Progress Report

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Development of scalable Silicon quantum computer technology[2] Development of middle-distance quantum link

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Progress until FY2025

1. Outline of the project

To realize large-scale quantum computers, quantum bits (qubits) must be arranged stably in the semiconductor chip and connected when needed. In this R&D Item, we are developing "middle-distance quantum links" that connect qubits located a few micrometers apart in the chip by using semiconductor technologies capable of scalable integration.

Fig.1
Figure 1: Conceptual illustration of two qubits connected by a middle-distance quantum link.

2. Outcome so far

1. Exploring Qubit Transfer Methods

To implement a method of transferring qubits through a semiconductor channel, we compared several approaches, including a bucket brigade method that passes an electron from site to site, a conveyor-mode method that carries an electron in a moving trap, and a method using exchange interaction.
For each approach we used numerical calculations of device electric fields and electron motion to clarify the main features, challenges, and operation conditions. A common requirement was to move the quantum state in an adiabatic way.

2. Testing Multi-Quantum-Dot Structures and Control Systems

To test middle-distance quantum links, both a structure for moving qubits and a measurement setup that can control it are required. We fabricated samples for studying qubit motion and prepared a low-temperature measurement system to examine how sample structure, applied voltages, and transfer speed affect device operation.

Fig.2
Figure 2: Measurement setup at The University of Osaka.
Fig.3
Figure 3: Electron microscope image of a multi-quantum-dot device made of gallium-arsenide measured at 10mK, with the quantum dot array showing individually confined single electrons in each dot (indicated by dashed lines labeled Dot1 to Dot7).
3. Developing Devices for Conveyor-Mode Method

These studies showed that the conveyor-mode method, which uses regularly arranged electrodes and periodic time-dependent voltages, is a promising approach to extend to larger circuits. Tests of prototypes also showed that stable transport requires a smooth electric landscape along the transfer path.
We will move the main demonstration to silicon devices, using prototypes developed with external partners, studies will show not only to move charge accurately, but also to keep the electron spin state from being disturbed during transport.

Fig.4
Figure 4: Example of a multi-quantum dot device under development (electron microscope image).

3. Future plans

The results obtained in this study provide a basis for building large-scale quantum processors. To connect distant qubit arrays while leaving space for control wiring, middle-distant quantum link technology compatible with industrial silicon integration will be needed.
We will focus on the conveyor-mode method and test whether electrons can be moved through silicon devices while preserving the spin state that carries quantum information. Through this work, we aim to establish scalable qubit-link technology.

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