@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},
}

@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},
}

@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},
}

@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},
}

@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},
}

@phdthesis{7996,
  abstract     = {Quantum computation enables the execution of algorithms that have exponential complexity. This might open the path towards the synthesis of new materials or medical drugs, optimization of transport or financial strategies etc., intractable on even the fastest classical computers. A quantum computer consists of interconnected two level quantum systems, called qubits, that satisfy DiVincezo’s criteria. Worldwide, there are ongoing efforts to find the qubit architecture which will unite quantum error correction compatible single and two qubit fidelities, long distance qubit to qubit coupling and 
 calability. Superconducting qubits have gone the furthest in this race, demonstrating an algorithm running on 53 coupled qubits, but still the fidelities are not even close to those required for realizing a single logical qubit.  emiconductor qubits offer extremely good characteristics, but they are currently investigated across different platforms. Uniting those good characteristics into a single platform might be a big step towards the quantum computer realization.
Here we describe the implementation of a hole spin qubit hosted in a Ge hut wire double quantum dot. The high and tunable spin-orbit coupling together with a heavy hole state character is expected to allow fast spin manipulation and long coherence times. Furthermore large lever arms, for hut wire devices, should allow good coupling to superconducting resonators enabling efficient long distance spin to spin coupling and a sensitive gate reflectometry spin readout. The developed cryogenic setup (printed circuit board sample holders, filtering, high-frequency wiring) enabled us to perform low temperature spin dynamics experiments. Indeed, we measured the fastest single spin qubit Rabi frequencies reported so far, reaching 140 MHz, while the dephasing times of 130 ns oppose the long decoherence predictions. In order to further investigate this, a double quantum dot gate was connected directly to a lumped element
resonator which enabled gate reflectometry readout. The vanishing inter-dot transition signal, for increasing external magnetic field, revealed the spin nature of the measured quantity.},
  author       = {Kukucka, Josip},
  issn         = {2663-337X},
  pages        = {178},
  publisher    = {Institute of Science and Technology Austria},
  title        = {{Implementation of a hole spin qubit in Ge hut wires and dispersive spin sensing}},
  doi          = {10.15479/AT:ISTA:7996},
  year         = {2020},
}

@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},
}

@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},
}