Kosuke Horibe

Key capacitive parameters for designing single-electron transistor charge sensors

Single-electron transistors (SETs) are efficient charge sensors for reading out spin or charge qubits confined in quantum dots (QDs). To investigate their capacitive parameters, which are related to the signal-to-noise ratio (SNR) during qubit readout, twin silicon single QDs were fabricated using a lithographic process on a silicon-on-insulator substrate. Since the configuration and dimensions of the QDs could be determined by direct imaging, the theoretical capacitive parameters could be compared to the measured values. Good agreement was found between the calculated and measured values, which confirms the validity of the calculation method. The results indicated that decreasing the SET diameter reduces the capacitive coupling between qubits but increases the signal-to-noise ratio for both dc and radio frequency single-shot measurements. Since these results are independent of the device materials, they are useful for establishing guidelines for the design of SET charge sensors in lateral QD-SET structur...

Lithographically defined few-electron silicon quantum dots based on a silicon-on-insulator substrate

Silicon quantum dot (QD) devices with a proximal single-electron transistor (SET) charge sensor have been fabricated in a metal-oxide-semiconductor structure based on a silicon-on-insulator substrate. The charge state of the QDs was clearly read out using the charge sensor via the SET current. The lithographically defined small QDs enabled clear observation of the few-electron regime of a single QD and a double QD by charge sensing. Tunnel coupling on tunnel barriers of the QDs can be controlled by tuning the top-gate voltages, which can be used for manipulation of the spin quantum bit via exchange interaction between tunnel-coupled QDs. The lithographically defined silicon QD device reported here is technologically simple and does not require electrical gates to create QD confinement potentials, which is advantageous for the integration of complicated constructs such as multiple QD structures with SET charge sensors for the purpose of spin-based quantum computing.

Back-action-induced excitation of electrons in a silicon quantum dot with a single-electron transistor charge sensor

Back-action in the readout of quantum bits is an area that requires a great deal of attention in electron spin based-quantum bit architecture. We report here back-action measurements in a silicon device with quantum dots and a single-electron transistor (SET) charge sensor. We observe the back-action-induced excitation of electrons from the ground state to an excited state in a quantum dot. Our measurements and theoretical fitting to the data reveal conditions under which both suitable SET charge sensor sensitivity for qubit readout and low back-action-induced transition rates (less than 1 kHz) can be achieved.

Coupled quantum dots on SOI as highly integrated Si qubits

Quantum computing is no longer a future technology. Recent advances in D-Wave computers based on quantum annealing and superconducting devices, and the demonstration of long spin decoherence times in isotopically-enriched Si qubits, have accelerated the research and development of this technology. The remaining challenge is large scale integration of qubits. Physically-defined coupled quantum dots (QDs) on silicon-on-insulator substrates represent potential multiple scaled qubits. This work demonstrated the fabrication of coupled QDs with control gates and charge sensor single-electron transistors, the observation of Pauli spin blockade and the control of a few electron regimes, as well as triple QDs and p-channel operation.

Perpendicular magnetic field dependence of triangular triple silicon quantum dot system

We have fabricated a triangular triple quantum dot system with 3 leads, which is made on a silicon-on-insulator wafer by dry etching and integrated with a single electron transistor as charge sensor. We have successfully monitored the charge transitions of each dot by charge sensor. We have observed charge degeneracy point and investigated a dependence of the point on perpendicular magnetic field.

Fabrication and evaluation of heavily P-doped Si quantum dot and back-gate induced Si quantum dot

We realized lithographically-defined electrically-tunable silicon quantum dot (QD) and charge sensor. Two types of device were fabricated and measured. One is heavily P-doped, and the other is back gate (BG)-induced undoped QD device. I-V characteristic of QD and charge sensor was clearly observed in both devices. Then, we estimate capacitance between the charge sensor and QD or two side gates (SGs) from the measurement and the simulation, and compared two devices in terms of their charging energy.

Charge sensing of a Si triple quantum dot system using single electron transistors

We fabricate a serial triple quantum dot (TQD) system, which is made on a silicon-on-insulator (SOI) wafer by dry etching and integrated with single electron transistors (SETs) as charge sensors. We observe charge transitions of a dot in the TQD in the characteristic of the charge sensor which is the furthest to the dot. It implies a SET charge sensor has a capability of sensing of all the charge transitions in TQD.

Spin-related tunneling in lithographically-defined silicon quantum dots

We realized lithographically-defined electrically-tunable silicon quantum dots (Si QDs) without unintentional localized potentials by improving device structures and fabrication techniques. Carrier density was tuned with a top gate and QD-potentials were controlled with the side gates. We succeeded in observing spin-related tunneling phenomena using the double QD device.