# Szemináriumok

## Yu-Shiba-Rusinov states in spin chains on superconductors

## Diversity of chemical synapses of the central nervous system

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

## Nanoscale skyrmions in magnets

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

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

## Entanglement and readout of superconducting circuits with light

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

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

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.