Szemináriumok

Fluctuations, uncertainty relations, and the geometry of quantum state manifolds

Időpont: 
2023. 10. 20. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Balázs Hetényi (BME)
A szeminárium részletei: 
https://physics.bme.hu/node/5569
I will discuss our recent study of the quantum geometry.  For a pedagogic presentation geometric tensor quantities will be introduced: metric tensor, Christoffel symbol, and briefly the four index Riemann curvature.  There is some mystery in the terms "connection" and "curvature", as they appear to refer to different quantites when used in the context of Riemannian geometry or in the case of adiabatic cycles in quantum systems. I will try to demystify this through a detailed derivation. Our main result [1] is that the fidelity can be used as a cumulant generating function: the first cumulant generates a Berry connection, the second cumulant generates the two two-index geometric quantities, one being the quantum metric the other the Berry curvature.  In the fidelity language this second cumulant is the fidelity susceptibility.  The series can be continued, the third cumulant (or skew) corresponding to what one would call the "quantum Christoffel symbol" (the real part of which corresponds to a true Christoffel symbol of the parameter space of the given quantum system), the fourth cumulant (kurtosis) giving a four index "quantum Riemann curvature tensor". The formalism will be applied to several model systems. For coupled quantum classical systems moving on a Born-Oppenheimer surface, we show that a complex Hermitian inverse mass tensor leads to a mixing of the "molecular electric" and "molecular magnetic" fields.  Requiring the determinant of the second cumulant to be greater than or equal to zero leads to uncertainty relations.  In the end I will discuss our calculations for coherent states (Glauber, SU(2) and SU(1,1)), where we find that the quantum metric tensor has a determinant of zero for minimum uncertainty states, meaning that the geometry is trivial, while for generalized coherent states this is not the case.
 
[1]: B Hetényi and P Lévay, Phys. Rev. A 108, 032218 (2023).

Yu-Shiba-Rusinov states in spin chains on superconductors

Időpont: 
2023. 10. 27. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Levente Rózsa (Wigner/BME)
A szeminárium részletei: 
https://physics.bme.hu/node/5574
Magnetic impurities in conventional superconductors locally break Cooper pairs, leading to the emergence of Yu-Shiba-Rusinov (YSR) bound states. Chains of YSR impurities have been theoretically predicted to give rise to Majorana bound states, which hold promises for realizing topological quantum computers. A fundamental understanding of the formation of YSR states in small atomic clusters is essential for revealing the topological properties of the YSR band structure. The accurate theoretical modelling of YSR states represents a considerable challenge, since it requires a simultaneous description of the electronic structure, the magnetic ordering of the impurities and superconductivity on significantly different energy scales.
Here, first-principles simulations are combined with tight-binding model calculations to determine the influence of the electronic and magnetic structure on the band structure of YSR states. In ferromagnetic chains the spin-orbit coupling opens a minigap in the bands around the Fermi energy where end states are formed, but the small size of this gap in typical material platforms is found to lead to a long-range extension and interactions between these precursors of Majorana bound states [1]. The minigap is larger in antiferromagnetic chains where the spin-orbit coupling is not necessary for its formation, but the well-localized end states in this minigap typically have a topologically trivial origin [2]. The theoretical concepts are illustrated by experimental realizations in specific materials.
 
[1] L. Schneider, P. Beck, J. Neuhaus-Steinmetz, L. Rózsa, T. Posske, J. Wiebe, and R. Wiesendanger, Nat. Nanotechnol. 17, 384 (2022).
[2] L. Schneider, P. Beck, L. Rózsa, T. Posske, J. Wiebe, and R. Wiesendanger, Nat. Commun. 14, 2742 (2023).

Diversity of chemical synapses of the central nervous system

Időpont: 
2023. 10. 31. 14:30
Hely: 
BME building F, lecture hall 13, second floor
Előadó: 
Zoltán Nusser (KOKI)
A szeminárium részletei: 
https://physics.bme.hu/node/5576
Upcoming lecture of the Szilárd Leó Colloquium of the BME Institute of Physics:
 
Understanding the mechanisms of chemical synaptic neurotransmission in the CNS has been in the focus on intense research for decades. It is now widely recognized that synapses between nerve cells are not uniform but show tremendous structural and functional heterogeneity. Revealing the molecular mechanisms underlying functional diversity of excitatory synapses of the rodent hippocampus, a brain area that is involved in learning, memory and spatial navigation, is the main interest of my laboratory. In my presentation, I will summarize our research efforts of the last decade demonstrating our progress in revealing the molecular mechanisms underlying functional diversity.

Monte Carlo option pricing on quantum computers: is a quantum advantage achievable?

Időpont: 
2023. 11. 03. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Zoltán Udvarnoki (ELTE)
A szeminárium részletei: 
https://physics.bme.hu/node/5575
Quantum computers have the potential to provide quadratic speedup for Monte Carlo methods currently used in various classical applications. We examined [1] the advantage of quantum computers for financial option pricing with the Monte Carlo method. Systematic and statistical errors were handled in a joint framework, and a relationship to quantum gate error was established. New metrics were introduced for the assessment of quantum advantage based on sample count and optimized error handling.  Our numerical calculations reveal the unpredictable nature of systematic errors, making consistent quantum advantage difficult with current quantum hardware. Our results indicate that very low noise levels, a two-qubit gate error rate below 10-6, are necessary for the quantum method to outperform the classical one, but a low number of logical qubits (ca. 20) may be sufficient to see quantum advantage already.
 
[1]: Z Udvarnoki, G Fáth and N Fogarasi, Quantum advantage of Monte Carlo option pricing, J. Phys. Commun. 7, 055001 (2023).

Nanoscale skyrmions in magnets

Időpont: 
2023. 11. 07. 14:30
Hely: 
BME building F, lecture hall 13, second floor
Előadó: 
Stefan Blügel (Jülich)
A szeminárium részletei: 
https://physics.bme.hu/node/5577

Upcoming lecture of the Szilárd Leó Colloquium of the BME Institute of Physics:

 

The soliton phenomenon was first described in 1834 by John Scott Russell who observed a wave in the Union Canal in Scotland that preserves its shape while propagating freely, at constant velocity, and recovers it even after collisions with other such localized wave packets. 1961 Tony Skyrme identified topologically stable three-dimensional configurations in a pion field as baryons. Nowadays we find a lot of similar phenomena in magnetic textures, but not only in one dimension as in domain walls or in the Union Canal, respectively, but also in two and three dimensions known as skyrmions and hopfions, respectively. Actually, we have now a zoo of particles predicted and also observed using a spectrum of experimental techniques such as spin-polarized scanning tunneling microscopy, Lorentz microscopy, electron microscopy with off-axis holography, or x-ray scattering. In this colloquium, I will introduce these particles, their topological nature, give arguments of their stability, discuss their lifetime, dynamics, their transport properties, and their potential field of applications. I relate their stability to underlying microscopic interactions and look at promising materials realizations by applying a multiscale simulation approach combining first-principles calculations with atomistic simulations.  

A simple electronic ladder model harbouring Z4 parafermions

Időpont: 
2023. 11. 10. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
László Oroszlány (ELTE)
A szeminárium részletei: 
https://physics.bme.hu/node/5580

Parafermions are anyons with the potential for realizing non-local qubits that are resilient to local perturbations. Compared to Majorana zero modes, braiding of parafermions implements an extended set of topologically protected quantum gates. This, however, comes at the price that parafermionic zero modes can not be realized in the absence of strong interactions whose theoretical description is challenging. In the present work, we construct a simple lattice model for interacting spinful electrons with parafermionic zero energy modes. The explicit microscopic nature of the considered model highlights new realization avenues for these exotic excitations in recently fabricated quantum dot arrays. By density matrix renormalization group calculations, we identify a broad range of parameters, with well-localized zero modes, whose parafermionic nature is substantiated by their unique 8 pi -  periodic Josephson spectrum.

Quantum interference effects in molecular nanoelectronics

Időpont: 
2023. 11. 17. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Angelo Valli (BME)
A szeminárium részletei: 
https://physics.bme.hu/node/5581
It is well established that quantum interference (QI) effects in molecular junctions can dramatically change electron transport by several orders of magnitude. As QI effects can be tailored by a variety of chemical and physical mechanisms and can survive even at room temperature, they hold great potential for nanoelectronics applications. I give an overview of experimental evidence of QI and characterize its distinctive features and origin from a theoretical perspective. As time allows, I will show how and why [1] QI can be exploited to enhance the performance of chemical sensors [2], thermoelectric, and spintronic [3] devices.
 
[1] A. Valli, T. Fabian, F. Libisch, and R. Stadler, Carbon 214, 118358 (2023)
[2] Ö. Şengül, J. Völkle, A. Valli, and R. Stadler, Phys. Rev. B  105, 165428 (2022)
[3] A. Valli, A. Amaricci, V. Brosco, and M. Capone, Nano Lett. 18, 2158 (2018)

Entanglement and readout of superconducting circuits with light

Időpont: 
2023. 11. 21. 14:30
Hely: 
BME building F, lecture hall 13, second floor
Előadó: 
Johannes Fink (ISTA)
A szeminárium részletei: 
https://physics.bme.hu/node/5582

The rapid development of superconducting quantum hardware is expected to run into significant I/O restrictions due to the need for large-scale error correction in a cryogenic environment. Photonics could be the key to both, i.e. optical multiplexing of many control and readout lines on the one hand, but also to realize distributed quantum computing with modules of manageable size each. We have developed an electro-optic interconnect that facilitates strong interactions between microwave and telecom wavelength light. It operates close to the quantum limit and offers new perspectives for a number of scaling, networking and sensing applications. We demonstrate ultra-low noise wavelength conversion, entanglement of microwave and optical fields, as well as an all-optical superconducting qubit readout that does not require any of the bulky cryogenic microwave components.

Exploring new physics in the electroweak sector at the LHC

Időpont: 
2023. 11. 28. 14:30
Hely: 
BME building F, lecture hall 13, second floor
Előadó: 
Gabriella Pásztor (ELTE)
A szeminárium részletei: 
https://physics.bme.hu/node/5583

Upcoming lecture of the Szilárd Leó Colloquium of the BME Institute of Physics:

 

The Large Hadron Collider (LHC) with its extensive and growing data set at record proton - proton collision energies — supported by the ever improving sophistication of experimental and theoretical methods — provides unique opportunities to explore the boundaries of the standard model (SM) of particle physics. The discovery of the Higgs boson completed the model and provided experimental confirmation of the mechanism at its heart that generates the masses of elementary particles. More than 10 years on, the research focuses on the precise investigation of the electroweak sector, including the measurements of the properties of the Higgs boson and the scattering processes of the gauge bosons. These are sensitive to contributions from new physics at high(er) energy scales and offer a model-agnostic way to look beyond the SM. These studies complement dedicated searches for exotic phenomena predicted by a wealth of extended ultraviolet-complete models that offer solutions to the shortcomings of the SM. I will highlight recent results from the LHC that probe the validity of the SM in search of potential signals of new physics.

Classification of complex 2D magnetic ground states using unsupervised contrastive learning

Időpont: 
2023. 12. 01. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Tim Mathies (Hamburg)
A szeminárium részletei: 
https://physics.bme.hu/node/5586

Phase diagrams capture the essential features of a system in many areas of physics. Distinguishing one phase from another is often done by hand-crafted selection rules and an automated approach could accelerate this process. Here, we use a machine learning technique called contrastive learning to classify 18,000 magnetic ground state configurations into 12 distinct clusters. This is done by using a hybrid approach of increasing the number of clusters given by the model to 40 and then merging these clusters into the 12 phases by hand. The ground states of two-dimensional magnetic atomic lattices on metallic substrate are generated by fitting a tight-binding model to a classical Heisenberg model and subsequent classical Monte Carlo calculations. The symmetries of the system are utilized as transformations to cluster identical phases together. Furthermore, we investigate the representation space created by the model as a quick overview for understanding large amounts of physical data. Because of the lack of labeled phases, we judge the quality of the phase diagrams by taking random samples of the resulting clusters. The approach contributes to a better understanding of the connection between magnetism and topological electronic matter. Our results are generalizable to the automated identification of phases in condensed matter physics and beyond.

Oldalak