Cooperation from Multiplicative Processes

Ponente: Lorenzo Fant. ICTP, Trieste.

Fecha y hora: jueves, 21 de abril de 2022 a las 10:00. 

Lugar: Seminario de Física Computacional, planta baja del edificio de Física (junto a las pantallas). Facultad de Ciencias. 

Questions on non-equilibrium statistical physics

Ponente: Christian Maes. Institute for Theoretical Physics, KU Leuven.

Fecha y hora: lunes, 14 de marzo de 2022 a las 11:00. 

Lugar: Seminario de Física Computacional, planta baja del edificio de Física (junto a las pantallas). Facultad de Ciencias. 

A macroecological law describes correlations as a function of phylogenetic similarity in microbial ecosystems

Fuente: Surkova Tatʹjana (Wikimedia Commons)

The question of how coexistence of species is generated and maintained is as old as ecology itself. Many ecological “forces” such as competition, cooperation, demographic fluctuation, environmental fluctuation and filtering, migration, predation etc. are expected to act together in ecosystems. Disentangling the effect and intensity of each of theses forces in natural communities is the focus of present research. Thanks to a recent revolution in the availability of high quality data, natural microbial ecosystems offer an invaluable possibility to tackle this question. Here, we explore if it is possible to discriminate a dominant force at a give resolution of genetic similarity. By using both relative-abundances and metagenomic data, we reveal the presence of a new macroecological law relating correlation and phylogenetic similarity. In particular, the average correlation of species abundance fluctuation decays with phylogenetic distance from positive to null values following a stretch exponential function consistently in all empirically analyzed biomes both across communities (hosts) and in temporal data for each community. By scrutinizing different ecological models, we show that competition cannot reproduce the observed pattern. Instead, the elucidated macroecological law is explained quantitatively by the “correlated stochastic logistic model” (CSLM) pointing to environmental filtering as the dominant ecological force at this resolution level. We conclude by arguing that in order to understand interactions in microbial ecosystems one needs to abandon the concept of species and study the system from different scales, much as done in physics exploiting renormalization-group ideas.

Ponente: Matteo Sireci. Universidad de Granada.

Fecha y hora: viernes, 4 de febrero de 2022 a las 12:00. 

Lugar: Seminario de Física Computacional, planta baja del edificio de Física (junto a las pantallas). Facultad de Ciencias. Online a través de Google Meet en el siguiente enlace: https://meet.google.com/vub-uoiw-piz

From integrability to chaos in quantum Liouvillians

Fuente: https://arxiv.org/abs/2102.13452

The dynamics of open quantum systems can be described by a Liouvillian, which in the Markovian approximation fulfills the Lindblad master equation. We present a family of integrable many-body Liouvillians based on Richardson-Gaudin models with a complex structure of the jump operators. Making use of this new region of integrability, we study the transition to chaos in terms of a two-parameter Liouvillian. The transition is characterized by the spectral statistics of the complex eigenvalues of the Liouvillian operators using the nearest neighbor spacing distribution and by the ratios between eigenvalue distances.

Ponente: Rafael A. Molina Fernández. Instituto de Estructura de la Materia, CSIC, Madrid.

Fecha y hora: miércoles, 24 de noviembre de 2021 a las 12:00. 

Lugar: Presencial: hasta completar aforo en el seminario de Física Computacional, planta baja del edificio de Física (junto a las pantallas), Facultad de Ciencias.  Online: a través de Google Meet en https://meet.google.com/kye-ynsm-vzd.

Modeling brain reorganization after hemispherectomy

Fuente: http://dx.doi.org/10.1016/j.celrep.2019.10.067

Hemispherectomy is a last-resource treatment for some neurological disorders. This radical intervention allows some patients to live normally, with better odds the earlier in life it happens. Somehow, the remaining hemisphere takes on the outstanding computational burden. Brain plasticity at smaller scales shows how functionality is adopted by adjacent tissue. In models of brain rewiring after stroke, circuits accepting new workload are close and similar to the damaged ones. Hemispherectomy demands more drastic changes, mixing far and functionally diverse regions. We lack mathematical models of this. We introduce a simple model of brain reorganization after hemispherectomy based on Self-Organized Maps (SOMs). We show how emerging representations in SOMs constrain brain reorganization after simulated hemispherectomy, resulting in some forbidden and some other favored rearrangement pathways, each with distinct symmetries and properties. We discuss what the enabled paths imply for the recovery of topographic maps and language functionality after hemispherectomy. We find how too much symmetry can be detrimental for the proper formation of representation systems. We also obtain results regarding the existence of window periods – a critical age after which hemispherectomy causes irreversible function loss. These findings illuminate various (hitherto unexplained) clinical facts about window periods for language recovery. Our model offers a powerful thinking tool and suggests simple guiding principles for large-scale brain plasticity – notably, that the geometry of emerging representations turns into topological constraints for large-scale brain rearrangement. This offers insights about why such an aggressive intervention results in highly functional brains nevertheless, and suggests specific treatments for simulated, pathological disorders observed in our SOM models.

Ponente: Luis Seoane. Centro Nacional de Biotecnología, CSIC.

Fecha y hora: jueves, 16 de septiembre de 2021 a las 12:00. 

Lugar:

  • Presencial (hasta completar aforo): seminario de Física Computacional, en la planta baja del edificio de Física (junto a las pantallas) de la Facultad de Ciencias.
  • Online: a través de Google Meet en https://meet.google.com/drx-hnze-fpo.

Overfitting mitigation in correlation matrices and applications to the study of brain connectivity

We compare various known and original strategies of overfitting mitigation in correlation matrices, in the context of brain functional connectivity. In particular, we infer a database of human brain activity from functional Magnetic Resonance Imaging (fMRI), beyond Maximum Likelihood inference and using the multivariate Gaussian as likelihood. We show that the relative algorithm performances are consistent across subjects, and across samples of a synthetic database of similar characteristics. We observe as well that the resulting cleaned correlation matrices, that are proposed as a refined model of functional connectivity, depend crucially on the cleaning algorithm. We discuss possible applications of these findings to network neuroscience.

Ponente: Dr. Miguel Ibáñez. ISTC-CNR (Italy).

Fecha y hora: lunes, 28 de junio de 2021 a las 12:00. 

Lugar: Seminario de Física Computacional, planta baja del edificio de Física (hasta completar aforo) y vía Google Meet: https://meet.google.com/ijd-agcd-rye.. 

Macroscopically nonlocal quantum correlations

Macroscopic Bell-type experiment (source).

It is usually believed that coarse-graining of quantum correlations leads to classical correlations in the macroscopic limit. Such a principle, known as macroscopic locality, has been proved for correlations arising from independent and identically distributed (IID) entangled pairs. In this work we consider the generic (non-IID) scenario. We find that the Hilbert space structure of quantum theory can be preserved in the macroscopic limit. This leads directly to a Bell violation for coarse-grained collective measurements, thus breaking the principle of macroscopic locality.

Reference: https://arxiv.org/abs/2104.03988

Ponente: Miguel Gallego Ballester. University of Vienna.

Fecha y hora: viernes, 11 de junio de 2021 a las 10:00. 

Lugar: Seminario de Física Computacional, planta baja del edificio de Física (hasta completar aforo) y vía Google Meet: https://meet.google.com/bqi-vemu-gro. 

Mpemba effect in Markovian open quantum systems

The Mpemba effect is the phenomenon for which a hotter liquid can cool at a faster rate than a colder one. This effect is nowadays receiving much attention; it has been predicted for different classical systems and it has even been observed experimentally for a trapped colloid particle. In this talk, I will propose an analogue of the Mpemba effect for Markovian open quantum systems. I will show that the relaxation dynamics of these systems can be exponentially accelerated by devising an optimal unitary transformation, that is applied to the quantum state immediately before the actual dynamics. This preliminary “rotation” is engineered in such a way that the state of the quantum system becomes orthogonal to the slowest decaying dynamical mode. I will illustrate potential applications of this effect by showing how it can be used to achieve an exponential speed-up in the convergence to stationarity in Dicke models as well as to avoid metastable regimes in an all-to-all interacting spin system.

Ponente: Dr. Federico Carollo. University of Tübingen.

Fecha y hora: viernes, 23 de abril de 2021 a las 12:00. 

Lugar: Online. Enlace: https://meet.google.com/smu-dsfx-mqk

From quantum simulations of fermionic models to continuous-variable quantum computation

Stephen Shankland/CNET

Firstly, I will introduce digital-analog and purely digital methods for quantum simulations of fermionic models. The digital-analog approach provides a higher degree of scalability than purely digital or analog techniques, and it is suitable for implementing quantum simulations of interacting fermions and bosons in condensed matter, and quantum field theories [1]. Purely digital quantum simulations [2,3,4] are grouped under standard discrete-variable quantum computation, while analog quantum simulators with bosonic fields can be described with the language of continuous-variable (CV) quantum computation.

The second part of the talk is devoted to analyzing the factors that may yield quantum advantage. In particular, Wigner negativity is known to be a necessary resource for computational advantage in several quantum-computing architectures, including those based on continuous variables [5,6]. However, it is not a sufficient resource, and it is an open question under which conditions CV circuits displaying Wigner negativity offer the potential for quantum advantage. I will present large families of circuits that display large Wigner negativity and yet are classically efficiently simulatable [7], although they were not recognized as such by previously available theorems.

Ponente: Laura García-Álvarez. Chalmers University of Technology.

Fecha y hora: viernes, 16 de abril de 2021 a las 10:30. 

Lugar: Online. Enlace: https://meet.google.com/wmk-nsik-qyz