Szemináriumok

Fermion and Boson Sampling: new developments

Időpont: 
2022. 03. 18. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Zoltán Zimborás (Wigner)
The paradigm of quantum computational advantage (also known as supremacy) aims to demonstrate that a programmable quantum device can solve a problem that no classical computer can solve in any feasible amount of time (irrespective of the usefulness of the problem). In this talk, we shortly describe two such schemes: Fermion Sampling and Boson Sampling. We present an overview of the recent developments concerning the Fermion Sampling scheme [1], and also review some of the parallel developments for Boson Sampling. In particular, we discuss hardness guarantees, experimental feasibility, certification and actual experimental results. Finally, we assess the potential of practically useful sampling schemes (for many-body, network theory and machine learning problems) based on Fermion and Boson Sampling [2].
 
[1] M. Oszmaniec, N. Dangniam, M. Morales, Z. Zimborás,
Fermion Sampling: a robust quantum computational advantage scheme 
using fermionic linear optics and magic input states
arXiv: 2012.15825
 
[2] D. Nagy, Z. Tabi, P. Hága, Z. Kallus, Z. Zimborás, 
Photonic Quantum Policy Learning in OpenAI Gym, 
IEEE Quantum Computing and Engineering 2021, pp. 123-129 (2021).

Kondo Cloud in a Superconductor

Időpont: 
2022. 04. 01. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics & online in Teams
Előadó: 
Gergely Zaránd (BME)
Magnetic impurities embedded in a metal are screened by the Kondo effect, signaled by the formation of an extended correlation cloud, the so-called Kondo or screening cloud. In a superconductor, the Kondo state turns into subgap Yu-Shiba-Rusinov states, and a quantum phase transition occurs between screened and unscreened phases, once the superconducting energy gap exceeds sufficiently the Kondo temperature. We show [1] that, although the Kondo state does not form in the unscreened phase, the Kondo cloud does exist in both quantum phases. However, while screening is complete in the screened phase, it is only partial in the unscreened phase. Compensation, a quantity introduced to characterize the integrity of the cloud, is universal, and shown to be related to the magnetic impurities’ g factor, monitored experimentally by bias spectroscopy.
 
[1]: CP Moca, I Weymann, MA Werner, and G Zaránd,  Phys. Rev. Lett. 127, 186804 (2021).

Twist, squeeze, stretch: topological physics in bilayer graphene

Időpont: 
2022. 04. 05. 16:15
Hely: 
online (Teams)
Előadó: 
Mike Zaletel (Berkeley)

Bilayer graphene, in both its  conventional and  twisted form, has emerged  as a facile platform for exploring topological and correlated phases. In this talk I will review routes for controlling bilayer graphene via twisting, pressure, strain, and proximity-induced spin orbit coupling. 

Egyensúlyi és nemegyensúlyi dinamika kétdimenziós kvantumtérelméletekben

Időpont: 
2022. 04. 08. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics & Online
Előadó: 
Kristóf Hódsági (BME)
- PhD házi védés -
 
Értekezésemben kétdimenziós kvantumtérelméleteket vizsgálok, melyek az alacsony dimenziós kvantumos soktestrendszerek univerzális tulajdonságait írják le. Az integrálható térelméletek eszköztárát alkalmazva különböző analitikus megközelítéseket fejlesztek ki és alkalmazok az egyensúlyi és nemegyensúlyi viselkedés modellezésére, továbbá egy numerikus módszert is, ami a konform Hilbert-tér csonkolásán alapul. A munka központi témája az Ising-térelmélet modellezése, amelynek keretein belül kiszámított egzakt eredményeim alátámasztják az egzotikus E8 részecskespektrum kísérleti megfigyelését. Emellett különböző nemegyensúlyi helyzetekben vizsgálódom - mint a kísérletileg is megvalósítható kvantumkvencs, valamint a paraméterek lassú, Kibble--Zurek-skálázásra vezető hangolása - és azonosítom a dinamika alapvető jellemzőit, a korai időfejlődés egyrészecske-oszcillációitól kezdve az univerzális dinamikai skálatörvényekig.
 
Témavezető: Kormos Márton, BME TTK Elm. Fiz. tanszék
házi bíráló: Lencsés Máté, BME TTK Elm. Fiz. tanszék / Wigner FK SZFI
 
Tézisfüzet magyarul:
 

 

Trade-off between reducing mutational accumulation and increasing commitment to differentiation determines tissue organization

Időpont: 
2022. 04. 12. 14:30
Hely: 
BME building F, 2nd floor, lecture hall 13
Előadó: 
Gergely Szöllősi (ELTE)

Species-specific differences control cancer risk across orders of magnitude variation in body size and lifespan, e.g., by varying the copy numbers of tumor suppressor genes. It is unclear, however, how different tissues within an organism can control somatic evolution despite being subject to markedly different constraints, but sharing the same genome. Hierarchical differentiation, characteristic of self-renewing tissues, can restrain somatic evolution both by limiting divisional load, thereby reducing mutation accumulation, and by increasing cells’ commitment to differentiation, which can “wash out” mutants. Here, we explore the organization of hierarchical tissues that have evolved to limit their lifetime incidence of cancer. Estimating the likelihood of cancer in the presence of mutations that enhance self-proliferation, we demonstrate that a trade-off exists between mutation accumulation and the strength of washing out. Our results explain differences in the organization of widely different hierarchical tissues, such as colon and blood.

Cancelled: Silicon-based quantum computing: Scaling strategies

Időpont: 
2022. 04. 19. 14:15
Hely: 
online (Teams)
Előadó: 
Fernando Gonzalez-Zalba (Quantum Motion)
1) Compilation and scaling strategies for a silicon quantum processor with sparse two-dimensional connectivity, O. Crawford, J. R. Cruise, N. Mertig, M. F. Gonzalez-Zalba, https://arxiv.org/abs/2201.02877
2) Fast high-fidelity single-shot readout of spins in silicon using a single-electron box, G. A. Oakes, ..., F. Gonzalez-Zalba, https://arxiv.org/abs/2203.06608
 
About the speaker: Fernando is an experimental physicist, working as the Lead Quantum Engineer at the quantum hardware company Quantum Motion Technologies, a part-time Associate Lecturer at University of Cambridge, and an Honorary Research Associate at University College London. After obtaining his nanotechnology PhD from Cambridge in 2013, he continued his research in Cambridge, working jointly with Hitachi Europe, where he was Head of Quantum Computing between 2018 and 2020. During these years, he has developed state-of-the-art experimental techniques, aiming at quantum computing with single electrons in silicon-based nanostructures. 

Antianyag a gyógyításban, avagy hogyan működik a PET

Időpont: 
2022. 04. 22. 16:00
Hely: 
BME TTK F épület, III. lépcsőház, F3213-as terem
Előadó: 
Nagy Kálmán (Mediso)
Hogyan találjunk meg néhány femtomol (minden 10^15 között 1) molekulát egy élő testben? Sőt, kövessük útját a sejtek között? A válasz során kiderül, miért vannak részecskegyorsítók a nagy kórházak pincéiben, és számbavesszük a módszer technikai megvalósításának részleteiben rejlő "kisördögöket".
 

Lightweight and recyclable polymer composites for the sustainability

Időpont: 
2022. 04. 24. 14:30
Hely: 
BME building F, 2nd floor, lecture hall 13
Előadó: 
Tamás Bárány (BME)

Polymer matrix composites are widely used in our everyday life, thanks to their low density, high specific strength and designable anisotropy, among others. From the matrix point of view, we can talk about thermoset and thermoplastic, while from the fibre reinforcement point of view, we can talk about continuous and short-fibre-reinforced polymer composites. For sustainability, further weight reduction and increased recyclability are key issues: single-polymer composites and thermoplastic polymer composites produced by thermoplastic resin transfer moulding are good solutions.

 

Page of the Szilárd Leó Colloquium: http://physics.bme.hu/kollokvium 

Hund's physics as a key to understanding emerging quantum phases in iron-pnictides

Időpont: 
2022. 04. 29. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics & Online
Előadó: 
Angelo Valli (TU Wien)
The origin of high-temperature superconductivity is one of the most elusive puzzles of modern solid-state physics. The discovery of materials with unprecedently high critical temperatures (copper oxides, fullerides, iron pnictides) which defy the conventional theory of superconductivity, has fueled an intense research activity for decades. Unconventional superconducting states arise in the proximity of Mott insulators, i.e., states in which the electrons are localized by strong Coulomb repulsion, and in the iron pnictides, overwhelming evidence identifies the normal state as bad metal characterized by orbital-selective correlations arising from the Hund’s exchange interaction. This scenario suggests that a prominent role is played by many-body effects. At the same time, approaches based on the exchange of bosons of electronic origin (spin and orbital fluctuations being the most popular candidates) can predict the emergence of a variety of phenomena, thus pointing toward a more conventional pairing mechanism. 
We discuss a strategy to reconcile this evidence and show that the nature of the electronic correlations in the normal state has non-trivial repercussions on the ordered phases. The inclusion of dynamical correlations allows the simultaneous description of electronic excitations living on different energy scales. Within this framework, the Hund's metal is substantially different from both a weakly interacting metal and an ordinary correlated metal with a strong effective mass renormalization. We reveal that Hund’s correlations are counterintuitively beneficial to boson-mediated superconductivity [1] and that experimental features observed in the nematic photoemission spectra of FeSe and FeAs compounds naturally emerge from the orbital-selective behavior [2]. Our analysis leads to questioning the popular approaches that mainly focus on “who is the driver?” of the electronic instabilities and calls instead for a more coherent theoretical description of iron pnictides which must be able to capture the essence of the correlated metal. 
 
 
[1] L. Fanfarillo, A. Valli, and M. Capone, Phys. Rev. Lett. 125, 177001 (2020)
[2] L. Fanfarillo, A. Valli, and M. Capone, arXiv:2203.01273 (2022)

ELI ALPS – The next generation of attosecond sources

Időpont: 
2022. 05. 03. 14:30
Hely: 
BME building F, 2nd floor, lecture hall 13
Előadó: 
Varjú Katalin (ELI-ALPS)
The Extreme Light Infrastructure – Attosecond Light Pulse Source (ELI-ALPS), the Hungarian pillar of ELI, is the first of its kind that operates by the principle of a user facility, supporting laser based fundamental and applied researches in physical, biological, chemical, medical and materials sciences at extreme short time scales.
 
This goal is realized by the combination of specialized primary lasers which drive nonlinear frequency conversion and acceleration processes in more than twelve different secondary sources. Thus a uniquely broad spectral range of the highest power and shortest light pulses becomes available for the study of dynamic processes on the femtosecond and attosecond time scale in atoms, molecules, condensed matter and plasmas.
 
The attosecond secondary sources are based on advanced techniques of Higher-order Harmonic Generation (HHG). Other secondary sources provide particle beams for plasma physics and radiobiology. A set of state-of-the-art endstations will be accessible to those users who do not have access or do not wish to bring along their own equipment.

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