(Keynote) Devices Architectures and Technology for Quantum Computing
Authors: Kodera Tetsuo , Horibe Kosuke , Oda Shunri 
Published: February 2020

There have been many reports on quantum computing hardware using various quantum two-level systems. In principle, any quantum two-level system can be used as a basic unit of information in quantum computing, which is called quantum bit (qubit). Many ways for implementing qubits have been studied to date, for example, trapped ions, cavity quantum electrodynamics, single defect centers, single photons, nuclear spin resonances, superconductor circuits, and semiconductor quantum dots (QDs). Among these, solid-state system has an advantage of scalability, which will be required for practical applications of quantum computers. Especially an electron spin in a QD is a well-defined quantum two-level system characterized by a spin-up and a spin-down state, which is one of the most promising candidates for a qubit, because of its long coherence time. Electron spin-based qubits have been studied using QDs in GaAs, which has a relatively small electron effective mass, m* = 0.067 m
0, the QD size required to obtain quantum effects is relatively large (~100 nm) and can be readily obtained using electron beam lithography. However, further research into quantum operations has found difficulties related to decoherence. The main source of the decoherence is hyperfine interaction with the nuclear spins of the Ga and As atoms.

To reduce this hyperfine interaction, the use of Si and SiGe has been investigated, since these materials have isotopes with zero nuclear spin. It is also possible to completely remove the hyperfine interaction through controlling the isotopic composition of the host. Si system is advantageous for future applications because of its potential for compatibility with conventional CMOS devices. However, it is necessary to fabricate much smaller QDs in Si in order to obtain quantum effects because of the larger electron effective mass in this material (m* = 0.19 m
0), and so a highly sophisticated fabrication technique is required.

To properly apply qubits based on electron spins, it is necessary to observe spin-related tunneling phenomena and to reduce the electron number in individual QDs to only a few, so as to ensure energetically well-defined spin states. To date, single-electron regimes in single QDs and Pauli-spin blockades (PSBs) in coupled QDs have been observed in Si/SiGe heterostructures. Single-electron regimes in Si QDs have also been observed in MOS structures using a double top-gate design, MOS fine-gate structures and nanowire structures. Thus far, the majority of these Si QD devices have had gate-defined QD structures in which confinement potentials are generated by electric fields with fine top gates. In contrast, the physically-defined Si QD devices described below do not require gates to create confinement potentials for the QDs. These devices thus represent a technological simplification owing to the reduced number of gates and may be advantageous with regard to the integration of qubit technology, since common fabrication processes normally used to produce Si MOS technologies may be applied to make the physically-defined Si QDs.

Recently, our group was able to obtain high-quality physically-defined Si QD structures by applying fabrication techniques such as electron beam lithography, oxidation and dry-etching. The resulting devices represented physically-defined coupled Si QDs with MOS structures incorporating a top gate for inducing inversion carriers and side gates for controlling potentials in the QDs. We observed spin-related tunneling phenomena [1] and reduced the electron number in the QDs to the few-electron regime [2,3]. Charge detection of change in number of carriers in the QDs, one by one, has also been successfully demonstrated [4]. These achievements are the important steps for realizing quantum computing devices,

[1] G. Yamahata, T. Kodera, H. O. H. Churchill, K. Uchida, C. M. Marcus, and S. Oda, Phys. Rev. B 86, 115322 (2012).

[2] K. Horibe, T. Kodera, T. Kambara, K. Uchida, and S. Oda, J. Appl. Phys. 111, 093715 (2012).

[3] K. Horibe, T. Kodera, and S. Oda, Appl. Phys. Lett. 106, 083111 (2015).

[4] K. Horibe, T. Kodera, and S. Oda, Appl. Phys. Lett. 106, 053119 (2015).