@unpublished{8831, abstract = {Holes in planar Ge have high mobilities, strong spin-orbit interaction and electrically tunable g-factors, and are therefore emerging as a promising candidate for hybrid superconductorsemiconductor devices. This is further motivated by the observation of supercurrent transport in planar Ge Josephson Field effect transistors (JoFETs). A key challenge towards hybrid germanium quantum technology is the design of high quality interfaces and superconducting contacts that are robust against magnetic fields. By combining the assets of Al, which has a long superconducting coherence, and Nb, which has a significant superconducting gap, we form low-disordered JoFETs with large ICRN products that are capable of withstanding high magnetic fields. We furthermore demonstrate the ability of phase-biasing individual JoFETs opening up an avenue to explore topological superconductivity in planar Ge. The persistence of superconductivity in the reported hybrid devices beyond 1.8 T paves the way towards integrating spin qubits and proximity-induced superconductivity on the same chip.}, author = {Aggarwal, Kushagra and Hofmann, Andrea C and Jirovec, Daniel and Prieto Gonzalez, Ivan and Sammak, Amir and Botifoll, Marc and Marti-Sanchez, Sara and Veldhorst, Menno and Arbiol, Jordi and Scappucci, Giordano and Katsaros, Georgios}, booktitle = {arXiv}, title = {{Enhancement of proximity induced superconductivity in planar Germanium}}, year = {2020}, } @misc{8834, abstract = {This data collection contains the transport data for figures presented in the supplementary material of "Enhancement of Proximity Induced Superconductivity in Planar Germanium" by K. Aggarwal, et. al. The measurements were done using Labber Software and the data is stored in the hdf5 file format. The files can be opened using either the Labber Log Browser (https://labber.org/overview/) or Labber Python API (http://labber.org/online-doc/api/LogFile.html). }, author = {Katsaros, Georgios}, publisher = {Institute of Science and Technology Austria}, title = {{Enhancement of proximity induced superconductivity in planar Germanium}}, doi = {10.15479/AT:ISTA:8834}, year = {2020}, } @article{7541, abstract = {Semiconductor nanowires have been playing a crucial role in the development of nanoscale devices for the realization of spin qubits, Majorana fermions, single photon emitters, nanoprocessors, etc. The monolithic growth of site‐controlled nanowires is a prerequisite toward the next generation of devices that will require addressability and scalability. Here, combining top‐down nanofabrication and bottom‐up self‐assembly, the growth of Ge wires on prepatterned Si (001) substrates with controllable position, distance, length, and structure is reported. This is achieved by a novel growth process that uses a SiGe strain‐relaxation template and can be potentially generalized to other material combinations. Transport measurements show an electrically tunable spin–orbit coupling, with a spin–orbit length similar to that of III–V materials. Also, charge sensing between quantum dots in closely spaced wires is observed, which underlines their potential for the realization of advanced quantum devices. The reported results open a path toward scalable qubit devices using nanowires on silicon.}, author = {Gao, Fei and Wang, Jian-Huan and Watzinger, Hannes and Hu, Hao and Rančić, Marko J. and Zhang, Jie-Yin and Wang, Ting and Yao, Yuan and Wang, Gui-Lei and Kukucka, Josip and Vukušić, Lada and Kloeffel, Christoph and Loss, Daniel and Liu, Feng and Katsaros, Georgios and Zhang, Jian-Jun}, issn = {0935-9648}, journal = {Advanced Materials}, number = {16}, publisher = {Wiley}, title = {{Site-controlled uniform Ge/Si hut wires with electrically tunable spin-orbit coupling}}, doi = {10.1002/adma.201906523}, volume = {32}, year = {2020}, } @misc{7689, abstract = {These are the supplementary research data to the publication "Zero field splitting of heavy-hole states in quantum dots". All matrix files have the same format. Within each column the bias voltage is changed. Each column corresponds to either a different gate voltage or magnetic field. The voltage values are given in mV, the current values in pA. Find a specific description in the included Readme file. }, author = {Katsaros, Georgios}, publisher = {Institute of Science and Technology Austria}, title = {{Supplementary data for "Zero field splitting of heavy-hole states in quantum dots"}}, doi = {10.15479/AT:ISTA:7689}, year = {2020}, } @misc{9222, author = {Katsaros, Georgios}, publisher = {Institute of Science and Technology Austria}, title = {{Transport data for: Site‐controlled uniform Ge/Si Hut wires with electrically tunable spin–orbit coupling}}, doi = {10.15479/AT:ISTA:9222}, year = {2020}, } @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}, } @article{317, abstract = {We replace the established aluminium gates for the formation of quantum dots in silicon with gates made from palladium. We study the morphology of both aluminium and palladium gates with transmission electron microscopy. The native aluminium oxide is found to be formed all around the aluminium gates, which could lead to the formation of unintentional dots. Therefore, we report on a novel fabrication route that replaces aluminium and its native oxide by palladium with atomic-layer-deposition-grown aluminium oxide. Using this approach, we show the formation of low-disorder gate-defined quantum dots, which are reproducibly fabricated. Furthermore, palladium enables us to further shrink the gate design, allowing us to perform electron transport measurements in the few-electron regime in devices comprising only two gate layers, a major technological advancement. It remains to be seen, whether the introduction of palladium gates can improve the excellent results on electron and nuclear spin qubits defined with an aluminium gate stack.}, author = {Brauns, Matthias and Amitonov, Sergey and Spruijtenburg, Paul and Zwanenburg, Floris}, journal = {Scientific Reports}, number = {1}, publisher = {Nature Publishing Group}, title = {{Palladium gates for reproducible quantum dots in silicon}}, doi = {10.1038/s41598-018-24004-y}, volume = {8}, year = {2018}, } @article{23, abstract = {The strong atomistic spin–orbit coupling of holes makes single-shot spin readout measurements difficult because it reduces the spin lifetimes. By integrating the charge sensor into a high bandwidth radio frequency reflectometry setup, we were able to demonstrate single-shot readout of a germanium quantum dot hole spin and measure the spin lifetime. Hole spin relaxation times of about 90 μs at 500 mT are reported, with a total readout visibility of about 70%. By analyzing separately the spin-to-charge conversion and charge readout fidelities, we have obtained insight into the processes limiting the visibilities of hole spins. The analyses suggest that high hole visibilities are feasible at realistic experimental conditions, underlying the potential of hole spins for the realization of viable qubit devices.}, author = {Vukušić, Lada and Kukucka, Josip and Watzinger, Hannes and Milem, Joshua M and Schäffler, Friedrich and Katsaros, Georgios}, issn = {15306984}, journal = {Nano Letters}, number = {11}, pages = {7141 -- 7145}, publisher = {American Chemical Society}, title = {{Single-shot readout of hole spins in Ge}}, doi = {10.1021/acs.nanolett.8b03217}, volume = {18}, year = {2018}, } @phdthesis{69, abstract = {A qubit, a unit of quantum information, is essentially any quantum mechanical two-level system which can be coherently controlled. Still, to be used for computation, it has to fulfill criteria. Qubits, regardless of the system in which they are realized, suffer from decoherence. This leads to loss of the information stored in the qubit. The upper bound of the time scale on which decoherence happens is set by the spin relaxation time. In this thesis I studied a two-level system consisting of a Zeeman-split hole spin confined in a quantum dot formed in a Ge hut wire. Such Ge hut wires have emerged as a promising material system for the realization of spin qubits, due to the combination of two significant properties: long spin coherence time as expected for group IV semiconductors due to the low hyperfine interaction and a strong valence band spin-orbit coupling. Here, I present how to fabricate quantum dot devices suitable for electrical transport measurements. Coupled quantum dot devices allowed the realization of a charge sensor, which is electrostatically and tunnel coupled to a quantum dot. By integrating the charge sensor into a radio-frequency reflectometry setup, I performed for the first time single-shot readout measurements of hole spins and extracted the hole spin relaxation times in Ge hut wires.}, author = {Vukušić, Lada}, issn = {2663-337X}, pages = {103}, publisher = {Institute of Science and Technology Austria}, title = {{Charge sensing and spin relaxation times of holes in Ge hut wires}}, doi = {10.15479/AT:ISTA:TH_1047}, year = {2018}, } @article{77, abstract = {Holes confined in quantum dots have gained considerable interest in the past few years due to their potential as spin qubits. Here we demonstrate two-axis control of a spin 3/2 qubit in natural Ge. The qubit is formed in a hut wire double quantum dot device. The Pauli spin blockade principle allowed us to demonstrate electric dipole spin resonance by applying a radio frequency electric field to one of the electrodes defining the double quantum dot. Coherent hole spin oscillations with Rabi frequencies reaching 140 MHz are demonstrated and dephasing times of 130 ns are measured. The reported results emphasize the potential of Ge as a platform for fast and electrically tunable hole spin qubit devices.}, author = {Watzinger, Hannes and Kukucka, Josip and Vukusic, Lada and Gao, Fei and Wang, Ting and Schäffler, Friedrich and Zhang, Jian and Katsaros, Georgios}, journal = {Nature Communications}, number = {3902 }, publisher = {Nature Publishing Group}, title = {{A germanium hole spin qubit}}, doi = {10.1038/s41467-018-06418-4}, volume = {9}, year = {2018}, } @phdthesis{49, abstract = {Nowadays, quantum computation is receiving more and more attention as an alternative to the classical way of computing. For realizing a quantum computer, different devices are investigated as potential quantum bits. In this thesis, the focus is on Ge hut wires, which turned out to be promising candidates for implementing hole spin quantum bits. The advantages of Ge as a material system are the low hyperfine interaction for holes and the strong spin orbit coupling, as well as the compatibility with the highly developed CMOS processes in industry. In addition, Ge can also be isotopically purified which is expected to boost the spin coherence times. The strong spin orbit interaction for holes in Ge on the one hand enables the full electrical control of the quantum bit and on the other hand should allow short spin manipulation times. Starting with a bare Si wafer, this work covers the entire process reaching from growth over the fabrication and characterization of hut wire devices up to the demonstration of hole spin resonance. From experiments with single quantum dots, a large g-factor anisotropy between the in-plane and the out-of-plane direction was found. A comparison to a theoretical model unveiled the heavy-hole character of the lowest energy states. The second part of the thesis addresses double quantum dot devices, which were realized by adding two gate electrodes to a hut wire. In such devices, Pauli spin blockade was observed, which can serve as a read-out mechanism for spin quantum bits. Applying oscillating electric fields in spin blockade allowed the demonstration of continuous spin rotations and the extraction of a lower bound for the spin dephasing time. Despite the strong spin orbit coupling in Ge, the obtained value for the dephasing time is comparable to what has been recently reported for holes in Si. All in all, the presented results point out the high potential of Ge hut wires as a platform for long-lived, fast and fully electrically tunable hole spin quantum bits.}, author = {Watzinger, Hannes}, issn = {2663-337X}, pages = {77}, publisher = {Institute of Science and Technology Austria}, title = {{Ge hut wires - from growth to hole spin resonance}}, doi = {10.15479/AT:ISTA:th_1033}, year = {2018}, } @article{5816, abstract = {Solid-state qubit manipulation and read-out fidelities are reaching fault-tolerance, but quantum error correction requires millions of physical qubits and therefore a scalable quantum computer architecture. To solve signal-line bandwidth and fan-out problems, microwave sources required for qubit manipulation might be embedded close to the qubit chip, typically operating at temperatures below 4 K. Here, we perform the first low temperature measurements of a 130 nm BiCMOS based SiGe voltage controlled oscillator at cryogenic temperature. We determined the frequency and output power dependence on temperature and magnetic field up to 5 T and measured the temperature influence on its noise performance. The device maintains its full functionality from 300 K to 4 K. The carrier frequency at 4 K increases by 3% with respect to the carrier frequency at 300 K, and the output power at 4 K increases by 10 dB relative to the output power at 300 K. The frequency tuning range of approximately 20% remains unchanged between 300 K and 4 K. In an in-plane magnetic field of 5 T, the carrier frequency shifts by only 0.02% compared to the frequency at zero magnetic field.}, author = {Hollmann, Arne and Jirovec, Daniel and Kucharski, Maciej and Kissinger, Dietmar and Fischer, Gunter and Schreiber, Lars R.}, issn = {00346748}, journal = {Review of Scientific Instruments}, number = {11}, publisher = {AIP Publishing}, title = {{30 GHz-voltage controlled oscillator operating at 4 K}}, doi = {10.1063/1.5038258}, volume = {89}, year = {2018}, } @article{5990, abstract = {A Ge–Si core–shell nanowire is used to realize a Josephson field‐effect transistor with highly transparent contacts to superconducting leads. By changing the electric field, access to two distinct regimes, not combined before in a single device, is gained: in the accumulation mode the device is highly transparent and the supercurrent is carried by multiple subbands, while near depletion, the supercurrent is carried by single‐particle levels of a strongly coupled quantum dot operating in the few‐hole regime. These results establish Ge–Si nanowires as an important platform for hybrid superconductor–semiconductor physics and Majorana fermions.}, author = {Ridderbos, Joost and Brauns, Matthias and Shen, Jie and de Vries, Folkert K. and Li, Ang and Bakkers, Erik P. A. M. and Brinkman, Alexander and Zwanenburg, Floris A.}, issn = {0935-9648}, journal = {Advanced Materials}, number = {44}, publisher = {Wiley}, title = {{Josephson effect in a few-hole quantum dot}}, doi = {10.1002/adma.201802257}, volume = {30}, year = {2018}, } @article{840, abstract = {Heavy holes confined in quantum dots are predicted to be promising candidates for the realization of spin qubits with long coherence times. Here we focus on such heavy-hole states confined in germanium hut wires. By tuning the growth density of the latter we can realize a T-like structure between two neighboring wires. Such a structure allows the realization of a charge sensor, which is electrostatically and tunnel coupled to a quantum dot, with charge-transfer signals as high as 0.3 e. By integrating the T-like structure into a radiofrequency reflectometry setup, single-shot measurements allowing the extraction of hole tunneling times are performed. The extracted tunneling times of less than 10 μs are attributed to the small effective mass of Ge heavy-hole states and pave the way toward projective spin readout measurements.}, author = {Vukusic, Lada and Kukucka, Josip and Watzinger, Hannes and Katsaros, Georgios}, issn = {15306984}, journal = {Nano Letters}, number = {9}, pages = {5706 -- 5710}, publisher = {American Chemical Society}, title = {{Fast hole tunneling times in germanium hut wires probed by single-shot reflectometry}}, doi = {10.1021/acs.nanolett.7b02627}, volume = {17}, year = {2017}, } @article{1328, abstract = {Hole spins have gained considerable interest in the past few years due to their potential for fast electrically controlled qubits. Here, we study holes confined in Ge hut wires, a so-far unexplored type of nanostructure. Low-temperature magnetotransport measurements reveal a large anisotropy between the in-plane and out-of-plane g-factors of up to 18. Numerical simulations verify that this large anisotropy originates from a confined wave function of heavy-hole character. A light-hole admixture of less than 1% is estimated for the states of lowest energy, leading to a surprisingly large reduction of the out-of-plane g-factors compared with those for pure heavy holes. Given this tiny light-hole contribution, the spin lifetimes are expected to be very long, even in isotopically nonpurified samples.}, author = {Watzinger, Hannes and Kloeffel, Christoph and Vukusic, Lada and Rossell, Marta and Sessi, Violetta and Kukucka, Josip and Kirchschlager, Raimund and Lausecker, Elisabeth and Truhlar, Alisha and Glaser, Martin and Rastelli, Armando and Fuhrer, Andreas and Loss, Daniel and Katsaros, Georgios}, journal = {Nano Letters}, number = {11}, pages = {6879 -- 6885}, publisher = {American Chemical Society}, title = {{Heavy-hole states in germanium hut wires}}, doi = {10.1021/acs.nanolett.6b02715}, volume = {16}, year = {2016}, }