@article{8911,
  abstract     = {In the worldwide endeavor for disruptive quantum technologies, germanium is emerging as a versatile material to realize devices capable of encoding, processing, or transmitting quantum information. These devices leverage special properties of the germanium valence-band states, commonly known as holes, such as their inherently strong spin-orbit coupling and the ability to host superconducting pairing correlations. In this Review, we initially introduce the physics of holes in low-dimensional germanium structures with key insights from a theoretical perspective. We then examine the material science progress underpinning germanium-based planar heterostructures and nanowires. We review the most significant experimental results demonstrating key building blocks for quantum technology, such as an electrically driven universal quantum gate set with spin qubits in quantum dots and superconductor-semiconductor devices for hybrid quantum systems. We conclude by identifying the most promising prospects
toward scalable quantum information processing. },
  author       = {Scappucci, Giordano and Kloeffel, Christoph and Zwanenburg, Floris A. and Loss, Daniel and Myronov, Maksym and Zhang, Jian-Jun and Franceschi, Silvano De and Katsaros, Georgios and Veldhorst, Menno},
  issn         = {2058-8437},
  journal      = {Nature Reviews Materials},
  pages        = {926–943 },
  publisher    = {Springer Nature},
  title        = {{The germanium quantum information route}},
  doi          = {10.1038/s41578-020-00262-z},
  volume       = {6},
  year         = {2021},
}

@inproceedings{9464,
  abstract     = {We firstly introduce the self-assembled growth of highly uniform Ge quantum wires with controllable position, distance and length on patterned Si (001) substrates. We then present the electrically tunable strong spin-orbit coupling, the first Ge hole spin qubit and ultrafast operation of hole spin qubit in the Ge/Si quantum wires.},
  author       = {Gao, Fei and Zhang, Jie Yin and Wang, Jian Huan and Ming, Ming and Wang, Tina and Zhang, Jian Jun and Watzinger, Hannes and Kukucka, Josip and Vukušić, Lada and Katsaros, Georgios and Wang, Ke and Xu, Gang and Li, Hai Ou and Guo, Guo Ping},
  booktitle    = {2021 5th IEEE Electron Devices Technology and Manufacturing Conference, EDTM 2021},
  isbn         = {9781728181769},
  location     = {Virtual, Online},
  publisher    = {IEEE},
  title        = {{Ge/Si quantum wires for quantum computing}},
  doi          = {10.1109/EDTM50988.2021.9420817},
  year         = {2021},
}

@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},
}

@article{23,
  abstract     = {The strong atomistic spin–orbit coupling of holes makes single-shot spin readout measurements difficult because it reduces the spin lifetimes. By integrating the charge sensor into a high bandwidth radio frequency reflectometry setup, we were able to demonstrate single-shot readout of a germanium quantum dot hole spin and measure the spin lifetime. Hole spin relaxation times of about 90 μs at 500 mT are reported, with a total readout visibility of about 70%. By analyzing separately the spin-to-charge conversion and charge readout fidelities, we have obtained insight into the processes limiting the visibilities of hole spins. The analyses suggest that high hole visibilities are feasible at realistic experimental conditions, underlying the potential of hole spins for the realization of viable qubit devices.},
  author       = {Vukušić, Lada and Kukucka, Josip and Watzinger, Hannes and Milem, Joshua M and Schäffler, Friedrich and Katsaros, Georgios},
  issn         = {15306984},
  journal      = {Nano Letters},
  number       = {11},
  pages        = {7141 -- 7145},
  publisher    = {American Chemical Society},
  title        = {{Single-shot readout of hole spins in Ge}},
  doi          = {10.1021/acs.nanolett.8b03217},
  volume       = {18},
  year         = {2018},
}

@article{77,
  abstract     = {Holes confined in quantum dots have gained considerable interest in the past few years due to their potential as spin qubits. Here we demonstrate two-axis control of a spin 3/2 qubit in natural Ge. The qubit is formed in a hut wire double quantum dot device. The Pauli spin blockade principle allowed us to demonstrate electric dipole spin resonance by applying a radio frequency electric field to one of the electrodes defining the double quantum dot. Coherent hole spin oscillations with Rabi frequencies reaching 140 MHz are demonstrated and dephasing times of 130 ns are measured. The reported results emphasize the potential of Ge as a platform for fast and electrically tunable hole spin qubit devices.},
  author       = {Watzinger, Hannes and Kukucka, Josip and Vukusic, Lada and Gao, Fei and Wang, Ting and Schäffler, Friedrich and Zhang, Jian and Katsaros, Georgios},
  journal      = {Nature Communications},
  number       = {3902 },
  publisher    = {Nature Publishing Group},
  title        = {{A germanium hole spin qubit}},
  doi          = {10.1038/s41467-018-06418-4},
  volume       = {9},
  year         = {2018},
}

@article{840,
  abstract     = {Heavy holes confined in quantum dots are predicted to be promising candidates for the realization of spin qubits with long coherence times. Here we focus on such heavy-hole states confined in germanium hut wires. By tuning the growth density of the latter we can realize a T-like structure between two neighboring wires. Such a structure allows the realization of a charge sensor, which is electrostatically and tunnel coupled to a quantum dot, with charge-transfer signals as high as 0.3 e. By integrating the T-like structure into a radiofrequency reflectometry setup, single-shot measurements allowing the extraction of hole tunneling times are performed. The extracted tunneling times of less than 10 μs are attributed to the small effective mass of Ge heavy-hole states and pave the way toward projective spin readout measurements.},
  author       = {Vukusic, Lada and Kukucka, Josip and Watzinger, Hannes and Katsaros, Georgios},
  issn         = {15306984},
  journal      = {Nano Letters},
  number       = {9},
  pages        = {5706 -- 5710},
  publisher    = {American Chemical Society},
  title        = {{Fast hole tunneling times in germanium hut wires probed by single-shot reflectometry}},
  doi          = {10.1021/acs.nanolett.7b02627},
  volume       = {17},
  year         = {2017},
}

@article{1328,
  abstract     = {Hole spins have gained considerable interest in the past few years due to their potential for fast electrically controlled qubits. Here, we study holes confined in Ge hut wires, a so-far unexplored type of nanostructure. Low-temperature magnetotransport measurements reveal a large anisotropy between the in-plane and out-of-plane g-factors of up to 18. Numerical simulations verify that this large anisotropy originates from a confined wave function of heavy-hole character. A light-hole admixture of less than 1% is estimated for the states of lowest energy, leading to a surprisingly large reduction of the out-of-plane g-factors compared with those for pure heavy holes. Given this tiny light-hole contribution, the spin lifetimes are expected to be very long, even in isotopically nonpurified samples.},
  author       = {Watzinger, Hannes and Kloeffel, Christoph and Vukusic, Lada and Rossell, Marta and Sessi, Violetta and Kukucka, Josip and Kirchschlager, Raimund and Lausecker, Elisabeth and Truhlar, Alisha and Glaser, Martin and Rastelli, Armando and Fuhrer, Andreas and Loss, Daniel and Katsaros, Georgios},
  journal      = {Nano Letters},
  number       = {11},
  pages        = {6879 -- 6885},
  publisher    = {American Chemical Society},
  title        = {{Heavy-hole states in germanium hut wires}},
  doi          = {10.1021/acs.nanolett.6b02715},
  volume       = {16},
  year         = {2016},
}