@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},
}