Coherent electron shuttling as an adiabatic transition in the open quantum system

2021. 12. 07. 14:15
online (Teams)
Jan Krzywda (Warsaw)
As predicted by the Landau-Zener model, isolated quantum system driven though avoided crossing remains in its ground state provided external drive is sufficiently slow. We show the opposite might be true in presence of coupling between the system and its environment, as the longer transition time allows for energy exchange. As an example we analyze, relevant for scaling of future quantum computer [1], coherent shuttling of electron in realistic Silicon and GaAs nanostructures [2,3].
[1] L. M. K.  Vandersypen et al., Interfacing spin qubits in quantum dots and donors—hot, dense, and coherent, npj Quantum Information 3, Article number: 34 (2017)
[2] Jan A. Krzywda, Łukasz Cywiński, Adiabatic electron charge transfer between two quantum dots in presence of 1/f noise, Phys. Rev. B 101, 035303 (2020)
[3] Jan A. Krzywda, Łukasz Cywiński, Interplay of charge noise and coupling to phonons in adiabatic electron transfer between quantum dots, Phys. Rev. B 104, 075439 (2021)

Quantum Spin Liquid Physics on a novel square-kagome lattice material

2021. 12. 10. 10:15
BME building F, seminar room of the Dept. of Theoretical Physics & Online
Yasir Iqbal (Chennai)
The search for quantum spin liquids is one of the most hotly pursued endeavors in condensed matter physics. In two dimensions, corner-sharing triangular geometries such as the kagome lattice have proved to be a fertile ground in realizing these exotic phases of quantum matter. In this talk, I will discuss the novel square-kagome lattice geometry as an ideal playground for realizing quantum spin liquids, being motivated by its recent first of a kind experimental realization in the spin S=1/2 system KCu6AlBiO4(SO4)5Cl. Towards understanding the rich quantum phase diagram of the square-kagome lattice, we employ[1] state-of-the-art quantum many-body numerical techniques such as variational Monte Carlo (VMC) with versatile Gutzwiller-projected Jastrow wave functions, unconstrained multi-variable variational Monte Carlo (mVMC), and pseudo-fermion/Majorana functional renormalization group (PF/PM-FRG) methods. We establish the presence of a quantum paramagnetic ground state and investigate its nature, by classifying symmetric and chiral quantum spin liquids, and inspecting their instabilities towards competing valence-bond-crystal (VBC) orders. Our VMC analysis reveals that a VBC with a pinwheel structure emerges as the lowest-energy variational ground state, and it is obtained as an instability of the U(1) Dirac spin liquid. Analogous conclusions are drawn from mVMC calculations employing accurate BCS pairing states supplemented by symmetry projectors, which confirm the presence of pinwheel VBC order by a thorough analysis of dimer-dimer correlation functions. Our work highlights the nontrivial role of accounting for further neighbor Heisenberg and/or Dzyaloshinkii-Moriya interactions towards explaining the experimental observations. 
[1]: arXiv:2110.08198 (2021), Pinwheel valence-bond-crystal ground state of the spin-1/2 Heisenberg antiferromagnet on the shuriken lattice, N Astrakhantsev, F Ferrari, N Niggemann, T Müller, A Chauhan, A Kshetrimayum, P Ghosh, N Regnault, R Thomale, J Reuther, T Neupert, Y Iqbal

Noise-protected superconducting qubits

2021. 12. 14. 16:15
online (Teams)
András Gyenis (Boulder)
Abstract: Artificial atoms realized by superconducting circuits offer unique opportunities to store and process quantum information with high fidelity. Among them, implementations of circuits that harness intrinsic noise protection have been rapidly developed in recent years. These noise-protected devices constitute a new class of qubits in which the computational states are largely decoupled from local noise channels. The main challenges in engineering such systems are simultaneously guarding against both bit- and phase-flip errors, and also ensuring high-fidelity qubit control. In this talk, we review the theoretical principles at the heart of these new qubits, describes recent experiments, and highlights the potential of robust encoding of quantum information in superconducting qubits.
PRX Quantum 2, 030101 (2021)
PRX Quantum 2, 010339 (2021)
Phys. Rev. Applied 14, 054033 (2020)
About the speaker: Andras is an assistant professor in electrical engineering at the University of Colorado Boulder. He received his BS and MS in experimental condensed matter physics at the Budapest University of Technology, Hungary. Prior to joining CU Boulder, Andras received his PhD in physics at Princeton University in 2016, investigating the surface and bulk properties of unconventional superconductors, strongly correlated electronic systems and topological materials using ultra-low temperature scanning tunneling microscopy. He continued as a postdoctoral researcher at the Department of Electrical Engineering at Princeton, focusing on the design, fabrication and measurement of superconducting quantum circuits. Between 2020 and 2021, he has extended his research focus by developing semiconductor-based quantum devices at the Niels Bohr Institute at the University of Copenhagen as a visiting assistant professor. The defining feature of his current research program at CU Boulder is to realize hybrid superconducting – semiconducting quantum devices that harness intrinsic protection to extend the lifetime of quantum processors.

Molecular Impurities as a Realization of Anyons on the Two-Sphere

2022. 01. 18. 14:15
online (Teams)
Enderalp Yakaboylu (Garching)

Studies on experimental realization of two-dimensional anyons in terms of quasiparticles have been restricted, so far, to only anyons on the plane. It is known, however, that the geometry and topology of space can have significant effects on quantum statistics for particles moving on it. In this talk, I will show that the emerging fractional statistics for particles restricted to move on the sphere, instead of on the plane, arises naturally in the context of quantum impurity problems. In particular, I will demonstrate a setup in which the lowest-energy spectrum of two linear bosonic molecules immersed in a quantum many particle environment can coincide with the anyonic spectrum on the sphere. This paves the way towards experimental realization of anyons on the sphere using molecular impurities. Finally, I will present an approach to interacting quantum many-body systems based on the notion of quantum groups, also known as q-deformed Lie algebras. In particular, I will exemplify this approach by considering a quantum rotor interacting with a bath of bosons.

Approximate quantum gate synthesis

2022. 02. 18. 10:15
online (Teams)
Péter Rakyta (ELTE)
I will describe a novel numerical approach to decompose quantum programs in terms of single- and two-qubit quantum gates with close-to-optimal CNOT gate count[1]. In particular, it turns out that 15 and 63 CNOT gates are sufficient to decompose a general 3- and 4-qubit unitary, respectively. In addition, the algorithm can be adapted to sparse inter-qubit connectivity architectures provided by current intermediate-scale quantum computers, needing only few additional CNOT gates to be implemented in the resulting quantum circuits. I will provide a benchmark comparison of the SQUANDER package implementing our algorithm with the state-of-the-art quantum synthesis tools by decomposing quantum programs accessible in online databases.
[1] P. Rakyta and Z. Zimboras, “Sequential quantum gate decomposer (SQUANDER),” 2021. [Online].

Spin-transfer torque in non-collinear antiferromagnetic junctions

2022. 02. 25. 10:15
online (Teams)
Jakub Zelezny (Prague)
Ferromagnetic spin-valves and tunneling junctions are among the most fundamental and important spintronics devices. Their functionality is based on two effects: the giant or tunneling magnetoresistance for electrical readout and the spin-transfer torque allowing for electrical switching. It has been predicted that the same functionality could be achieved with antiferromagnetic junctions, which would provide various advantages over ferromagnets, such as a much faster switching speed. However, these devices are very sensitive to disorder and have never been experimentally demonstrated [1].
Here we show that the key to obtaining robust spin-transfer torque and magnetoresistance in antiferromagnets is utilizing lower symmetry antiferromagnets, in which electrical current is spin-polarized [2]. We consider junctions composed of non-collinear antiferromagnets and find a spin-transfer torque and magnetoresistance with a magnitude and robustness against disorder comparable to ferromagnetic junctions [3]. Furthermore, our calculations reveal novel aspects of the torque in non-collinear junctions. In particular, we find that apart from the conventional spin-transfer torque, a novel self-generated torque appears. This torque is similar to a spin-orbit torque but has a non-relativistic origin. We also find that a torque appears for any configuration of the junction, in contrast to ferromagnetic junctions where the torque vanishes in the parallel or antiparallel configurations.
[1] J. Železný et al., Nature Physics 14, 220–228 (2018)
[2] J. Železný et al., Phys. Rev. Lett. 119, 187204 (2017)
[3] S. Ghosh et al., arXiv:2109.01399 (2021)

Orvosi fizikától a 4. generációs reaktorokig - Nukleáris kutatások a BME-n

2022. 02. 25. 16:00
online (Teams)
Dr. Czifrus Szabolcs (BME TTK, Nukleáris Technikai Intézet)
Szeretettel hívunk meg minden érdeklődőt a BME TTK ScienceCampus
tudománynépszerűsítő előadássorozat következő előadására:
Dr. Czifrus Szabolcs (BME TTK, Nukleáris Technikai Intézet):
Orvosi fizikától a 4. generációs reaktorokig - Nukleáris kutatások a BME-n
február 25. (péntek) 16.00-17.00
helyszín: online, a Microsoft Teamsben,
Hogyan valósítsuk meg a csillagok energiatermelését a Földön, mik a fúziós berendezések kihívásai? Hogyan lesznek biztonságosabbak, hatékonyabbak, környezetbarátabbak a nukleáris erőművek? Hogyan pillanthatunk be az emberi testbe a radioaktív sugárzások és az atommagok viselkedésének megértése által? Ilyen és ehhez hasonló kérdésekre keressük egy-egy bepillantás erejéig a választ.
További információ:
Az előadássorozat FB oldala:
Az előadásra a természettudományok iránt érdeklődő középiskolásokat és minden más érdeklődőt szeretettel várunk!
Asbóth János
BME TTK Fizikai Intézet, Science Campus koordinátor


Birth Quota of Non-Generic Degeneracy Points

2022. 03. 04. 10:15
BME building F, seminar room of the Dept. of Theoretical Physics
Gergő Pintér (BME)
Weyl points are generic and stable features in the energy spectrum of Hamiltonians 
that depend on a three-dimensional parameter space. Non-generic isolated two-fold 
degeneracy points, such as multi-Weyl points, split into Weyl points upon a generic 
perturbation that removes the fine-tuning or protecting symmetry. The number of the 
resulting Weyl points is at least |Q|, where Q is the topological charge associated to 
the non-generic degeneracy point. Here[1], we show that such a non-generic degeneracy 
point also has a birth quota, i.e., a maximum number of Weyl points that can be born 
from it upon any perturbation. The birth quota is a local multiplicity associated to 
the non-generic degeneracy point, an invariant of map germs known from singularity 
theory. This holds not only for the case of a three-dimensional parameter space with 
a Hermitian Hamiltonian, but also for the case of a two-dimensional parameter space
with a chiral symmetric Hamiltonian. We illustrate the power of this result for electronic 
band structures of two- and three-dimensional crystals. 
Our work establishes a strong connection between singularity theory and 
topological band structures, and more broadly, parameter-dependent quantum systems.
[1]: Birth Quota of Non-Generic Degeneracy Points, 
Gergő Pintér, György Frank, Dániel Varjas, András Pályi, arXiv:2202.05825