Szemináriumok

Energetikai kihívások és kitörési pontok a háború árnyékában

Időpont: 
2023. 04. 14. 16:00
Hely: 
BME building F, lecture hall F29
Előadó: 
Aszódi Attila (BME)
A szeminárium részletei: 
https://physics.bme.hu/node/5439
Szeretettel hívunk meg minden érdeklődőt a BME TTK ScienceCampus
tudománynépszerűsítő előadássorozat következő előadására:
 
Aszódi Attila (BME TTK Nukleáris Technikai Intézet): 
Napelemek a háztetőn, kis moduláris reaktorok a hátsó kertben? Energetikai kihívások és kitörési pontok a háború árnyékában
 
április 14. péntek 16.00-17.00
helyszín: BME TTK F épület, F29-es terem
 
Energiaellátásunk régóta alkalmazott technológiai elemei a közelmúltban alapvetően kérdőjeleződtek meg:
- A szén, olaj és földgáz mint fosszilis energiahordozók felhasználásával fel akarunk hagyni a klímavédelem miatt.
- Az Európába importált nagy mennyiségű földgáz rendelkezésre állása és ára az orosz-ukrán háború miatt is megkérdőjeleződött.
- A nap- és szélenergia időjárástól való függősége az ellátás folyamatosságát nem engedi.
- Az atomenergia alkalmazását számos országban politikai viták övezik.
 
Hogyan segíthetnek a korszerű tudományos módszerek, mint a gépi tanulás és a mesterséges neurális hálózatok a műszaki problémák megoldásában? Milyen megújuló energia és nukleáris technológiák látszanak ma a piacon és a kutatás-fejlesztésben? Hogyan lehet ezeket ötvözni? Mi lehet a szerepe az energiatárolásnak? Milyen feladatok várnak a fiatal szakemberekre ezen a forradalmi átalakulásban lévő területen?
 
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!
 
Asbóth János
BME TTK Fizikai Intézet, Science Campus koordinátor

Theory of hole spin qubits in silicon and germanium quantum dots

Időpont: 
2023. 04. 21. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Stefano Bosco (Basel)
A szeminárium részletei: 
https://physics.bme.hu/node/5440

Hole spin qubits in silicon and germanium quantum dots are promising platforms for large-scale quantum computers because of their large intrinsic spin-orbit interaction, which permits efficient and ultrafast all-electric qubit control without additional components.

I will present schemes to engineer this interaction in different architectures, e.g. in the squeezed Ge quantum dots proposed in [1], aiming to optimize quantum information processing. A large spin-orbit interaction mediates a strong coupling between hole spins and microwave photons. Hole spin-photon coupling is not only strong but is also electrically tunable and can be engineered to be longitudinal [2], where the microwave field couples to the phase of the spin. This type of coupling enables exact protocols for fast and high-fidelity two-qubit gates that could even work at high temperatures.

On the other hand, the spin-orbit interaction also couples the spin to charge noise, causing the qubit to decohere. To overcome this issue, I will discuss qubit designs that enable sweet spots where charge noise can be completely removed [3]. These sweet spots appear in hole spin qubits encoded in silicon fin field-effect transistors, devices commonly used in the modern semiconductor industry. In these qubits, the noise caused by hyperfine interactions with nuclear spins -another leading source of decoherence in spin qubits- is also strongly suppressed, greatly enhancing their coherence, and reducing the need for expensive isotopically purified materials [4].

Moreover, the large spin-orbit interaction in hole quantum dots enables phenomena that are out of reach in competing architectures. For example, in these systems the exchange interactions between nearby spins can be highly anisotropic, even at zero magnetic fields, opening the way to novel protocols to enhance the speed and fidelity of two-qubit gates in future quantum processors. 

 

[1] Bosco et al (2021) PRB 104

[2] Bosco et al (2022) PRL 129

[3] Bosco Hetenyi Loss (2021) PRX Quantum 2

[4] Bosco and Loss (2021) PRL 127

Tailored error correction codes for spin qubits

Időpont: 
2023. 04. 28. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Bence Hetényi (Basel)
A szeminárium részletei: 
https://physics.bme.hu/node/5441

Spin qubits in semiconductor structures bring the promise of large-scale 2D integration, with the possibility to incorporate the control electronics on the same chip. Recent spin-qubit experiments demonstrate gate operations and readout well within 1% error rate. This error rate, based on the error threshold of the surface code under circuit-level noise, has become a standard figure of merit for gate errors in the spin-qubit community. The isotropic circuit-level noise model, however, assumes that gate errors, measurement errors, and data qubit errors during ancilla measurements occur with the same probability suggesting that the requirements might be less stringent for some of these ingredients. Furthermore, recent developments in quantum error correction codes present opportunities to improve the error threshold and reduce connectivity requirements compared to Kitaev’s surface code. In this work we consider state-of-the-art error-correction codes that require only nearest-neighbour two-qubit gates, and study their performance under anisotropic circuit-level noise that accounts for distinct error rates for gates, measurement and qubit decoherence during idling. We present the spin-qubit layout required for each of the error correction codes, accounting for additional elements required by spin-qubit architectures such as auxiliary quantum dots for spin-blockade-assisted readout.

The sound of tiny guitars approaching the quantum regime

Időpont: 
2023. 05. 02. 14:30
Hely: 
BME building F, 2nd floor, lecture hall 13
Előadó: 
Adrian Bachtold (Barcelona)
A szeminárium részletei: 
https://physics.bme.hu/node/5442

Mechanics has historically played a pivotal role in science by providing the basis for classical physics. Today, with the advent of nanoscale mechanical devices combined with quantum electronic devices, we are witnessing a renaissance in the field of mechanics. After an introduction on the mesoscopic physics of nanomechanical resonators, I will discuss our recent advances on mechanical resonators based on carbon nanotubes. The nanotube in these devices vibrates as a guitar string. Single-electron tunneling enables coupling the mechanical vibrations to electrons by a large amount. I will show how to use this coupling to create a nonlinear mechanical oscillator approaching the quantum regime, where the resulting quantum energy levels of the mechanical oscillator are no longer evenly spaced. Using mechanical nanotubes hosting multiple quantum dots, we expect that our approach may enable the realization of a mechanical qubit and a quantum simulator of quantum matters featuring strong electron-phonon correlations.

Quantum Thermal State Preparation

Időpont: 
2023. 05. 12. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
András Gilyén (Rényi Inst.)
A szeminárium részletei: 
https://physics.bme.hu/node/5445
Preparing ground states and thermal states is of key importance to simulating quantum systems on a quantum computer. Despite the hope for practical quantum advantage in quantum simulation, popular approaches like variational circuits or adiabatic algorithms appear to face serious difficulties. Monte-Carlo style quantum Gibbs samplers have emerged as an alternative, but prior proposals have been unsatisfactory due to technical obstacles related to energy-time uncertainty. We introduce [1] simple continuous-time quantum Gibbs samplers that overcome these obstacles by efficiently simulating Nature-inspired quantum Master Equations (Liouvillians) utilizing the operator Fourier transform. In addition, we construct the first provably accurate and efficient algorithm for preparing certain purified Gibbs states (called thermal field double states in high-energy physics) of rapidly thermalizing systems; this algorithm also benefits from a Szegedy-type quadratic improvement with respect to the mixing time. Our algorithms' cost has a favorable dependence on temperature, accuracy, and the mixing time (or spectral gap) of the relevant Liouvillians. We contribute to the theory of thermalization by developing a general analytic framework that handles energy uncertainty through non-asymptotic secular approximation and approximate detailed balance, establishing our approximation guarantees and, as a byproduct yielding the first rigorous proof of finite-time thermalization for physically derived Liouvillians. Given the success of the classical Metropolis algorithm and the ubiquity of thermodynamics, we anticipate that quantum Gibbs sampling will become an indispensable tool in quantum computing.
 
[1]: C-F Chen, MJ Kastoryano, FGSL Brandão, A Gilyen: https://arxiv.org/abs/2303.18224

Thermodynamics and order beyond equilibrium

Időpont: 
2023. 05. 16. 14:30
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Roderich Moessner (MPIPKS Dresden)
A szeminárium részletei: 
https://physics.bme.hu/node/5446
Title: Thermodynamics and order beyond equilibrium — from eigenstate thermalisation to time crystals
 
Abstract: The field of thermodynamics is one of the crown jewels of classical physics. Thanks to the advent of experiments in cold atomic systems with long coherence times, our understanding of the connection of thermodynamics to quantum statistical mechanics has seen remarkable progress.
 
Extending these ideas and concepts to the non-equilibrium setting is a challenging topic, in itself of perennial interest. Here, we present perhaps the simplest non-equilibrium class of quantum problems, namely Floquet systems, i.e. systems whose Hamiltonians depend on time periodically, H(t + T) = H(t). For these, there is no energy conservation, and hence not even a natural concept of temperature.
 
We find that certain structures from equilibrium thermodynamics are lost, while entirely new non-equilibrium phenomena can arise, including a spectacular spatiotemporal `time-crystalline' form of order, recently observed experimentally on google AI's sycamore NISQ platform.

Exact bounds on the energy gap of transverse-field Ising chains by mapping to random walks

Időpont: 
2023. 05. 19. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Róbert Juhász (Wigner)
A szeminárium részletei: 
https://physics.bme.hu/node/5447
Based on a relationship with continuous-time random walks [1], we derive [2] exact lower and upper bounds on the lowest energy gap of open transverse-field Ising chains, which are explicit in the parameters and are generally valid for arbitrary sets of possibly random couplings and fields. In the homogeneous chain and in the random chain with uncorrelated parameters, both the lower and upper bounds show the same finite-size scaling in the ferromagnetic phase and at the critical point, demonstrating the ability of these bounds to infer the correct finite-size scaling of the critical gap. Applying the bounds to random transverse-field Ising chains with coupling-field correlations, a model which is relevant for adiabatic quantum computing, the finite-size scaling of the gap is shown to be related to that of sums of independent random variables. We determine the critical dynamical exponent of the model and reveal the existence of logarithmic corrections at special points.
 
[1]: F Iglói, L Turban, and H Rieger, Phys. Rev. E 59, 1465 (1999)
[2]: R Juhász, Phys. Rev. B 106, 064204 (2022)

Many-body quantum physics with cold atoms and molecules

Időpont: 
2023. 05. 23. 10:15
Hely: 
BME building F, lecture hall 13, second floor
Előadó: 
Hanns-Christoph Nägerl (Innsbruck)
A szeminárium részletei: 
https://physics.bme.hu/node/5450

I will give an overview over my group's activities in the field of cold atoms and molecules, past, present, and future. Cold atoms have shown to offer an ideal and very rich experimental platform to study diverse phenomena in quantum many-body physics, ranging from superfluidity and ground-state properties to transport and dynamical processes. In recent years, we have used samples of atoms initially in the state of a Bose-Einstein condensate cooled to nano-Kelvin temperatures as a source for a diverse set of experiments, e.g., on correlated tunneling, low-dimensional transport and the inhibition thereof, and on the formation of low-entropy samples of molecules. One of our experimental efforts presently focusses on impurity dynamics in a one-dimensional setting, which evidently allows us to simulate the behavior of any-ons, to some extent. Other activities in my groups are geared towards the generation of exotic many-body states on the basis of long-range interactions, as e.g. given by dipolar molecules. I will start with an overview over the various cold-atom tools and tricks, and I will close with an outlook on the future perspectives, e.g. coupling of quantum light to an interacting many-body quantum system.

 

Web page of the Szilárd Colloquium: https://physics.bme.hu/kollokvium?language=hu

Extending the geometric phase formalism to cycles with isolated degeneracy points

Időpont: 
2023. 05. 26. 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/5454
Balázs Hetényi (BME): Extending the geometric phase formalism to cycles with isolated degeneracy points and how to implement finite size scaling in the modern polarization theory
 
The modern theory of polarization casts the dielectric polarization as a geometric phase (Zak-Berry phase).   Due to the fact that this quantity is not a simple operator expectation value, traditional finite size scaling approaches are not applicable to it.  In systems where a local order parameter exists, the Binder cumulant (a ratio of statistical cumulants) is guaranteed to locate classical and quantum phase transition points via the finite size scaling hypothesis.  In this talk, it will be shown that the so-called gauge invariant cumulants associated with the geometric phase can be used to construct the analog of the Binder cumulant for adiabatic cycles.  The formalism is general, in the sense that such "Berry-Binder cumulants" can be constructed for any adiabatic cycle with isolated degeneracy points, and they take particular finite values at gap closure.   We apply the formalism to the location of gap closure points in a variety of systems in one and two dimensions, including topological, disordered, and correlated systems.  Our approach is sensitive to gap closure, even in cases where the Fermi surface is down by two dimensions compared to the dimension of the system (Dirac points in graphene or the topological Haldane model).  We also develop a renormalization scheme based on the modern polarization theory, and apply it to disordered systems in one, two, and three dimensions.  In one and three dimensions our approach concurs with the famous "gang-of-four" results, in two dimensions we run into system size limitations, but our preliminary results are not inconsistent with the scaling theory of localization.
 
[1]: B. Hetényi and B. Dóra, "Quantum phase transitions from analysis of the polarization amplitude", Phys. Rev. B 99 085126 (2018).
[2]: B. Hetényi, "Interaction-driven polarization shift in the t-V-V' lattice fermion model at half filling: emergent Haldane phase", Phys. Rev. Research, 2 023266 (2020).
[3]: B. Hetényi, S. Parlak, and M. Yahyavi, "Scaling and renormalization in the modern theory of polarization: application to disordered systems", Phys. Rev. B 104 214207 (2021).
[4]: B. Hetényi and S. Cengiz, "Geometric cumulants associated with adiabatic cycles crossing degeneracy points: Application to finite size scaling of metal-insulator transitions in crystalline electronic systems" 106 195151 (2022).

Cost-effective explicitly correlated coupled-cluster approaches

Időpont: 
2023. 05. 30. 14:30
Hely: 
BME building F, lecture hall 13, second floor
Előadó: 
Mihály Kállay (BME)
A szeminárium részletei: 
https://physics.bme.hu/node/5453
If accurate results are desired in quantum chemistry, the coupled-cluster (CC) approach is the method of choice. The accuracy provided by the most successful CC approach, the CC singles, doubles, and perturbative triples [CCSD(T)] method, is adequate for most chemical applications, and it is often referred to as the “gold standard” of quantum chemistry. Besides the level of correlation, the other factor seriously limiting the accuracy of computational results is the quality of the atomic orbital basis set. Large basis sets including functions of high angular momentum are required to reduce the error stemming from the incompleteness of the basis below an acceptable magnitude. A partial solution to this problem is offered by explicitly correlated approaches, which are based on wave functions that explicitly contain the interelectronic distances thereby significantly reducing the basis set requirements. Unfortunately, calculations with explicitly correlated CCSD(T) are still relatively time consuming. Recently, we have developed a reduced-cost explicitly correlated CCSD(T) approach that is as accurate as the parent method. The approach is based on well-established data compression techniques, which were also successfully utilized to speed up conventional CC methods. The approach significantly extends the scope of explicitly correlated CCSD(T) and enables the accurate solution of the electronic Schrödinger equation for a couple of dozens of atoms.
 
 
Web page of the Szilárd Colloquium: https://physics.bme.hu/kollokvium

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