The main R&D theme of this project is to develop cryogenic circuit and implementation techniques for large-scale silicon quantum computers. This enables high-fidelity control and high-density implementation of a large number of silicon qubits, thereby contributing to the realization of error-tolerant quantum computers for Moonshot Goal 6.
In order to achieve this, we are tackling the following three challenging themes. First, we are developing ultra-small, low-power cryogenic analog circuits which enable control of numerous qubits inside a dilution refrigerator. Second, we are creating innovative packaging by integrating qubit chips and their interface functionality on an interposer to accommodate large-scale qubits within the refrigerator. Furthermore, we are establishing a monitoring technique that observes quantum environmental noise affecting the accuracy of qubits and provides feedback to the control circuit.

• ①Design of cryogenic DA/AD converters for qubit control
• ②Development of multi-chip packages for stacking qubit chips
• ③Implementation of environmental sensor near the qubits

In the above outcome①, we developed an IC chip to control silicon qubits from the 4K stage of a dilution refrigerator. Specifically, we worked on the design of a 16-bit cryogenic DAC that generates the bias voltages for qubits, and a cryogenic ADC which measures each control signal inside the chip. The cryogenic DAC is required to be small area since a large number of channels are installed for controlling 64 qubits. Therefore, we invented novel architecture which effectively utilizes cryogenic characteristics and calibration technique for mismatch, integrating a 59-channel voltage generator into a single chip. Furthermore, we completed performance evaluations at 4K and confirmed that it operates correctly.

In ②, we developed silicon interposers for flip-chip mounting of the silicon qubit chips. We have constructed a multi-chip package by stacking qubit chips from HITACHI, Tokyo Institute of Technology and RIKEN, which are part of the same project, and we are currently measuring the qubit characteristics. In addition, we are developing a manufacturing process which achieves efficient signal routing and heat dissipation by forming through-silicon vias in the silicon interposers.
In ③, we designed cryogenic sensor circuit to acquire quantum environmental noise that affects the control precision of qubits, such as the temperature, power supply noise and control signal waveform near the qubits. This sensor circuit is placed at the 100 mK stage, thus we proposed a new sensor architecture that can operate with only 1 μW. We mounted this sensor on the interposer developed in ②, and confirmed that it properly operates at room temperature.