a natural baseline for assessing quantum complexity. In this talk, based on [arXiv:2506.00116], I will briefly introduce the idea of magic state resource theories and then focus on a framework for quantifying fermionic magic resources, also known as fermionic non-Gaussianity. I will describe the algebraic structure of the fermionic commutant and introduce fermionic antiflatness (FAF)—an efficiently computable and experimentally accessible measure of non-Gaussianity with a clear physical interpretation in terms of Majorana fermion correlation functions. I will argue that FAF detects phase transitions, reveals universal features of critical points, and identifies special solvable points in many-body systems. Extending to out-of-equilibrium settings, I will show that fermionic magic resources proliferate in highly excited eigenstates, and I will describe the growth and saturation of FAF under ergodic dynamics, emphasizing how conservation laws and locality constrain the increase of non-Gaussianity during unitary evolution. The main goal of this talk is to present fermionic non-Gaussianity—alongside entanglement and non-stabilizerness—as a resource relevant not only for foundational studies but also for experimental platforms aiming at quantum advantage.

Asbtract: Understanding how nuclei respond to external probes is essential for connecting nuclear structure to fundamental interactions. In this talk, I will present a new framework to calculate integrated properties of the nuclear response starting directly from nuclear forces. Using the in-medium similarity renormalization group (IMSRG), we evaluate expectation values of operators that encode the multipole response of nuclei. I will show applications to the monopole, quadrupole, and dipole response in closed-shell nuclei from helium to nickel, and discuss the impact of many-body correlations beyond the random-phase approximation. Our IMSRG calculations provide an improved description of experimental data in oxygen and calcium isotopes, including a successful reproduction of the Thomas–Reiche–Kuhn sum-rule enhancement. Finally, I will outline how this approach can serve as a benchmark for other ab initio methods that describe nuclear response functions through explicit excited states.

[1] Porro, A., Schwenk, A., & Tichai, A. (2025). Impact of ground-state correlations on the multipole response of nuclei: Ab initio calculations of moment operators. arXiv preprint arXiv:2507.20665.

Abstract: Ab initio nuclear many-body methods have become powerful tools for describing nuclei across the nuclear chart, providing predictive insights into nuclear structure and dynamics from realistic QCD-based interactions. This talk reviews the basic principles of these approaches, highlights recent advances in their extension to deformed nuclei, including the newly developed deformed self-consistent Green’s function method, and discusses strategies to mitigate their demanding computational cost, with a particular emphasis on emulators.

Abstract: Quantum computers hold the promise of revolutionizing algorithmic approaches to some problems that are provably hard for classical computers.   One such possibility is in the simulation of many-body quantum systems.  Here, the exponential scaling of the Hilbert space spanned by a many-qubit system as the number of qubits grows linearly, alongside the natural way of encoding entanglement, are the key factors which make this method viable.  

In this presentation, nuclear structure, as an archetypical many-body quantum problem, is explored on quantum computer.  We discuss methods of encoding the nuclear Hamiltonian onto quantum computer, ways to express nuclear wave functions by their qubit proxies, algorithms for discovering ground and excited states of the nuclear system, and show some recent results of the Surrey group obtained using tens of qubits on current IBM hardware.  We end with some thoughts on prospects for calculations of future generations of quantum hardware.

Abstract: Artificial intelligence has been ballooning as a promise to accelerate progress in scientific research and as a new tool for teaching science. The lessons from the use of social networks, however, warn us that reliance on these tools may increase biased opinions and reinforce prior beliefs in discussions within communities that become increasingly isolated and intolerant. I will discuss the example of asking Deepseek about the origin of the difference between the proton and neutron masses according to QCD, where the answers of AI to the physics questions seem clear, elegant and compelling, but may simply follow from the way the user is asking questions and may end up reinforcing wrong ideas and beliefs that may be incorrect as physical explanations. The talk will discuss both on the QCD physics of the proton and neutron mass difference and how AI may or may not be useful to learn on questions like this.