@article{10920, abstract = {The spin-orbit interaction permits to control the state of a spin qubit via electric fields. For holes it is particularly strong, allowing for fast all electrical qubit manipulation, and yet an in-depth understanding of this interaction in hole systems is missing. Here we investigate, experimentally and theoretically, the effect of the cubic Rashba spin-orbit interaction on the mixing of the spin states by studying singlet-triplet oscillations in a planar Ge hole double quantum dot. Landau-Zener sweeps at different magnetic field directions allow us to disentangle the effects of the spin-orbit induced spin-flip term from those caused by strongly site-dependent and anisotropic quantum dot g tensors. Our work, therefore, provides new insights into the hole spin-orbit interaction, necessary for optimizing future qubit experiments.}, author = {Jirovec, Daniel and Mutter, Philipp M. and Hofmann, Andrea C and Crippa, Alessandro and Rychetsky, Marek and Craig, David L. and Kukucka, Josip and Martins, Frederico and Ballabio, Andrea and Ares, Natalia and Chrastina, Daniel and Isella, Giovanni and Burkard, Guido and Katsaros, Georgios}, issn = {1079-7114}, journal = {Physical Review Letters}, number = {12}, publisher = {American Physical Society}, title = {{Dynamics of hole singlet-triplet qubits with large g-factor differences}}, doi = {10.1103/PhysRevLett.128.126803}, volume = {128}, year = {2022}, } @article{8909, abstract = {Spin qubits are considered to be among the most promising candidates for building a quantum processor. Group IV hole spin qubits have moved into the focus of interest due to the ease of operation and compatibility with Si technology. In addition, Ge offers the option for monolithic superconductor-semiconductor integration. Here we demonstrate a hole spin qubit operating at fields below 10 mT, the critical field of Al, by exploiting the large out-of-plane hole g-factors in planar Ge and by encoding the qubit into the singlet-triplet states of a double quantum dot. We observe electrically controlled X and Z-rotations with tunable frequencies exceeding 100 MHz and dephasing times of 1μs which we extend beyond 15μs with echo techniques. These results show that Ge hole singlet triplet qubits outperform their electronic Si and GaAs based counterparts in speed and coherence, respectively. In addition, they are on par with Ge single spin qubits, but can be operated at much lower fields underlining their potential for on chip integration with superconducting technologies.}, author = {Jirovec, Daniel and Hofmann, Andrea C and Ballabio, Andrea and Mutter, Philipp M. and Tavani, Giulio and Botifoll, Marc and Crippa, Alessandro and Kukucka, Josip and Sagi, Oliver and Martins, Frederico and Saez Mollejo, Jaime and Prieto Gonzalez, Ivan and Borovkov, Maksim and Arbiol, Jordi and Chrastina, Daniel and Isella, Giovanni and Katsaros, Georgios}, issn = {1476-4660}, journal = {Nature Materials}, number = {8}, pages = {1106–1112}, publisher = {Springer Nature}, title = {{A singlet triplet hole spin qubit in planar Ge}}, doi = {10.1038/s41563-021-01022-2}, volume = {20}, year = {2021}, } @article{8911, abstract = {In the worldwide endeavor for disruptive quantum technologies, germanium is emerging as a versatile material to realize devices capable of encoding, processing, or transmitting quantum information. These devices leverage special properties of the germanium valence-band states, commonly known as holes, such as their inherently strong spin-orbit coupling and the ability to host superconducting pairing correlations. In this Review, we initially introduce the physics of holes in low-dimensional germanium structures with key insights from a theoretical perspective. We then examine the material science progress underpinning germanium-based planar heterostructures and nanowires. We review the most significant experimental results demonstrating key building blocks for quantum technology, such as an electrically driven universal quantum gate set with spin qubits in quantum dots and superconductor-semiconductor devices for hybrid quantum systems. We conclude by identifying the most promising prospects toward scalable quantum information processing. }, author = {Scappucci, Giordano and Kloeffel, Christoph and Zwanenburg, Floris A. and Loss, Daniel and Myronov, Maksym and Zhang, Jian-Jun and Franceschi, Silvano De and Katsaros, Georgios and Veldhorst, Menno}, issn = {2058-8437}, journal = {Nature Reviews Materials}, pages = {926–943 }, publisher = {Springer Nature}, title = {{The germanium quantum information route}}, doi = {10.1038/s41578-020-00262-z}, volume = {6}, year = {2021}, } @phdthesis{10058, abstract = {Quantum information and computation has become a vast field paved with opportunities for researchers and investors. As large multinational companies and international funds are heavily investing in quantum technologies it is still a question which platform is best suited for the task of realizing a scalable quantum processor. In this work we investigate hole spins in Ge quantum wells. These hold great promise as they possess several favorable properties: a small effective mass, a strong spin-orbit coupling, long relaxation time and an inherent immunity to hyperfine noise. All these characteristics helped Ge hole spin qubits to evolve from a single qubit to a fully entangled four qubit processor in only 3 years. Here, we investigated a qubit approach leveraging the large out-of-plane g-factors of heavy hole states in Ge quantum dots. We found this qubit to be reproducibly operable at extremely low magnetic field and at large speeds while maintaining coherence. This was possible because large differences of g-factors in adjacent dots can be achieved in the out-of-plane direction. In the in-plane direction the small g-factors, on the other hand, can be altered very effectively by the confinement potentials. Here, we found that this can even lead to a sign change of the g-factors. The resulting g-factor difference alters the dynamics of the system drastically and produces effects typically attributed to a spin-orbit induced spin-flip term. The investigations carried out in this thesis give further insights into the possibilities of holes in Ge and reveal new physical properties that need to be considered when designing future spin qubit experiments.}, author = {Jirovec, Daniel}, issn = {2663-337X}, keywords = {qubits, quantum computing, holes}, pages = {151}, publisher = {Institute of Science and Technology Austria}, title = {{Singlet-Triplet qubits and spin-orbit interaction in 2-dimensional Ge hole gases}}, doi = {10.15479/at:ista:10058}, year = {2021}, } @unpublished{10066, abstract = {The potential of Si and SiGe-based devices for the scaling of quantum circuits is tainted by device variability. Each device needs to be tuned to operation conditions. We give a key step towards tackling this variability with an algorithm that, without modification, is capable of tuning a 4-gate Si FinFET, a 5-gate GeSi nanowire and a 7-gate SiGe heterostructure double quantum dot device from scratch. We achieve tuning times of 30, 10, and 92 minutes, respectively. The algorithm also provides insight into the parameter space landscape for each of these devices. These results show that overarching solutions for the tuning of quantum devices are enabled by machine learning.}, author = {Severin, B. and Lennon, D. T. and Camenzind, L. C. and Vigneau, F. and Fedele, F. and Jirovec, Daniel and Ballabio, A. and Chrastina, D. and Isella, G. and Kruijf, M. de and Carballido, M. J. and Svab, S. and Kuhlmann, A. V. and Braakman, F. R. and Geyer, S. and Froning, F. N. M. and Moon, H. and Osborne, M. A. and Sejdinovic, D. and Katsaros, Georgios and Zumbühl, D. M. and Briggs, G. A. D. and Ares, N.}, booktitle = {arXiv}, title = {{Cross-architecture tuning of silicon and SiGe-based quantum devices using machine learning}}, doi = {10.48550/arXiv.2107.12975}, year = {2021}, } @unpublished{10065, abstract = {We study double quantum dots in a Ge/SiGe heterostructure and test their maturity towards singlet-triplet ($S-T_0$) qubits. We demonstrate a large range of tunability, from two single quantum dots to a double quantum dot. We measure Pauli spin blockade and study the anisotropy of the $g$-factor. We use an adjacent quantum dot for sensing charge transitions in the double quantum dot at interest. In conclusion, Ge/SiGe possesses all ingredients necessary for building a singlet-triplet qubit.}, author = {Hofmann, Andrea C and Jirovec, Daniel and Borovkov, Maxim and Prieto Gonzalez, Ivan and Ballabio, Andrea and Frigerio, Jacopo and Chrastina, Daniel and Isella, Giovanni and Katsaros, Georgios}, booktitle = {arXiv}, title = {{Assessing the potential of Ge/SiGe quantum dots as hosts for singlet-triplet qubits}}, doi = {10.48550/arXiv.1910.05841}, year = {2019}, }