Quantum Computing

There are alternatives for the next-generation computing technology such as quantum computing and spintronics. Quantum computing utilizes quantum bits (qubits) to solve specific problems which cannot be tackled by conventional computers. By increasing the qubit number and fidelity, quantum supremacy might be achieved and universal quantum computers are intensive development by universities, research institutes, industry, and start-ups. Among all qubit platforms, solid-state quantum computers attract much attention because of their scalability and compatibility to Si VLSI technology. Si-based quantum dots (QDs) are a promising candidate due to the extremely long decoherence time provided by its enriched 28Si isotopes and the similar fabrication processes to state of the art Si CMOS technology. Furthermore, Ge-based QDs might be even more suitable for large-scale qubit systems thanks to their fast operational speeds (> 100 MHz) by spin-orbit-coupling (SOC) effects and the strong interaction between different spins. Our group is working on GeSn-based QDs, which are promising to be integrated with other electronic, optoelectronic, and spintronic devices.

Group-IV QD devices are based on the Si/SiGe or Ge/GeSi heterostructures, where we discovered strong surface tunneling of 2DEG/2DHG carriers to the surface channel at the oxide/surface interface. This surface tunneling effect screens the remote impurity scattering from the buried 2D carriers, which enhances the mobility and leads to a bilayer conduction (2017 Physical Review Materials DOI, 2018 Applied Physics Letters DOI, 2019 Journal of Applied Physics DOI) and its transient properties (2024 Applied Physics Letters DOI). We also characterize its capacitive characteristics (2022 IEEE Transactions on Electron Devices DOI) and utilizes this bilayer feature to create a flash memory device (2022 ACS Applied Electronic Materials DOI) fully compatible to the Si/SiGe or Ge/GeSi QDs (2019 Nanotechnology DOI) with high endurance and rentions. This enables a quantum system on a chip (QSOC) where conventional logic, memory, and qubit devices are integrated on the same material platform. Physics of group-IV heterostructures for quantum technologies and applications has been addressed in our review paper in Materials for Quantum Technology (2024, DOI).

For a practical quantum computer, large-scale qubits are required. Thus far, not too many works focus on the large-scale qubit control. We demonstrate a meanderline structure to serve as a electron-spin-resonance (ESR) line to drive Si-based qubits. The design shows it can drive more than 30 qubits  (2021 Applied Physics Letters DOI). We also proposed a simple hyperbolic-shape micromagnet to drive Si-based qubits via electric-dipole-spin resonance (EDSR) for large-scale qubits to achieve fast Rabi oscillation with high addressaboility  (2024 IEEE Electron Device Letters DOI). We also developed an in-house code for QD design for large-scale qubit applications (2025 Applied Physics Letters DOI). While high-fidelity qubits have been demonstrated on Si-based and superconducting platforms, for large-scale qubits, effective electronic control at cryogenic temperatures is critical to reduce heat latency and signal noise to realize a full-scale quantum computer. We are focusing on cryo-CMOS devices on Si MOS and Si/SiGe or Ge/GeSi heterostructures such as low-noise amplifiers (LNAs), cryogenic flash memories, and on-chip filters/electrostatic discharge (ESD) devices.