Bence Hetényi
ContactDepartment of PhysicsUniversity of Basel Klingelbergstrasse 82 CH4056 Basel, Switzerland

Short CV
2018—present:  PhD student in the Condensed Matter Theory & Quantum Computing Group at the University of Basel, supervised by Prof. Dr. Jelena Klinovaja and Prof. Dr. Daniel Loss 
2016 — 2018:  MSc in Physics with honours at Eötvös Loránd University Thesis: Quantumbits in silicon nanostructures, supervised by András Pályi 
2012 — 2016: 
BSc in Physics at Eötvös Loránd University Thesis: Optomechanical multistbility in elastic structures, supervised by János Asbóth 
Publications
Show all abstracts.1.  Anomalous zerofield splitting for hole spin qubits in Si and Ge quantum dots 
Bence Hetényi, Stefano Bosco, and Daniel Loss. arxiv:2205.02582
An anomalous energy splitting of spin triplet states at zero magnetic field has recently been measured in germanium quantum dots. This zerofield splitting could crucially alter the coupling between tunnelcoupled quantum dots, the basic building blocks of stateoftheart spinbased quantum processors, with profound implications for semiconducting quantum computers. We develop an analytical model linking the zerofield splitting to spinorbit interactions that are cubic in momentum. Such interactions naturally emerge in hole nanostructures, where they can also be tuned by external electric fields, and we find them to be particularly large in silicon and germanium, resulting in a significant zerofield splitting in the μeV range. We confirm our analytical theory by numerical simulations of different quantum dots, also including other possible sources of zerofield splitting. Our findings are applicable to a broad range of current architectures encoding spin qubits and provide a deeper understanding of these materials, paving the way towards the next generation of semiconducting quantum processors.
 
2.  Crossed Andreev reflection in spinpolarized chiral edge states due to Meissner effect 
Tamás Haidekker Galambos, Flavio Ronetti, Bence Hetényi, Daniel Loss, and Jelena Klinovaja. arXiv:2203.05894
We consider a hybrid quantum Hallsuperconductor system, where a superconducting finger with oblique profile is wedged into a twodimensional electron gas in the presence of a perpendicular magnetic field, as considered by Lee et al., Nat. Phys. 13, 693 (2017). The electron gas is in the quantum Hall regime at filling factor ν=1. Due to the Meissner effect, the perpendicular magnetic field close to the quantum Hallsuperconductor boundary is distorted and gives rise to an inplane component of the magnetic field. This component enables nonlocal crossed Andreev reflection between the spinpolarized chiral edge states running on opposite sides of the superconducting finger, thus opening a gap in the spectrum of the edge states without the need of spinorbit interaction or nontrivial magnetic textures. We compute numerically the transport properties of this setup and show that a negative resistance exists as consequence of nonlocal Andreev processes. We also obtain numerically the zeroenergy local density of states, which systematically shows peaks stable to disorder. The latter result is compatible with the emergence of Majorana bound states.
 
3.  Hole spin qubits in Si FinFETs with fully tunable spinorbit coupling and sweet spots for charge noise 
Stefano Bosco, Bence Hetényi, and Daniel Loss. PRX Quantum 2, 010348 (2021); arXiv:2011.09417.
The strong spinorbit coupling in hole spin qubits enables fast and electrically tunable gates, but at the same time enhances the susceptibility of the qubit to charge noise. Suppressing this noise is a significant challenge in semiconductor quantum computing. Here, we show theoretically that hole Si FinFETs are not only very compatible with modern CMOS technology, but they present operational sweet spots where the charge noise is completely removed. The presence of these sweet spots is a result of the interplay between the anisotropy of the material and the triangular shape of the FinFET crosssection, and it does not require an extreme finetuning of the electrostatics of the device. We present how the sweet spots appear in FinFETs grown along different crystallographic axes and we study in detail how the behaviour of these devices change when the crosssection area and aspect ratio are varied. We identify designs that maximize the qubit performance and could pave the way towards a scalable spinbased quantum computer.
 
4.  Strong spinorbit interaction and gfactor renormalization of hole spins in Ge/Si nanowire quantum dots 
F. N. M. Froning, M. J. Rančić, B. Hetényi, S. Bosco, M. K. Rehmann, A. Li, E. P. A. M. Bakkers, F. A. Zwanenburg, D. Loss, D. M. Zumbühl, and F. R. Braakman. Phys. Rev. Research 3, 013081 (2021); arXiv:2007.04308.
The spinorbit interaction lies at the heart of quantum computation with spin qubits, research on topologically nontrivial states, and various applications in spintronics. Hole spins in Ge/Si core/shell nanowires experience a spinorbit interaction that has been predicted to be both strong and electrically tunable, making them a particularly promising platform for research in these fields. We experimentally determine the strength of spinorbit interaction of hole spins confined to a double quantum dot in a Ge/Si nanowire by measuring spinmixing transitions inside a regime of spinblockaded transport. We find a remarkably short spinorbit length of ∼65 nm, comparable to the quantum dot length and the interdot distance. We additionally observe a large orbital effect of the applied magnetic field on the hole states, resulting in a large magnetic field dependence of the spinmixing transition energies. Strikingly, together with these orbital effects, the strong spinorbit interaction causes a significant enhancement of the gfactor with magnetic field.The large spinorbit interaction strength demonstrated is consistent with the predicted direct Rashba spinorbit interaction in this material system and is expected to enable ultrafast Rabi oscillations of spin qubits and efficient qubitqubit interactions, as well as provide a platform suitable for studying Majorana zero modes.
 
5.  Exchange interaction of holespin qubits in double quantum dots in highly anisotropic semiconductors 
Bence Hetényi, Christoph Kloeffel, and Daniel Loss. Phys. Rev. Research 2, 033036 (2020); arXiv:2004.07658.
We study the exchange interaction between two holespin qubits in a double quantum dot setup in a silicon nanowire in the presence of magnetic and electric fields. Based on symmetry arguments we show that there exists an effective spin that is conserved even in highly anisotropic semiconductors, provided that the system has a twofold symmetry with respect to the direction of the applied magnetic field. This finding facilitates the definition of qubit basis states and simplifies the form of exchange interaction for twoqubit gates in coupled quantum dots. If the magnetic field is applied along a generic direction, cubic anisotropy terms act as an effective spinorbit interaction introducing novel exchange couplings even for an inversion symmetric setup. Considering the example of a silicon nanowire double dot, we present the relative strength of these anisotropic exchange interaction terms and calculate the fidelity of the rootSWAP gate. Furthermore, we show that the anisotropyinduced spinorbit effects can be comparable to that of the direct Rashba spinorbit interaction for experimentally feasible electric field strengths.
 
6.  Hyperfineassisted decoherence of a phosphorus nuclearspin qubit in silicon 
Bence Hetényi, Péter Boross, and András Pályi. Phys. Rev. B 100, 115435 (2019); arXiv:1903.01102.
The nuclear spin of a phosphorus atom in silicon has been used as a quantum bit in various quantuminformation experiments. It has been proposed that this nuclearspin qubit can be efficiently controlled by an ac electric field, when embedded in a twoelectron dotdonor setup subject to intrinsic or artificial spinorbit interaction. Exposing the qubit to control electric fields in that setup exposes it to electric noise as well. In this work, we describe the effect of electric noise mechanisms, such as phonons and 1/f charge noise, and estimate the corresponding decoherence timescales of the nuclearspin qubit. We identify a promising parameter range where the electrical singlequbit operations are at least an order of magnitude faster then the decoherence. In this regime, decoherence is dominated by dephasing due to 1/f charge noise. Our results facilitate the optimized design of nanostructures to demonstrate electrically driven nuclearspin resonance.
