Szemináriumok

Correlations at PT-symmetric quantum critical point

Időpont: 
2022. 05. 13. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics & Online
Előadó: 
Balázs Dóra (BME)
We consider a PT-symmetric Fermi gas with an exceptional point, 
representing the critical point between PT-symmetric and symmetry 
broken phases. The low energy spectrum remains linear in momentum 
and is identical to that of a hermitian Fermi gas. The fermionic Green's 
function decays in a power law fashion for large distances, as expected 
from gapless excitations, albeit the exponent is reduced from −1 due to 
the quantum Zeno effect. In spite of the gapless nature of the excitations, 
the ground state entanglement entropy saturates to a finite value, 
independent of the subsystem size due to the non-hermitian correlation 
length intrinsic to the system. Attractive or repulsive interaction drives 
the system into the PT-symmetry broken regime or opens up a gap and 
protects PT-symmetry, respectively. Our results challenge the concept of 
universality in non-hermitian systems, where quantum criticality can be 
masked due to non-hermiticity.
 
 
[1]: B Dóra, D Sticlet, CP Moca: Phys. Rev. Lett. 128, 146804 (2022)

2D magnetic materials

Időpont: 
2022. 05. 17. 14:30
Hely: 
BME building F, 2nd floor, lecture hall 13
Előadó: 
Alberto Morpurgo (Geneva)

I will discuss our research on 2D magnetic materials and heterostructures. After a short introduction, I will present results on atomically thin multilayers of different magnetic semiconductors such as CrI3, CrCl3 (layered antiferromagnets), MnPS3 (antiferromagnetic within individual layers), and CrBr3 (ferromagnetic semiconductors). Using atomically thin, exfoliated crystals, we form tunnel barriers that enable magnetism to be probed by magnetotransport measurements. Examples of observed phenomena include: i) a giant tunneling magnetoresistance in CrI3; ii) a full characterization of the magnetic phase diagram of CrCl3 multilayers; iii) the observation of a spin-flop transition in MnPS3 persisting to the ultimate thickness of an individual monolayer, and iv) the demonstration that the tunneling magnetoresistance of ferromagnetic CrBr3 barriers depends on magnetic field and temperature only through the magnetization (from well above to well below the Curie temperature). We conclude that measurements of the temperature and magnetic field dependence of the tunneling magnetoresistance allow precise information about the magnetic state of atomically thin crystals to be obtained, something impossible to do with most conventional experimental techniques, not sufficiently sensitive when used on such a small amount of material.

Simulating Lindbladian evolution with non-abelian symmetries

Időpont: 
2022. 05. 20. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics & Online
Előadó: 
Miklós Werner (BME)
We develop [1] a non-Abelian time evolving block decimation (NA-TEBD) approach to study of open systems governed by Lindbladian time evolution, while exploiting an arbitrary number of abelian or non-abelian symmetries. We illustrate this method in a one-dimensional fermionic SU(2) Hubbard model on a semi-infinite lattice with localized particle loss at one end. We observe a ballistic front propagation with strongly renormalized front velocity, and a hydrodynamic current density profile. For large loss rates, a suppression of the particle current is observed, as a result of the quantum Zeno effect. Operator entanglement is found to propagate faster than the depletion profile, preceding the latter.
 
[1]: CP Moca, MA Werner, Ö Legeza, T Prosen, M Kormos, G Zaránd: arXiv:2112.15342

Post-quantum cryptography

Időpont: 
2022. 05. 24. 14:15
Hely: 
online (Teams)
Előadó: 
Péter Kutas (ELTE)

The security of public key cryptography is based on the hardness of certain algorithmic problems. Schemes we use today rely on the hardness of factoring and computing discrete logarithms in elliptic curve groups. Unfortunately, these are no longer secure once a large-scale quantum computer is built. Thus we have to switch (not instantly but gradually) our currently used protocols (e.g., TLS) to ensure quantum resistance. In this talk I will describe how the abelian hidden subgroup problem relates to factoring and discrete logarithms and will present hard algorithmic problems that we presume are intractable even for a quantum computer

Brutális fizika a szabadban

Időpont: 
2022. 05. 27. 16:00
Hely: 
BME Z épület parkolója
Előadó: 
Härtlein Károly (BME)
Härtlein Károly (BME TTK Fizika Tanszék): 
Brutális fizika a szabadban
 
május 27. péntek 16.00-17.00
helyszín:  a BME Z épület parkolója
 
A tavaszi félév utolsó Science Campus eseménye hagyományosan a Brutális Fizika, ahol Härtlein Károly, a BME TTK mesteroktatója mutat be látványos kísérleteket. Ezúttal a Paksi Atomerőmű tűzoltóinak segítségével, 50 m magasságból, a BME Z épület parkolójában! Ízelítő: Vízzel teli lyukas hordó ejtése - Torricelli kísérlete hordót roppant - 50 m magas vízoszlop nyomása - Magnus-effektus forgó kosárlabdával.
 
További információ és megközelítés:
 
Az előadássorozat FB oldala:
 
Az előadásokkal elsősorban a természettudományok iránt érdeklődő középiskolás korosztályt célozzuk meg, de természetesen minden érdeklődőt szeretettel
várunk!
 
Kapcsolat: Asbóth János, BME TTK Fizikai Intézet, Science Campus koordinátor

Silicon-based quantum computing: Scaling strategies

Időpont: 
2022. 05. 31. 14:15
Hely: 
online (Teams)
Előadó: 
Fernando Gonzalez-Zalba (Quantum Motion)
References: 
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.

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