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Monday, 07 June 2021

Holes in germanium as promising spin qubits for quantum computing

An article just published in “Nature Materials” discusses the implementation of a qubit system based on the interacting spins of holes, confined in a silicon-germanium quantum device. These qubits can be operated with small magnetic fields. Members of the ICN2 Advanced Electron Nanoscopy Group participated in the characterization of the fabricated structures. 

Quantum computers are expected to provide the future paradigm shift in the realm of information processors. Based on a different principle compared with the current devices –i.e. the use of quantum properties of matter–, they promise tremendous processing capabilities and speeds, as well as the ability to solve computational problems currently not manageable. Much research in this field is centred in developing quantum processors based on hole spin qubits, where the qubit is the fundamental information unit that, as for the “classical” bit, can assume only the values “0” and “1”.

The qubit can be created by using the spin of so-called holes. In particular, outstanding candidates to act as spin qubits are holes in some materials, as Germanium (Ge), belonging to the IV group of the chemical table, the same group of the golden element of electronics, i.e. Silicon (Si). Holes are positive charge carriers, since they represent the absence of an electron; even though they are not real particles, they share with electrons many properties, including spin.

As explained in a paper recently published in Nature Materials, a team of researchers led by Prof. Georgios Katsaros and Daniel Jirovec, from the Institute of Science and Technology of Austria, has successfully implemented spin qubits in a nanoscale semiconductor substrate by controlling the interacting spins of a pair of germanium holes. Contributed to this research ICREA Prof. Jordi Arbiol, leader of the ICN2 Advanced Electron Nanoscopy Group, and doctoral student Marc Botifoll, who analysed and characterized the quantum device, using cutting-edge nanoscopy techniques –such as atomic resolution Scanning Transmission-Electronic-Microscopy (STEM) and Electron Energy Loss Spectroscopy (EELS).

The authors of this study built a nanostructure made of various layers of silicon and germanium (mixed with different proportions), which allowed them to confine holes in a region so thin to be considered just bi-dimensional. The movement of the holes along such layer could be controlled by applying voltage to electrical wires (called “gates” as in transistors) connected to the structure. The researchers used this technique to push two holes close to each other, so that their spins interacted. By doing so, they were able to create a spin qubit. Even more importantly, they managed to make this two-hole based system work with a very small magnetic field, of less than 10 mT, by taking advantage of an intrinsic property of hole states in germanium.

These hole spin qubits are very promising for efficient quantum computing since they can be manipulated at record speed, allowing quantum processors to complete millions of operations per second, and are stable over long time intervals, up to 150 microseconds. The study here discussed proved that Ge hole qubits based on this system can compete with state-of-the-art qubits technology using silicon and Gallium Arsenide (GaAs). In addition, the possibility to operate them at much lower magnetic field allow their integration with superconducting technologies.

 

Reference article:

Daniel Jirovec, Andrea Hofmann, Andrea Ballabio, Philipp M. Mutter, Giulio Tavani, Marc Botifoll, Alessandro Crippa, Josip Kukucka, Oliver Sagi, Frederico Martins, Jaime Saez-Mollejo, Ivan Prieto, Maksim Borovkov, Jordi Arbiol, Daniel Chrastina, Giovanni Isella & Georgios Katsaros, A singlet-triplet hole spin qubit in planar Ge. Nat. Mater. (2021). DOI: 10.1038/s41563-021-01022-2