Hole-spin qubits in silicon and germanium quantum dots

For decades, silicon and germanium have played an essential role in semiconductor-based information technology. They are also excellent host materials for new devices in quantum computation and spintronics, due to the natural abundance (> 90%) of non-magnetic nuclei and the consequent reduction of the decoherence due to hyperfine interaction. Si and Ge devices benefit from established industrial fabrication techniques, allowing for scalability and for the achievement of homogeneous building blocks for the qubits and control electronics of a quantum computer. CMOS qubits have been implemented by modifying transistors currently fabricated in foundry processes. 

Within the project IQUBITS, our group deals with the theoretical characterization of one- and two-hole Si and Ge quantum dots, for their use as qubits. We combine the multi-band “k dot p” description of confined single-hole states with the full-configuration-interaction treatment of interactions, and with semi-analytical methods. In this talk, I will present our results on single and double quantum dots, including the effects of interband Coulomb interactions, signals of Wigner crystallization, and the mapping of double-dot states on a four-band Hubbard model. The latter allows us to describe in a transparent way the two-hole low-energy states, which display a high degree of mixing of angular-momentum eigenstates, even with different permutational symmetries, in contrast with the common case of single-band systems. We also discuss how, in the regime of weak entanglement between band and orbital degrees of freedom, it is possible to reduce the four-band Hubbard model to an effective pseudospin-1/2 model.  

The seminar is realized in the framework of the IQubits project. This project has received funding from the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement No 829005.

[1] A. Secchi, L. Bellentani, A. Bertoni, and F. Troiani, “Inter- and intraband Coulomb interactions between holes in silicon nanostructures”, Phys. Rev. B 104, 205409 (2021).

[2] A. Secchi, L. Bellentani, A. Bertoni, and F. Troiani, “Interacting holes in Si and Ge double quantum dots: From a multiband approach to an effective spin picture”, Phys. Rev. B 104, 035302 (2021).

[3] L. Bellentani et al., “Toward Hole-Spin Qubits in Si p-MOSFETs within a Planar CMOS Foundry Technology”, Phys. Rev. Applied 16, 054034 (2021).

See the SLIDES of the presentation.