The main R&D Item of this project is to develop cryogenic circuits 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.

We have developed a cryogenic quantum control circuit, including 16-bit bias voltage generators, for controlling silicon qubits from the 4K stage of a dilution refrigerator. To achieve further precise control, we also developed a cryogenic analog-to-digital converter (Cryo-ADC) that enables signal acquisition and qubit readout in the 100 mK temperature, which is closer to the qubits. For this ADC, a new circuit architecture was designed to simultaneously meet stringent power constraints and achieve wideband performance. A prototype chip based on this design was fabricated, and it was confirmed to operate correctly at 100 mK with a power consumption of 30 uW. Furthermore, the Cryo-ADC was applied to the quantum environmental monitoring system. By employing an equivalent sampling technique, we successfully achieved pulse signal acquisition with 1ns time resolution in close to the qubits.

As part of the development of cryogenic multi-chip packaging technology, an advanced chip implementation structure was developed to integrate qubit and interface chips. In addition, chip-level thermal management techniques were explored to enable efficient dissipation of heat generated during qubit operation. A cryogenic packaging was developed in which through-silicon vias (TSVs) were formed in a silicon interposer, enabling both signal transmission and heat dissipation. To construct a high-thermal-conductivity path, Cu pillars were formed on the TSVs and bonded to a Cu substrate via Cu–Cu bonding. Thermal cycling tests were conducted using a cryogenic refrigerator to evaluate the reliability. We confirmed the connectivity remained through the cross-sectional view even after the tests. Currently, we prepare the evaluation for thermal dissipation characteristics of this structure by employing the temperature monitoring technique.