Szemináriumok

Non-ergodicity in kinetically constrained models: localization, fragmentation, and confinement

Időpont: 
2024. 06. 25. 14:30
Hely: 
Schay room, BME Building F, stairway I, first floor
Előadó: 
Riccardo J. Valencia-Tortora (Mainz)
Riccardo J. Valencia-Tortora
Johannes Gutenberg-Universität, Mainz, Germany
 
Non-ergodicity in kinetically constrained models: localization, fragmentation, and confinement
 
Strongly interacting systems are generally expected to thermalize, making local information of the initial state scattered into highly non-local degrees of freedom that are challenging to access. However, various mechanisms exist that could prevent thermalization and, therefore, aid in passively protecting coherence and quantum information.
 
In this talk, I will show how different ergodicity-breaking mechanisms manifest in a single class of models, known as kinetically constrained models, which can be realized in current platforms based on Rydberg arrays [1] and superconducting circuits [2]. Then, I will discuss a novel dynamical phase transition between an ergodic phase and a non-ergodic one that these models can host, which can be diagnosed through the complexity of simulating the dynamics using tensor networks [3].
 
[1]  R. J. Valencia-Tortora, et al., Phys. Rev. Lett. 132, 223201
[2]  R. J. Valencia-Tortora, et al., PRX Quantum 3, 020346
[3]  Manthan Badbaria, et al., in preparation

Quantum computing with single-electron quantum bits

Időpont: 
2024. 09. 24. 14:30
Hely: 
BME building F, lecture hall 13, second floor
Előadó: 
András Pályi (BME)

Today’s most advanced quantum computer prototypes, based on superconducting circuits and trapped ions or atoms, consist of a few hundred quantum bits and feature logical operations with precision above 99%. In this talk, I will describe an alternative architecture for quantum computing, which is based on the semiconductor technology that enables most of our (classical) information technology today. In the semiconductor quantum computing architecture, each qubit is realized by the spin of a single electron. I will discuss how to initialize, control, and read out these qubits, the difficulties in building and operating such a device, as well as the experimental state of the art and the most exciting contemporary research directions.

 

Website of the Szilárd Colloquium: https://physics.bme.hu/kollokvium?language=hu 

Adapting academic physics methods to improve solar cell performance from femtoseconds to gigaseconds

Időpont: 
2024. 10. 08. 14:30
Hely: 
BME building F, lecture hall 13, second floor
Előadó: 
Gergely Zimányi (UC Davis)

This talk reviews four different areas where adapting academic physics methods and knowledge proved to be very useful for developing better solar cells. (1) Our knowledge of strongly interacting electrons helped boosting the efficiency of energy conversion in quantum dot solar cells by “multiple electron generation”. (2) Work on the Metal-Insulator Transition was useful to improve charge extraction from quantum dot solar cells. (3) Ideas from quantum glassy dynamics helped mitigating the performance degradation of the next generation heterojunction silicon solar cells. In this study Machine Learning techniques were very helpful to model defect dynamics in silicon solar cells. (4) The theory of phase separation formed the basis for understanding how the next generation topcon solar cells degrade.

 

Website of the Szilárd Colloquium: https://physics.bme.hu/kollokvium?language=hu 

Bizonyítás, az emberin túl (Science Campus előadás)

Időpont: 
2024. 10. 18. 16:00
Hely: 
BME building F, lecture hall 13, second floor
Előadó: 
Molnár Zoltán Gábor (BME)
A gépi tételbizonyítás sikere vitát indított el a matematikai közösségben. A fő kérdés: vajon teljes mértékben megbízhatunk-e egy olyan bizonyításban, amelyet nem tudunk teljes egészében az emberi percepcióra és a tiszta észre hagyatkozva ellenőrizni? Molnár Zoltán Gábor előadása filozófiai problémákat is felvet, miközben bevezetést kapunk a Lean és Coq programnyelvekbe, ill. a bizonyításasszisztens szoftverek elvébe, érintve a mesterséges intelligencia jövőbeni fejlődését.
 
 
Regisztráció: https://lu.ma/mcxs5t6c
 
 
A ScienceCampus előadásorozat oldala: http://felvi.physics.bme.hu/sciencecampus
 

Magnetic microscopy of 2D and chiral magnetism

Időpont: 
2024. 10. 22. 14:30
Hely: 
BME building F, lecture hall 13, second floor
Előadó: 
Martino Poggio (Basel)
The ability to map magnetic field sensitively and on the nanometer-scale – unlike global magnetization or transport measurements – overcomes ensemble or spatial inhomogeneity in systems ranging from arrays of nanometer-scale magnets, to superconducting thin films, to strongly correlated states in van der Waals heterostructures. Local imaging of nanometer-scale magnetization, Meissner currents, or current in edge-states is the key to unraveling the microscopic mechanisms behind a wealth of new and poorly understood condensed matter phenomena. I will discuss efforts in our group aimed at developing and applying high-sensitivity, high-resolution, non-invasive magnetic scanning probes. In particular, we have been developing scanning probes based on nanometer-scale superconducting quantum interference devices and ultra-sensitive magnetic force microscopy sensors. I will discuss recent imaging experiments with these tools on 2D and chiral magnets, which yield new insights into their underlying magnetism.
 
Webpage of the Szilárd Colloquium: https://physics.bme.hu/kollokvium?language=hu 

Stability of Weyl node merging processes under symmetry constraints

Időpont: 
2024. 11. 08. 10:15
Hely: 
BME building F, Schay room
Előadó: 
Dániel Varjas, Gergő Pintér (BME)
Weyl semimetals are materials with topologically protected point-like degeneracies (Weyl nodes) in their band structures. When tuning external parameters, such as magnetic field or strain, isolated nodes move in momentum space, but small perturbations cannot gap them out. Changes in the number of Weyl nodes occur through merging processes, usually involving a pair of oppositely charged nodes. More complicated processes involving multiple Weyl nodes are also possible, but they typically require fine tuning and are thus less stable. In this work, we use singularity-theoretical tools to study how symmetries affect the allowed merging processes and their stability, focusing on the combination of a two-fold rotation and time-reversal (C2T) symmetry. We find in this case that, counter-intuitively, processes involving a merging of three nodes are more generic than processes involving only two nodes. Our work suggests that multi-Weyl-merging may be observed in a large variety of quantum materials, and we discuss MoTe2, SrSi2, and bilayer graphene as potential candidates.
 
Reference: arXiv:2403.08518 (PRL in press)

Quantum Complexity and scrambling entropy

Időpont: 
2024. 11. 12. 14:30
Hely: 
BME building F, lecture hall 13, second floor
Előadó: 
Alioscia Hamma (Naples)

A long-standing problem in quantum information is: what are the resources that make quantum computers able to perform computational tasks in a way that outperforms any kind of classical Turing machine? The flip side of the same question is: what makes quantum mechanics so hard to simulate? These questions are at the core of the notion of quantum complexity. In this talk we will show that quantum advantage and quantum complexity arise from the conspiracy and interplay of two entropic resources, Entanglement Entropy and Stabilizer Entropy. In particular, quantum complex behavior arises when stabilizer entropy gets scrambled around, giving rise to Scrambling Entropy (SE). We will provide an introduction to the resource theory of SE and applications in quantum thermodynamics, quantum metrology, black-hole physics, and foundations of quantum mechanics.

Optical signatures of dynamical excitonic condensates

Időpont: 
2024. 11. 15. 10:15
Hely: 
BME building F, Schay room
Előadó: 
Alexander Osterkorn (Ljubljana)
Excitons, or bound electron-hole pairs, can condense into an excitonic insulator state, similarly to Cooper pairs in superconductors. A non-equilibrium carrier concentration, such as the one transiently induced by photo-doping or sustained by a tuneable bias voltage in bilayers, can create a dynamical excitonic insulator state, yet proving phase coherence in such setups remains challenging. 
We examine the condensate phase behavior theoretically and show that optical spectroscopy can distinguish between phase-trapped and phase-delocalized dynamical regimes. In the weak-bias regime, trapped phase dynamics result in an in-gap absorption peak nearly independent of bias voltage, while at higher biases its frequency increases approximately linearly.
Both regimes exhibit pronounced second harmonic responses. In the large bias regime, the response current grows strongly under the application of a weak electric probe leading to negative weight in the optical response, which we analyze relative to predictions from a minimal model for the phase. This work opens new avenues for experimentally probing coherence in excitonic condensates and the detection of their dynamical regimes.
 
Reference: arxiv.org/abs/2410.22116

Mit mond a fizika a valóságról? (Science Campus előadás)

Időpont: 
2024. 11. 15. 16:00
Hely: 
BME building F, lecture hall 13, second floor
Előadó: 
Takács Gábor (BME)
Kedves Kollégák, Diákok!
 
Szeretettel hívunk meg minden érdeklődőt a BME TTK ScienceCampus
tudománynépszerűsítő előadássorozat következő előadására:
 
Takács Gábor (BME TTK Elméleti Fizika Tanszék):
Mit mond a fizika a valóságról?
 
november 15. péntek 16:00-17:15
BME TTK FIII 213-as terem
 
"A modern tudomány azonban egy olyan világot fed fel, amely merőben eltér a csecsemők világmodelljétől. Ha újra megnyílunk a világra, kíváncsian és előítéletektől mentesen – ha megengedjük magunknak az újjászületés luxusát –, másképp fogjuk érteni a világot." (F. Wilczek: Alapelvek - A fizika tíz kulcsa a valósághoz). Az előadásban Frank Wilczek Nobel-díjas fizikus könyvét követve áttekintjük, hogyan változtatta meg a modern fizika a világról alkotott elképzelésünket. Fizikai ismereteink a jelenségek elképesztően tág skáláját leírják, ugyanakkor egy, a hétköznapi szemléletünktől gyökeresen eltérő világképet közvetítenek. Wilczek tíz alapelve egyfelől kitűnően összefoglalja a modern fizika legfontosabb megállapításait, másfelől pedig éles megvilágításba helyezi a fennmaradt rejtélyeket és nyitott kérdéseket.
 
További információ és megközelítés:
 
Az érdeklődőket kérjük, lehetőleg regisztráljanak előre, itt:
 
Az előadásokkal elsősorban a természettudományok iránt érdeklődő középiskolás korosztályt célozzuk meg, de természetesen minden érdeklődőt szeretettel várunk!
 
Asbóth János
BME TTK Fizikai Intézet, Science Campus koordinátor

Sine-Gordon model at finite temperature: the method of random surfaces

Időpont: 
2024. 11. 22. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Miklós Tóth (BME)
The sine-Gordon model is a paradigmatic integrable field theory which provides the low-energy description of various 1D gapped quantum systems. Due to its integrability, many exact results are available, but its finite temperature thermodynamics and correlation functions can be challenging to calculate. We propose [1] the method of random surfaces (MRS), a numerical approach to compute such physical quantities at finite temperature, and compare it to other analytical and numerical predictions. With the method we are also able to calculate the finite temperature multi-point functions, where such comparisons are no longer possible.
 
[1]: M. Tóth, J. H. Pixley, D. Szász-Schagrin, G. Takács, M. Kormos, arXiv:2408.08828

Oldalak