@misc{19885, abstract = {This .zip file contains the data to reproduce the figures and supplementary figures of "Automated All-RF Tuning for Spin Qubit Readout and Control" by Cornelius Carlsson and Jaime Saez-Mollejo et al.}, author = {Saez Mollejo, Jaime}, publisher = {Institute of Science and Technology Austria}, title = {{Automated All-RF Tuning for Spin Qubit Readout and Control}}, doi = {10.15479/AT:ISTA:19885}, year = {2025}, } @article{20730, abstract = {Radio-frequency measurements could satisfy DiVincenzo’s readout criterion in future large-scale solid-state quantum processors, as they allow for high bandwidths and frequency multiplexing. However, the scalability potential of this readout technique can only be leveraged if quantum device tuning is performed using exclusively radio-frequency measurements, that is, without resorting to current measurements. We demonstrate an algorithm that performs automatic coarse tuning of double quantum dots with only radio-frequency measurements by exploiting their bandwidth and impedance matching. The tuning was completed within a few minutes with minimal prior knowledge about the device. Our results show that it is possible to eliminate the need for transport measurements for quantum-dot tuning, paving the way for more scalable device architectures.}, author = {Van Straaten, Barnaby and Fedele, Federico and Vigneau, Florian and Hickie, Joseph and Jirovec, Daniel and Ballabio, Andrea and Chrastina, Daniel and Isella, Giovanni and Katsaros, Georgios and Ares, Natalia}, issn = {2331-7019}, journal = {Physical Review Applied}, number = {5}, publisher = {American Physical Society}, title = {{All-rf-based coarse-tuning algorithm for quantum devices using machine learning}}, doi = {10.1103/v11m-dbhm}, volume = {24}, year = {2025}, } @article{19401, abstract = {High kinetic inductance superconductors are gaining increasing interest for the realisation of qubits, amplifiers and detectors. Moreover, thanks to their high impedance, quantum buses made of such materials enable large zero-point fluctuations of the voltage, boosting the coupling rates to spin and charge qubits. However, fully exploiting the potential of disordered or granular superconductors is challenging, as their inductance and, therefore, impedance at high values are difficult to control. Here, we report a reproducible fabrication of granular aluminium resonators by developing a wireless ohmmeter, which allows in situ measurements during film deposition and, therefore, control of the kinetic inductance of granular aluminium films. Reproducible fabrication of circuits with impedances (inductances) exceeding 13 kΩ (1 nH per square) is now possible. By integrating a 7.9 kΩ resonator with a germanium double quantum dot, we demonstrate strong charge-photon coupling with a rate of gc/2π = 566 ± 2 MHz. This broadly applicable method opens the path for novel qubits and high-fidelity, long-distance two-qubit gates.}, author = {Janik, Marian and Roux, Kevin Etienne Robert and Borja Espinosa, Carla N and Sagi, Oliver and Baghdadi, Abdulhamid and Adletzberger, Thomas and Calcaterra, Stefano and Botifoll, Marc and Garzón Manjón, Alba and Arbiol, Jordi and Chrastina, Daniel and Isella, Giovanni and Pop, Ioan M. and Katsaros, Georgios}, issn = {2041-1723}, journal = {Nature Communications}, publisher = {Springer Nature}, title = {{Strong charge-photon coupling in planar germanium enabled by granular aluminium superinductors}}, doi = {10.1038/s41467-025-57252-4}, volume = {16}, year = {2025}, } @misc{18886, abstract = {Research Data for publication 'Strong charge-photon coupling in planar germanium enabled by granular aluminium superinductors'}, author = {Janik, Marian}, publisher = {Institute of Science and Technology Austria}, title = {{Research data for publication 'Strong charge-photon coupling in planar germanium enabled by granular aluminium superinductors'}}, doi = {10.15479/AT:ISTA:18886}, year = {2025}, } @misc{19409, abstract = {This .zip file contains the data to reproduce the figures and supplementary figures of "Exchange anisotropies in microwave-driven singlet-triplet qubits" by Jaime Saez-Mollejo et al. }, author = {Saez Mollejo, Jaime}, publisher = {Institute of Science and Technology Austria}, title = {{Exchange anisotropies in microwave-driven singlet-triplet qubits}}, doi = {10.15479/AT:ISTA:19409}, year = {2025}, } @article{19424, abstract = {Hole spin qubits are rapidly emerging as the workhorse of semiconducting quantum processors because of their large spin-orbit interaction, enabling fast all-electric operations at low power. However, spin-orbit interaction also causes non-uniformities in devices, resulting in locally varying qubit energies and site-dependent anisotropies. While these anisotropies can be used to drive single-spins, if not properly harnessed, they can hinder the path toward large-scale quantum processors. Here, we report on microwave-driven singlet-triplet qubits in planar germanium and use them to investigate the anisotropy of two spins in a double quantum dot. We show two distinct operating regimes depending on the magnetic field direction. For in-plane fields, the two spins are largely anisotropic, and electrically tunable, which enables to measure all the available transitions; coherence times exceeding 3 $\mu$s are extracted. For out-of-plane fields, they have an isotropic response but preserve the substantial energy difference required to address the singlet-triplet qubit. Even in this field direction, where the qubit lifetime is strongly affected by nuclear spins, we find 400 ns coherence times. Our work adds a valuable tool to investigate and harness the anisotropy of spin qubits and can be implemented in any large-scale NxN device, facilitating the path towards scalable quantum processors.}, author = {Saez Mollejo, Jaime and Jirovec, Daniel and Schell, Yona A and Kukucka, Josip and Calcaterra, Stefano and Chrastina, Daniel and Isella, Giovanni and Rimbach-Russ, Maximilian and Bosco, Stefano and Katsaros, Georgios}, issn = {2041-1723}, journal = {Nature Communications}, publisher = {Springer Nature}, title = {{Exchange anisotropies in microwave-driven singlet-triplet qubits}}, doi = {10.1038/s41467-025-58969-y}, volume = {16}, year = {2025}, } @phdthesis{19836, abstract = {Over the past century, researchers have been fascinated by the quantum nature of the physical world, initially striving to understand its fundamental principles and consequences, and eventually progressing toward engineering systems that can control and manipulate quantum properties. Today, we stand at the dawn of the quantum technology era. While some quantum technologies follow well-defined roadmaps, others are still in the exciting and uncertain early stages of development. In the fields of quantum computing and quantum simulation, research is being conducted across a wide variety of platforms. Each of these demonstrates control over quantum properties but also faces challenges in scaling up to the level of a mature technology. This thesis explores some of the fundamental properties of hole spin qubits in planar germanium. Semiconductor spin qubits are considered strong candidates for the realization of quantum processors, owing to their long relaxation and coherence times, as well as their compatibility with existing semiconductor industry infrastructure. Among these, hole spin qubits in planar germanium are particularly promising. Their advantages include a large effective mass, which eases fabrication constraints; inherent protection from hyperfine noise; and strong spin-orbit interaction, which enables fast and purely electrical control. However, spin-orbit coupling also introduces site-dependent variability across qubits, particularly in the g-tensors and spin-flip tunneling, which might cause that the quantization axes are not aligned. In this thesis, we investigate the tilt between the quantization axes of two hole spins hosted in a double quantum dot as a function of both the magnetic field direction and various electrostatic configurations, demonstrating that both parameters influence this tilt. We conclude by introducing a machine-learning-assisted routine to automatically tune baseband spin qubits. This approach may prove to be a powerful tool for characterizing spin-orbit effects and gaining deeper insight into the physics governing spin qubit behavior. }, author = {Saez Mollejo, Jaime}, issn = {2663-337X}, pages = {175}, publisher = {Institute of Science and Technology Austria}, title = {{Singlet-triplet qubits in planar Germanium : From exchange anisotropies to autonomous tuning }}, doi = {10.15479/AT-ISTA-19836}, year = {2025}, } @article{18602, abstract = {Semiconductor quantum dots (QDs) in planar germanium (Ge) heterostructures have emerged as front-runners for future hole-based quantum processors. Here, we present strong coupling between a hole charge qubit, defined in a double quantum dot (DQD) in planar Ge, and microwave photons in a high-impedance (Zr = 1.3 kΩ) resonator based on an array of superconducting quantum interference devices (SQUIDs). Our investigation reveals vacuum-Rabi splittings with coupling strengths up to g0/2π = 260 MHz, and a cooperativity of C ~ 100, dependent on DQD tuning. Furthermore, utilizing the frequency tunability of our resonator, we explore the quenched energy splitting associated with strong Coulomb correlation effects in Ge QDs. The observed enhanced coherence of the strongly correlated excited state signals the presence of distinct symmetries within related spin functions, serving as a precursor to the strong coupling between photons and spin-charge hybrid qubits in planar Ge. This work paves the way towards coherent quantum connections between remote hole qubits in planar Ge, required to scale up hole-based quantum processors.}, author = {De Palma, Franco and Oppliger, Fabian and Jang, Wonjin and Bosco, Stefano and Janik, Marian and Calcaterra, Stefano and Katsaros, Georgios and Isella, Giovanni and Loss, Daniel and Scarlino, Pasquale}, issn = {2041-1723}, journal = {Nature Communications}, publisher = {Springer Nature}, title = {{Strong hole-photon coupling in planar Ge for probing charge degree and strongly correlated states}}, doi = {10.1038/s41467-024-54520-7}, volume = {15}, year = {2024}, } @article{18653, abstract = {Charge sensing is a sensitive technique for probing quantum devices, of particular importance for spin-qubit readout. To achieve good readout sensitivities, the proximity of the charge sensor to the device to be measured is a necessity. However, this proximity also means that the operation of the device affects, in turn, the sensor tuning and ultimately the readout sensitivity. We present an approach for compensating for this crosstalk effect allowing for the gate voltages of the measured device to be swept in a 1-V × 1-V window while maintaining a sensor configuration chosen by a Bayesian optimizer. Our algorithm will hopefully be a major contribution to the suite of fully automated solutions required for the operation of large quantum device architectures.}, author = {Hickie, Joseph and Van Straaten, Barnaby and Fedele, Federico and Jirovec, Daniel and Ballabio, Andrea and Chrastina, Daniel and Isella, Giovanni and Katsaros, Georgios and Ares, Natalia}, issn = {2331-7019}, journal = {Physical Review Applied}, number = {6}, publisher = {American Physical Society}, title = {{Automated long-range compensation of an rf quantum dot sensor}}, doi = {10.1103/PhysRevApplied.22.064026}, volume = {22}, year = {2024}, } @article{17389, 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 and each device realisation requires a different tuning protocol. We demonstrate that it is possible to automate the tuning of a 4-gate Si FinFET, a 5-gate GeSi nanowire and a 7-gate Ge/SiGe heterostructure double quantum dot device from scratch with the same algorithm. We achieve tuning times of 30, 10, and 92 min, respectively. The algorithm also provides insight into the parameter space landscape for each of these devices, allowing for the characterization of the regions where double quantum dot regimes are found. 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 de Kruijf, M. and Carballido, M. J. and Svab, S. and Kuhlmann, A. V. 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.}, issn = {2045-2322}, journal = {Scientific Reports}, publisher = {Springer Nature}, title = {{Cross-architecture tuning of silicon and SiGe-based quantum devices using machine learning}}, doi = {10.1038/s41598-024-67787-z}, volume = {14}, year = {2024}, } @article{17202, abstract = {Gate-tunable transmons (gatemons) employing semiconductor Josephson junctions have recently emerged as building blocks for hybrid quantum circuits. In this study, we present a gatemon fabricated in planar Germanium. We induce superconductivity in a two-dimensional hole gas by evaporating aluminum atop a thin spacer, which separates the superconductor from the Ge quantum well. The Josephson junction is then integrated into an Xmon circuit and capacitively coupled to a transmission line resonator. We showcase the qubit tunability in a broad frequency range with resonator and two-tone spectroscopy. Time-domain characterizations reveal energy relaxation and coherence times up to 75 ns. Our results, combined with the recent advances in the spin qubit field, pave the way towards novel hybrid and protected qubits in a group IV, CMOS-compatible material.}, author = {Sagi, Oliver and Crippa, Alessandro and Valentini, Marco and Janik, Marian and Baghumyan, Levon and Fabris, Giorgio and Kapoor, Lucky and Hassani, Farid and Fink, Johannes M and Calcaterra, Stefano and Chrastina, Daniel and Isella, Giovanni and Katsaros, Georgios}, issn = {2041-1723}, journal = {Nature Communications}, publisher = {Springer Nature}, title = {{A gate tunable transmon qubit in planar Ge}}, doi = {10.1038/s41467-024-50763-6}, volume = {15}, year = {2024}, } @phdthesis{18076, abstract = {The new era of Ge has opened up new possibilities in quantum computing. The maturity of Ge spin qubits is unquestioned, while hybrid semiconductor-superconductor Ge circuits are on track to enter the game. Gate-tunable transmons (gatemons) employing semiconductor Josephson junctions have recently emerged as building blocks for such hybrid quantum circuits. In this thesis, we present a gatemon fabricated in planar Germanium. We induce superconductivity in a two-dimensional hole gas by evaporating aluminum atop a thin spacer, which separates the superconductor from the Ge quantum well. The Josephson junction is then integrated into an Xmon circuit and capacitively coupled to a transmission line resonator. We showcase the qubit tunability in a broad frequency range with resonator and two-tone spectroscopy. Time-domain characterizations reveal energy relaxation and coherence times up to 75 ns. Our results, combined with the recent advances in the spin qubit field, pave the way towards novel hybrid and protected qubits in a group IV, CMOS-compatible material.}, author = {Sagi, Oliver}, issn = {2663-337X}, pages = {111}, publisher = {Institute of Science and Technology Austria}, title = {{Hybrid circuits on planar Germanium}}, doi = {10.15479/at:ista:18076}, year = {2024}, } @misc{17196, abstract = {This .zip File contains the data for the figures presented in the main text and supplementary material of "A gate tunable transmon qubit in planar Ge" by O.Sagi et al. The measurements were done using Qcodes. The description of the files and the instructions on opening the data can be found in the Readme. An additional Jupyter Notebook is attached that walks through the data analysis.}, author = {Sagi, Oliver}, publisher = {Institute of Science and Technology Austria}, title = {{A gate-tunable transmon in planar Ge}}, doi = {10.15479/AT:ISTA:17196}, year = {2024}, } @phdthesis{18129, abstract = {State-of-the-art quantum computers, with roughly a thousand qubits, face a crucial technological challenge of scaling up. Spins confined in quantum dots (QDs) are a promising candidate for qubits due to their long coherence, tunability, control, and readout. However, their natural coupling is the short-ranged (∼ 100 nm) exchange interaction, limited to nearest neighbours. Long-ranged (∼ 1 mm) qubit interactions mediated by a photon could be engineered through a coherent spin-photon coupling. Achieving a strong coupling to a photon is inherently challenging in QDs due to the small dipole moment of the confined charge. However, the potential of high-impedance resonators to compensate for this has gained significant attention in the past decade. Nevertheless, previous QD circuit quantum electrodynamics implementations have not exceeded the impedance of ∼ 3.8 kΩ, leaving opportunities for significant improvement. The large kinetic inductance of granular aluminium (grAl) could provide an order-of-magnitude enhancement. However, fully exploiting the potential of disordered or granular superconductors is challenging as their impedances close to the superconductor-to-insulator transition are difficult to control reproducibly. We report on the realization of a wireless ohmmeter which allows in situ resistance measurements during film deposition and, therefore, indirect control of the kinetic inductance of grAl films. This allows us to reproducibly fabricate resonators with characteristic impedance exceeding the resistance quantum, even reaching 22.3 kW, due to the large sheet kinetic inductance of up to 3 nH □−1 . By integrating an 8 kW resonator with a germanium double QD, we demonstrate a strong charge-photon coupling with the highest rate reported, 566 MHz. The demonstrated method and grAl properties make these resonators suitable for boosting the spin-photon coupling strength, a crucial requirement for fast, high-fidelity, long-distance two-qubit gates. }, author = {Janik, Marian}, issn = {2663-337X}, pages = {164}, publisher = {Institute of Science and Technology Austria}, title = {{Strong charge-photon coupling in Germanium enabled by granular aluminium superinductors}}, doi = {10.15479/at:ista:18129}, year = {2024}, } @unpublished{18144, abstract = {High kinetic inductance superconductors are gaining increasing interest for the realisation of qubits, amplifiers and detectors. Moreover, thanks to their high impedance, quantum buses made of such materials enable large zero-point fluctuations of the voltage, boosting the coupling rates to spin and charge qubits. However, fully exploiting the potential of disordered or granular superconductors is challenging, as their inductance and, therefore, impedance at high values are difficult to control. Here we have integrated a granular aluminium resonator, having a characteristic impedance exceeding the resistance quantum, with a germanium double quantum dot and demonstrate strong charge-photon coupling with a rate of $g_\text{c}/2\pi= (566 \pm 2)$ MHz. This was achieved due to the realisation of a wireless ohmmeter, which allows \emph{in situ} measurements during film deposition and, therefore, control of the kinetic inductance of granular aluminium films. Reproducible fabrication of circuits with impedances (inductances) exceeding 13 k$\Omega$ (1 nH per square) is now possible. This broadly applicable method opens the path for novel qubits and high-fidelity, long-distance two-qubit gates.}, author = {Janik, Marian and Roux, Kevin Etienne Robert and Borja Espinosa, Carla N and Sagi, Oliver and Baghdadi, Abdulhamid and Adletzberger, Thomas and Calcaterra, Stefano and Botifoll, Marc and Manjón, Alba Garzón and Arbiol, Jordi and Chrastina, Daniel and Isella, Giovanni and Pop, Ioan M. and Katsaros, Georgios}, booktitle = {arXiv}, title = {{Strong charge-photon coupling in planar germanium enabled by granular aluminium superinductors}}, doi = {10.48550/arXiv.2407.03079}, year = {2024}, } @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}, }