We study hardware and underlying physics for quantum information processing with quantum control of solid-state quantum states. Quantum information processing is an ideal information technology whose operation accompanies low-energy dissipation and high information security. The purpose of our group is to demonstrate the ability of the solid-state information processing based on concepts of quantum coherence and entanglement in semiconductor, topological phases, and photons to spin conversion, and finally outline a path for the realization. The specific research targets are "quantum computing" constructed by gate-based quantum logic operations, "quantum interface" useful for quantum repeaters, "quantum devices" relevant for the quantum control, and “topological control” providing new concepts of quantum information. We investigate best suited quantum circuits and devices, and fundamental physics of quantum entanglement, quantum dynamics, and topological particles.

### An ultra-high fidelity quantum dot qubit in a silicon quantum dot

The building block of quantum computers, or the smallest unit of quantum information, is called a qubit. A very large number of high-quality qubits will be needed to build a quantum computer. Single electron spins in quantum dots are a strong candidate as qubits, as they likely benefit from modern electronics integration technology, once sufficient quality is reached.

Enhancing the qubit quality means improving both coherence time and control time, challenging the trade-off commonly observed between these. In this work, we fabricated a quantum dot on an isotopically-clean silicon wafer to increase the coherence time, and deposited a micromagnet nearby to decrease the control time by its slanting magnetic field. With the coherence time ten times longer and the control time two orders of magnitude shorter than conventional within a single device, we implement highly coherent qubit operations (Fig. B). We further demonstrate >99.9% control fidelity (precision), above the fault-tolerance threshold.

Our work offers a promising route to large-scale, ultra-high-fidelity spin-qubit systems in silicon.

(Fig. A) Schematic view of the quantum-dot qubit device

(Fig. B) Chevron resulting from ultra-high fidelity spin rotations