@article{22323, abstract = {Arrays of Josephson junctions can be tuned through anomalous metallic, quantum-critical, and insulating regimes. We introduce an alternative experimental probe, capturing microwave radiation across all three regimes, using a two-dimensional array of superconductor-semiconductor hybrid Josephson junctions as a model system. Our approach allows calibration of the sample’s circuit parameters and provides isolation from measurement back-action effects. We measure the radiation temperature of the anomalous metal and find that it is hotter than both the quantum-critical and insulating regimes. We further show that the anomalous metallic regime is more susceptible to additional heating than other regimes, explaining its emergence in otherwise thermalized systems. Turning to the quantum-critical regime, we discover nonlinear scaling of radiative noise with applied bias, consistent with theoretical predictions of universal nonequilibrium behavior at quantum-critical points.}, author = {Galvin, Kristen W and Bubis, Anton and Mikalsen, Melissa and Schiela, William F. and Elfeky, Bassel H. and Strickland, William M. and Phan, Duc T and Shabani, Javad and Higginbotham, Andrew P}, issn = {2331-7019}, journal = {Physical Review Applied}, publisher = {American Physical Society}, title = {{Microwave radiometry of a quantum-critical hybrid Josephson array}}, doi = {10.1103/75bl-mm3b}, volume = {26}, year = {2026}, } @article{22619, abstract = {Planar germanium is currently the only semiconducting platform where high-coherence spin qubits and proximity-induced superconductivity have each been demonstrated. Recent research into spin qubits in Ge/SiGe heterostructures has focused on increasing the thickness of the SiGe capping layer, reporting improvements in the electrostatic noise levels. Meanwhile, heterostructures with thinner capping layers remain rather unexplored, despite the potential advantages for proximity-induced superconductivity. Here, we study a Ge/SiGe heterostructure with a thin SiGe cap d - 4nm and investigate its viability to host low-noise quantum dots. To keep the thermal budget compatible with superconducting layers, low-temperature oxide deposition processes were developed and implemented for the gate dielectrics. The charge noise level of the fabricated devices is estimated to be 1.8  +- 1.0 μeV/ square HZ⁠, comparable to devices fabricated on shallow heterostructures (⁠ d - 20nm⁠) with high-temperature deposited oxides. Low charge noise levels, together with the straightforward integration of superconductors, make this heterostructure an attractive platform for prototyping hybrid semiconducting–superconducting devices.}, author = {Borovkov, Maksim and Schell, Yona A and Sokolova, Dina and Roux, Kevin Etienne Robert and Falthansl-Scheinecker, Paul and Fabris, Giorgio and Shah, Devashish C and Saez Mollejo, Jaime and Previdi, Rodolfo and Taha, Inas and Genç, Aziz and Arbiol, Jordi and Calcaterra, Stefano and Oliveira, Afonso De Cerdeira and Chrastina, Daniel and Isella, Giovanni and Bubis, Anton and Katsaros, Georgios}, issn = {1077-3118}, journal = {Applied Physics Letters}, number = {3}, publisher = {AIP Publishing}, title = {{Low-noise quantum dots in ultra-shallow Ge/SiGe heterostructures for prototyping hybrid semiconducting–superconducting devices}}, doi = {10.1063/5.0333142}, volume = {129}, year = {2026}, } @misc{22242, abstract = {This deposit contains the data and analysis code accompanying the publication "Low-Noise Quantum Dots in Ultra-Shallow Ge/SiGe Heterostructures for Prototyping Hybrid Semiconducting–Superconducting Devices" (Borovkov et al.). The deposit includes the raw transport and current-noise measurements of three gate-defined quantum-dot devices as QCodes SQLite databases, the master table of the charge-noise (flank-method) analysis with the pointers linking every analyzed PSD trace to the raw data, the toy-model noise simulation datasets behind the supplementary figures, the archived analysis figures (PSD fits and lever-arm extractions), and the Python code reproducing the full analysis and all figures. The code is also maintained at https://github.com/ISTA-Nanoelectronics/noise_paper_public; instructions are provided in the README files.}, author = {Borovkov, Maksim}, publisher = {Institute of Science and Technology Austria}, title = {{Low-Noise Quantum Dots in Ultra-Shallow Ge/SiGe Heterostructures for Prototyping Hybrid Semiconducting-Superconducting Devices}}, doi = {10.15479/AT-ISTA-22242}, year = {2026}, } @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{19597, abstract = {Superconductor–semiconductor hybrid systems play a crucial role in realizing nanoscale quantum devices, including hybrid qubits, Majorana bound states, and Kitaev chains. For such hybrid devices, subgap states play a prominent role in their operation. In this paper, we study these subgap states via Coulomb and tunneling spectroscopy through a superconducting island defined in a semiconductor nanowire fully coated by a superconductor. We systematically explore regimes ranging from an almost decoupled island to the open configuration. In the weak-coupling regime, the experimental observations are very similar in the absence of a magnetic field and when one flux quantum pierces the superconducting shell. Conversely, in the strong-coupling regime, significant distinctions emerge between the two cases. We attribute this distinct behavior to the existence of subgap states at one flux quantum, which become observable only for sufficiently strong coupling to the leads. We support our interpretation using a simple model to describe transport through the island. Our study highlights the importance of studying a broad range of tunnel couplings for understanding the rich physics of hybrid devices.}, author = {Valentini, Marco and Souto, Rubén Seoane and Borovkov, Maksim and Krogstrup, Peter and Meir, Yigal and Leijnse, Martin and Danon, Jeroen and Katsaros, Georgios}, issn = {2643-1564}, journal = {Physical Review Research}, number = {2}, publisher = {American Physical Society}, title = {{Subgap transport in superconductor-semiconductor hybrid islands: Weak and strong coupling regimes}}, doi = {10.1103/PhysRevResearch.7.023022}, volume = {7}, year = {2025}, } @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{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{14793, abstract = {Superconductor/semiconductor hybrid devices have attracted increasing interest in the past years. Superconducting electronics aims to complement semiconductor technology, while hybrid architectures are at the forefront of new ideas such as topological superconductivity and protected qubits. In this work, we engineer the induced superconductivity in two-dimensional germanium hole gas by varying the distance between the quantum well and the aluminum. We demonstrate a hard superconducting gap and realize an electrically and flux tunable superconducting diode using a superconducting quantum interference device (SQUID). This allows to tune the current phase relation (CPR), to a regime where single Cooper pair tunneling is suppressed, creating a sin(2y) CPR. Shapiro experiments complement this interpretation and the microwave drive allows to create a diode with ≈ 100% efficiency. The reported results open up the path towards integration of spin qubit devices, microwave resonators and (protected) superconducting qubits on the same silicon technology compatible platform.}, author = {Valentini, Marco and Sagi, Oliver and Baghumyan, Levon and de Gijsel, Thijs and Jung, Jason and Calcaterra, Stefano and Ballabio, Andrea and Aguilera Servin, Juan L and Aggarwal, Kushagra and Janik, Marian and Adletzberger, Thomas and Seoane Souto, Rubén and Leijnse, Martin and Danon, Jeroen and Schrade, Constantin and Bakkers, Erik and Chrastina, Daniel and Isella, Giovanni and Katsaros, Georgios}, issn = {2041-1723}, journal = {Nature Communications}, publisher = {Springer Nature}, title = {{Parity-conserving Cooper-pair transport and ideal superconducting diode in planar germanium}}, doi = {10.1038/s41467-023-44114-0}, volume = {15}, year = {2024}, } @phdthesis{13286, abstract = {Semiconductor-superconductor hybrid systems are the harbour of many intriguing mesoscopic phenomena. This material combination leads to spatial variations of the superconducting properties, which gives rise to Andreev bound states (ABSs). Some of these states might exhibit remarkable properties that render them highly desirable for topological quantum computing. The most prominent and hunted of such states are Majorana zero modes (MZMs), quasiparticles equals to their own quasiparticles that they follow non-abelian statistics. In this thesis, we first introduce the general framework of such hybrid systems and, then, we unveil a series of mesoscopic phenomena that we discovered. Firstly, we show tunneling spectroscopy experiments on full-shell nanowires (NWs) showing that unwanted quantum-dot states coupled to superconductors (Yu-Shiba-Rusinov states) can mimic MZMs signatures. Then, we introduce a novel protocol which allowed the integration of tunneling spectroscopy with Coulomb spectroscopy within the same device. Employing this approach on both full-shell NWs and partial-shell NWs, we demonstrated that longitudinally confined states reveal charge transport phenomenology similar to the one expected for MZMs. These findings shed light on the intricate interplay between superconductivity and quantum confinement, which brought us to explore another material platform, i.e. a two-dimensional Germanium hole gas. After developing a robust way to induce superconductivity in such system, we showed how to engineer the proximity effect and we revealed a superconducting hard gap. Finally, we created a superconducting radio frequency driven ideal diode and a generator of non-sinusoidal current-phase relations. Our results open the path for the exploration of protected superconducting qubits and more complex hybrid devices in planar Germanium, like Kitaev chains and hybrid qubit devices.}, author = {Valentini, Marco}, issn = {2663-337X}, pages = {184}, publisher = {Institute of Science and Technology Austria}, title = {{Mesoscopic phenomena in hybrid semiconductor-superconductor nanodevices : From full-shell nanowires to two-dimensional hole gas in germanium}}, doi = {10.15479/at:ista:13286}, year = {2023}, } @unpublished{13312, abstract = {Superconductor/semiconductor hybrid devices have attracted increasing interest in the past years. Superconducting electronics aims to complement semiconductor technology, while hybrid architectures are at the forefront of new ideas such as topological superconductivity and protected qubits. In this work, we engineer the induced superconductivity in two-dimensional germanium hole gas by varying the distance between the quantum well and the aluminum. We demonstrate a hard superconducting gap and realize an electrically and flux tunable superconducting diode using a superconducting quantum interference device (SQUID). This allows to tune the current phase relation (CPR), to a regime where single Cooper pair tunneling is suppressed, creating a $ \sin \left( 2 \varphi \right)$ CPR. Shapiro experiments complement this interpretation and the microwave drive allows to create a diode with $ \approx 100 \%$ efficiency. The reported results open up the path towards monolithic integration of spin qubit devices, microwave resonators and (protected) superconducting qubits on a silicon technology compatible platform.}, author = {Valentini, Marco and Sagi, Oliver and Baghumyan, Levon and Gijsel, Thijs de and Jung, Jason and Calcaterra, Stefano and Ballabio, Andrea and Servin, Juan Aguilera and Aggarwal, Kushagra and Janik, Marian and Adletzberger, Thomas and Souto, Rubén Seoane and Leijnse, Martin and Danon, Jeroen and Schrade, Constantin and Bakkers, Erik and Chrastina, Daniel and Isella, Giovanni and Katsaros, Georgios}, booktitle = {arXiv}, keywords = {Mesoscale and Nanoscale Physics}, title = {{Radio frequency driven superconducting diode and parity conserving Cooper pair transport in a two-dimensional germanium hole gas}}, doi = {10.48550/arXiv.2306.07109}, year = {2023}, }