Fault-tolerant quantum computing requires high-fidelity qubits. This has been achieved in various solid-state systems, including isotopically purified silicon, but is yet to be accomplished in industry-standard natural (unpurified) silicon, mainly as a result of the dephasing caused by residual nuclear spins. This high fidelity can be achieved by speeding up the qubit operation and/or prolonging the dephasing time, that is, increasing the Rabi oscillation quality factor Q (the Rabi oscillation decay time divided by the π rotation time). In isotopically purified silicon quantum dots, only the second approach has been used, leaving the qubit operation slow. We apply the first approach to demonstrate an addressable fault-tolerant qubit using a natural silicon double quantum dot with a micromagnet that is optimally designed for fast spin control. This optimized design allows access to Rabi frequencies up to 35 MHz, which is two orders of magnitude greater than that achieved in previous studies. We find the optimum Q = 140 in such high-frequency range at a Rabi frequency of 10 MHz. This leads to a qubit fidelity of 99.6% measured via randomized benchmarking, which is the highest reported for natural silicon qubits and comparable to that obtained in isotopically purified silicon quantum dot–based qubits. This result can inspire contributions to quantum computing from industrial communities.

We have studied photoluminescence (PL) properties of surface-oxidized Si nanocrystals fabricated with a ${\mathrm{SiH}}_4$ plasma cell. The size dependence of the PL spectrum, the PL decay dynamics, and site-selective excitation spectroscopy show that the efficient and broad PL band around ∼1.65 eV originates from excitons localized at the interface between crystalline Si and the ${\mathrm{SiO}}_2$ surface layer. The PL from the crystalline Si core state in large nanocrystals appears in the infrared spectral region. By comparison between surface-oxidized Si nanocrystals and as-prepared porous Si, the size and surface dependence of the coupling between electronic and vibrational excitations are discussed.

We have investigated the formation of nanocrystalline Si (nc-Si) in SiH₄ plasma using a pulsed-H₂ gas supply by very-high-frequency (VHF; 144 MHz) excitation. Nanocrystalline Si is formed in the gas phase of a SiH₄ plasma cell by the coalescence of radicals. The particle size of nc-Si is determined by the growth time of nc-Si in the plasma cell. Supplying H₂ into SiH₄ plasma enhances nucleation of nc-Si. When the H₂ gas supply is turned off, nucleated nc-Si particles grow larger in SiH₄ plasma. In the next cycle of H₂ gas supply, nc-Si particles grown in the previous cycle are pushed out of the cell into the deposition chamber. Using this method, fabrication of nc-Si with a diameter around 8 nm and a narrow spread (±1 nm) of particle size was realized.

Temperature and frequency dependent electrical properties of ${\mathrm{SiO}}_2$ /nanocrystalline Si $({\mathrm{nc-Si)/SiO}}_2$ sandwich structures have been studied. A clear shift of the capacitance–voltage and conductance–voltage characteristics toward positive gate voltage suggests electron trapping in an nc-Si dot. The role of interface states and deep traps in our devices has also been thoroughly examined and shown to be unimportant on the overall device performance. The discharging process is found to be logarithmic with time and weakly temperature dependent. The long memory retention time and the logarithmic time dependence of charge loss in the dots are explained by a buildup of opposing electric field in the tunnel oxide, which hinders the discharge of electrons remaining in the dots.