Szemináriumok

Numerical obstacles in Monte Carlo simulations: The overlap and sign problem

Időpont: 
2022. 12. 02. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Kornél Kapás (BME)
A szeminárium részletei: 
https://physics.bme.hu/node/5197
A major breakthrough in the study of the strong interaction at low energies was achieved in 1974, when the foundations of lattice formalism were laid. This led to a very efficient, systematically correct and, most importantly, non-perturbative method. With the help of appropriate mathematical tools, the original theory can be reduced to a finite dimensional, finite volume statistical physical system in which the desired observables can be computed via Monte Carlo simulations. The limitations are typically the numerical difficulties, for which the obvious antidote would be to infinitely increase the computation time. This means that the algorithms should in principle be able to produce the exact results, but with finite machine time, very severe obstacles can arise, e.g. the notorious overlap and sign problem [1-3]. In my presentation I give a detailed insight into these numerical difficulties and show how they can be alleviated.
 
[1] M. Giordano, K. Kapás, S. D. Katz, D. Nógrádi and A. Pásztor. “Effect of stout smearing on the phase diagram from multiparameter reweighting in lattice QCD”. Physical Review D 102.3 (2020).
[2] M. Giordano, K. Kapás, S. D. Katz, D. Nógrádi és A. Pásztor. “New approach to lattice QCD at finite density; results for the critical end point on coarse lattices”. JHEP 05 (2020)
[3] M. Giordano, K. Kapás, S. D. Katz, A. Pásztor és Z. Tulipánt. “Exponential reduction of the sign problem at finite density in the 2+1D XY model via contour deformations”. Phys. Rev. D 106 (2022)

Kvantumos munkastatiszika, univerzalitás és adiabatikus időfejlődés

Időpont: 
2023. 02. 03. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Grabarits András
A szeminárium részletei: 
https://physics.bme.hu/node/5409
Grabarits András (BME Elméleti Fizika Tanszék)
Kvantumos munkastatiszika, univerzalitás és adiabatikus időfejlődés kaotikus fermionikus rendszerekben
PhD házi védés
 
Értekezésemben gerjesztett nemkölcsönható kaotikus Fermi-rendszerek nemegyensúlyi folyamatait vizsgálom mind véletlen mátrix leírásmódot alkalmazva, mind kétdimenziós Anderson modellekben. Munkámban kitüntetett figyelmet fordítok a rendszeren végzett munka statisztikus tulajdonságaira, az univerzalitás nemegyensúlyi rendszerekre való kiterjesztésére és az adiabatikus időfejlődésre. Levezetek egy determináns formulát a munkastatisztika karakterisztikus függvényének és az adiabaticitás valószínűségének kiszámítására.
Ezen felül egy klasszikus energiatérbeli diffúziós modell segítségével megmutatom, hogy a munkastatisztika és az adiabaticitás valószínűsége csak az átlagos munkától és a kezdeti hőmérséklettől függenek, míg bozonizációs és átlagtér módszerek segítségével analitikus közelítő kifejezéseket vezetek le a munkastatisztikára zérus hőmérsékleten. Ezen felül megvizsgálom az alacsony és magas hőmérsékletű határeseteket az adiabatikus időfejlődés és a munkastatisztika szemszögéből.

Numerical renormalization group and its applications to time-dependent problems

Időpont: 
2023. 02. 09. 14:00
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Ireneusz Weymann (Poznan)
A szeminárium részletei: 
https://physics.bme.hu/node/5412

During this talk, I will give a basic introduction to the physics of quantum impurities and the numerical renormalization group (NRG) method - a versatile tool allowing for very accurate treatment of general quantum impurity problems. I will also discuss the extension of NRG to the time domain and present some recent results on the quantum quench dynamics of quantum dot systems, involving also superconducting contacts.

Tensor, entanglement and dual varieties in Fermionic Fock spaces

Időpont: 
2023. 02. 24. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Frédéric Holweck (UTBM Belfort-Montbeliard)
A szeminárium részletei: 
https://physics.bme.hu/node/5422

In the past 20 years classical concepts of algebraic geometry such as the notion of dual varieties have been introduced in the quantum information literature to distinguish different classes of entanglement, a quantum property recognized as a resource in quantum information processing. In this talk, after introducing the connection between the basics of quantum information and the geometry of tensors, I will explain how new equations of the duals of homogeneous varieties can be obtained from graded simple Lie algebras. I will in particular focus on the dual of the spinor varieties $\mathbb{S}_{16}$, the projectivization of the highest weight orbit of the 128 dimensional spin module and its connection with what is known in physics as Fermionic Fock spaces.

Coherent errors and readout errors in surface code

Időpont: 
2023. 03. 10. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
János Asbóth (BME/Wigner)
A szeminárium részletei: 
https://physics.bme.hu/node/5427
The surface code is the leading candidate for quantum error correction. However, the level of protection it offers against coherent errors, i.e., nonfluctuating phase rotations on the physical qubits, is not well known. We consider[1] the combined effect of readout errors and coherent errors on the surface code. We use a recently developed numerical approach, via a mapping of the physical qubits to Majorana fermions[2]. We find a threshold for this combination of errors, with an error rate close to the threshold of the corresponding incoherent error channel (random Pauli-Z and readout errors). Below the threshold, scaling up the code leads to the rapid loss of coherence of errors on the logical level, but error rates that are greater than those of the corresponding incoherent error channel. We also vary the coherent and readout error rates independently, and find that the surface code is more sensitive to coherent errors than to readout errors. Our work extends the recent results on coherent errors with perfect readout [2] to the experimentally more realistic situation where readout errors also occur.
 
[1]: A Marton and JK Asboth: Coherent errors and readout errors in surface code, arXiv:2303.04672
[2]: S Bravyi et al: Correcting coherent errors with surface codes, npj Quantum Information, vol. 4, no. 55 (2018)

Multicriticality in Yang--Lee edge singularity

Időpont: 
2023. 03. 17. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Máté Lencsés (Wigner RCP)
A szeminárium részletei: 
https://physics.bme.hu/node/5430
The Yang--Lee edge singularity is related to the anomalous behaviour of zeroes of the partition function in the two-dimensional Ising model. It was shown by Cardy, that this behaviour is described by the M(2,5) nonunitary minimal model of conformal field theory, known as the Yang--Lee model. This model can be realised as the Ising model above the critical temperature in an imaginary external magnetic field, as the point where the PT symmetry becomes spontaneously broken. In this talk, I will consider the analogue of this scenario in the tricritical Ising model[1], where a three-parameter family of nonunitary PT-symmetric flows exists. I will present our findings regarding massless flows ending in the Yang--Lee model and in a nonunitary "tricritical" point identified as M(2,7). Finally, I will discuss our conjecture on higher nonunitary multicritical points, connected by PT-symmetric nonunitary flows.
 
[1]: M Lencsés, A Miscioscia, G Mussardo, G Takács: Multicriticality in Yang-Lee edge singularity, arXiv:2211.01123

Finite temperature spin diffusion in the Hubbard model in the strong coupling limit

Időpont: 
2023. 03. 24. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Arthur Hutsalyuk (ELTE)
A szeminárium részletei: 
https://physics.bme.hu/node/5433
We investigate finite temperature spin transport in one spatial dimension by considering the spin-spin correlation function of the Hubbard model in the limiting case of infinitely strong repulsion. We find that in the absence of bias the transport is diffusive, and derive the spin diffusion constant. Our approach is based on asymptotic analysis of a Fredholm determinant representation, which is entirely analytic and free of phenomenological assumptions of Generalized Hydrodynamics.
 
[1]:  O Gamayun, A Hutsalyuk, B Pozsgay, MB Zvonarev: Finite temperature spin diffusion in the Hubbard model in the strong coupling limit, arXiv:2301.13840

Weyl points in ball-and-spring mechanical systems

Időpont: 
2023. 03. 31. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Zoltán Guba (BME)
A szeminárium részletei: 
https://physics.bme.hu/node/5437
Degeneracy points of parameter-dependent Hermitian matrices play a fundamental role in quantum physics, as illustrated by the concept of Berry phase in quantum dynamics, the Weyl semimetals in condensed-matter physics, and the robust ground-state degeneracies in topologically ordered quantum systems. Here[1], we construct simple ball-and-spring mechanical systems, whose eigenfrequency degeneracies mimic the behaviour of degeneracy points of electronic band structures. These classical-mechanical arrangements can be viewed as de-quantized versions of Weyl Josephson circuits, i.e., superconducting nanostructures proposed recently to mimic band structure effects of Weyl semimetals. In the mechanical setups we study, we identify degeneracy patterns beyond simple Weyl points, including the chirality flip effect and a quadratic degeneracy point. Our theoretical work is a step toward simple and illustrative table-top experiments exploring topological and differential geometrical aspects of physics.
 
[1]: Z Guba, Gy Frank, G Pintér, A Pályi: Weyl points in ball-and-spring mechanical systems,  arXiv:2302.08241

Miért telik az idő? (Science Campus előadás)

Időpont: 
2023. 03. 31. 16:00
Hely: 
BME building F, second floor, lecture hall F3213
Előadó: 
Takács Gábor
A szeminárium részletei: 
https://physics.bme.hu/node/5434
Közeledik a következő Science Campus előadás! Most pénteken, 16:00-kor az F3213 teremben és a Galileo Webcaston: https://www.galileowebcast.hu/live/live_20230331.html  
Takács Gábor, a BME TTK Elméleti Fizika tanszék professzora a fizika egyik alapvető kérdését teszi fel: Miért telik az idő? Gyertek, hallgassátok meg!
 
Időpont: 2023. március 31., péntek, 16:00
Helyszín: BME F épület, Budafoki út 8., 2. emelet F3213-as terem. 
 
A témában Takács Gábor nemrég szerepelt az MTV Multiverzum műsorában is: https://www.youtube.com/watch?v=b4xwxV4Srtk&t=966s
 

Aharonov–Bohm Effect in Multiband Electron Systems

Időpont: 
2023. 04. 14. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Németh Róbert (ELTE)
A szeminárium részletei: 
https://physics.bme.hu/node/5438
In 1959, Yakir Aharonov and David Bohm discovered theoretically that the quantum mechanical scattering of charged particles on an infinitesimally thin solenoid differs significantly from its classical counterpart. After its experimental verification by Robert Chambers in 1960, the Aharonov–Bohm effect became a widely known phenomenon supporting quantum theory. In their original paper, Aharonov and Bohm presented the analytical solution of the scattering problem for spinless charged particles. In our paper [1], we present a unified treatment for two-dimensional multiband electronic systems possessing isotropic band structures. We propose an integral representation of the scattering state and derive its asymptotic form to obtain the differential cross section. A rather surprising result emerges: this cross section is the same for all isotropic systems and agrees with the original formula by Aharonov and Bohm. To demonstrate the generality of our theory, we apply it to several specific multiband systems relevant in condensed matter physics.
 
[1]: R Németh, J Cserti: Aharonov–Bohm Effect in Multiband Electron Systems, arXiv:2302.07050

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