Ambipolar quantum dots in planar silicon

Floris Zwanenburg
Associate Professor
University of Twente

Abstract
We create ambipolar quantum dots in planar silicon nanoscale transistors. We first investigate the conformity of Al, Ti and Pd nanoscale gates by means of transmission electron microscopy [1]. Next we define low-disorder electron quantum dots with Pd gates [2], and depletion-mode hole quantum dots in undoped silicon [3]. For the latter we use fixed charge in a SiO₂/Al₂O₃ dielectric stack to induce a 2DHG at the Si/SiO₂ interface. The depletion-mode design avoids complex multilayer architectures requiring precision alignment and allows directly adopting best practices already developed for depletion dots in other material systems. Finally, I will show ambipolar charge sensing: we have fabricated a single-electron transistor next to a single-hole transistor, and tuned both quantum dots to simultaneously sense charge transitions of the other quantum dot. Using active charge sensing the single-hole transistor can detect the few-charge regime in the electron quantum dot.[1] P. C. Spruijtenburg et al., Nanotechnology, (2018).[2] M. Brauns et al., Scientific Reports 8, 5690, (2018).[3] S. V. Amitonov et al., Applied Physics Letters 112, 023102 (2018).

Biography
Floris Zwanenburg (1976) studied applied physics at the TU Delft. In 2008 he received his PhD for research on semiconductor nanowires with Leo Kouwenhoven. As a post-doc at UNSW in Sydney he worked with silicon quantum dots. This system has a unique fabrication scheme offering unprecedented control over all relevant parameters, as he demonstrated by reaching the single-electron regime in a highly tunable Si quantum dot. With his team at UNSW he has also used this system to read out the spin of a single electron (Nature, 2010) and to create a nuclear spin qubit (Nature, 2013). In 2011, he returned to the Netherlands for a tenure track position at the University of Twente. After initial collaborative efforts with the Dzurak team from UNSW on silicon quantum-dot technology, his team has extended this design to an ambipolar circuit, with which he has defined electron and hole quantum dots in a single device. Since 2013 he has had a new project on quantum dots and superconductivity in Ge/Si core/shell nanowires. In the past ten years he has become an expert in silicon quantum electronics: the quantum mechanical behaviour of single electron or hole spins confined to (artificial) atoms in silicon.

Diamond for Quantum Computing

Oliver Ambacher
Director
Fraunhofer Institute

Abstract
Quantum computers exploit the phenomenon of quantum superposition, or the counterintuitive ability of small particles to inhabit contradictory physical states at the same time. An electron, for instance, can be said to be in more than one location simultaneously, or to have both of two opposed magnetic orientations. Where a bit in a conventional computer can represent zero or one, a qubit can represent zero, one, or both at the same time. It’s the ability of strings of qubits to simultaneously explore multiple solutions to a problem that promises computational speedups.Diamond-defect qubits result from the combination of “vacancies,” which are locations in the diamond’s crystal lattice where there should be a carbon atom but there isn’t one, and “dopants,” like nitrogen atoms placed in direct neighborhood to the vacancy. Together, the dopant and the vacancy create a donor-vacancy center, which has a free electron associated with it. The electrons magnetic orientation, or spin, which can be in superposition, constitutes the qubit. Donor-vacancy centers in diamond potentially can work at room temperature and are therefore considered a very attractive technology for building quantum networks. The biggest drawback to donor-vacancy centers in diamond is the difficulty of fabrication. Researchers either look for naturally-occurring defects in diamond, or fire atoms at a piece of diamond at high energy, creating defects in modulation doped lattice. We review the remarkable progress made in the past years in controlling electrons, atomic nuclei, and light at the single-quantum level in diamond. We also discuss prospects and challenges for the use of donor-vacancy centers in future quantum technologies.

Biography
Oliver Ambacher received the title of a Doctor of Natural Sciences at the Ludwig-Maximilians and the Technische Universität München in 1989 and 1993 with honors. In 1993 he received a position as scientific assistant at the Walter Schottky Institute of the Technical University of Munich. In 1995, he focused his research on the processing of GaN-based electronic and optical devices. He was instrumental in the investigation of low-dimensional electron systems in GaN-based heterostructures and quantum wells. In 1998/99 he received the opportunity to deepen his work in the field of AlGaN/ GaN-based power electronics as Feodor Lynen Fellow of the Alexander von Humboldt Foundation at Cornell University (USA). Following his habilitation in Experimental Physics 2000 and his promotion to Senior Assistant in 2001, he was appointed Professor of Nanotechnology at the Technical University Ilmenau a year later. In 2002, he was elected Director of the Institute of Solid State Electronics and two years later appointed Director of the Center for Micro- and Nanotechnologies of the TU Ilmenau. Since October 2007, Oliver Ambacher has been a professor at the Albert-Ludwigs-Universität Freiburg and director of the Fraunhofer Institute for Applied Solid State Physics.