@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}, } @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}, } @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}, } @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}, } @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}, } @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}, } @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}, } @article{8203, abstract = {Using inelastic cotunneling spectroscopy we observe a zero field splitting within the spin triplet manifold of Ge hut wire quantum dots. The states with spin ±1 in the confinement direction are energetically favored by up to 55 μeV compared to the spin 0 triplet state because of the strong spin–orbit coupling. The reported effect should be observable in a broad class of strongly confined hole quantum-dot systems and might need to be considered when operating hole spin qubits.}, author = {Katsaros, Georgios and Kukucka, Josip and Vukušić, Lada and Watzinger, Hannes and Gao, Fei and Wang, Ting and Zhang, Jian-Jun and Held, Karsten}, issn = {1530-6992}, journal = {Nano Letters}, number = {7}, pages = {5201--5206}, publisher = {American Chemical Society}, title = {{Zero field splitting of heavy-hole states in quantum dots}}, doi = {10.1021/acs.nanolett.0c01466}, volume = {20}, 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}, }