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Engineering electronic quantum coherence
and correlations in hybrid nanostructures

Final Report Summary - HYBRIDNANO (Engineering electronic quantum coherence and correlations in hybrid nanostructures)

Electrons possess a nonzero angular momentum, the so-called electron spin, and hence a finite magnetic moment proportional to it. In a magnetic field, the electron magnetic moment gets polarized resulting in a two-level quantum system, with ground and excited states corresponding to parallel and antiparallel polarization. Such a two-level system can thus encode an elementary bit of quantum information, a so-called qubit. The ability to manipulate and couple spin qubits is the key to a variety quantum devices: from low-consumption spin-based transistors to the logic gates of a quantum computer, quantum memories, etc. HybridNano has explored some original approaches to these types of devices. It focused mainly on p-type Si-based nanostructures, exploiting confined holes (i.e. missing electrons in the valence band of the semiconductor), as well as small-band-gap III-V nanostructures, exploiting confined electrons. These materials have the property of possessing a strong spin-orbit coupling, which allows electron (or hole) spins to be more easily manipulated with all-electrical drive signals. Bottom-up nanodevices fabricated from self-assembled SiGe quantum dots confining holes were studied during the first half of the project, whereas alternative bottom-up devices, obtained from InAs-based and InSb semiconductor nanowires, as well as top-down devices, fabricated from Si-based wafers, where considered in the second half.
The main results of the project have been:
• The observation of anisotropic hole g-factors with strong electrical tunability in Si-based nanostructures (both SiGe self-assembled quantum dots and Si-based nano MOSFETs.
• The first experimental demonstration of a hole spin qubit device issued from an industry-standard CMOS platform. This proof-of-concept demonstration leveraged the properties of hole g-factors mentioned above.
• Andreev-level spectroscopy in hybrid superconductor-semiconductor nanowire quantum dots, with i) the observation of a nontrivial magnetic-field behavior of the lowest-energy sub-gap excitations (Andreev levels) of a quantum dot strongly coupled to a superconducting contact, and ii) the observation of the scaling behavior relating the energy of these excitations to the ratio between the Kondo temperature and the superconducting energy gap.
These scientific achievements bring a significant contribution to the developing field of quantum spintronics. The results on silicon-based devices open a new route for the development of a scalable quantum processor leveraging silicon technology. The results on superconductor-semiconductor nanowire systems bare relevance to the field of topological superconductivity where a clear understanding of the underlying physics and technology is required to foster progress toward topologically protected qubits based on Majorana fermions or, more generically, parafermions.