Progress Report
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Large-scale Silicon Quantum Computer[3] Hot silicon qubits
Progress until FY2025
1. Outline of the project
In this Item, we aim to achieve "high-temperature" operation of silicon qubits, referred to as “hot silicon qubits” (Fig. 1). This involves operating the qubits around 1 Kelvin (K), equivalent to around -272 degrees Celsius. This is higher than the typical operating temperature of solid-state qubits, which is a few tens of millikelvin (mK). By achieving hot silicon qubits, we can improve the permissible circuit power consumption (heat dissipation) and enable closer placement of cryogenic control circuitry. This contributes to the development of large-scale integrated silicon quantum computers. To achieve high-performance hot silicon qubits, we are exploring the "sweet spot" where quantum coherence time is maximized and comparing electron spin and hole spin systems.
2. Outcome so far
Development of elemental techniques for the realization of hot silicon qubits.
① Manipulation: As a 2024 milestone, we observed the coherent time evolution of hole spins at ~1K. In addition, we are advancing technical developments to achieve high-fidelity operation at higher temperatures. For the first time, we demonstrated extension effect of spin phase relaxation time (T2Rabi) using phase-modulated microwaves on hole spins (Figure 2).
② Readout: Radio-frequency (RF) reflectometry measurement method, which is considered promising for high-temperature readout, is a technique used to read the state of a qubit by analyzing the reflectivity of RF signals applied to the qubit system. We have been advancing technical developments, such as the sensitivity evaluation of this method and the enhancement of its sensitivity. [R. Mizokuchi, et al., Sci. Rep. 11, 5863 (2021)] We achieved the evaluation of spin relaxation time(T1), the length of time that quantum information can be stored in a qubit, using the technique we developed (Figure 3). [C. Kondo, et al, Jpn. J. Appl. Phys. 64, 01SP09 (2025)]
③ Efforts toward scalability: We are conducting research aimed at scalability from multiple approaches. To realize integrated qubit structures, we proposed a two-dimensional array structure that allows for high-speed and individual control despite its high-density configuration. Additionally, we developed an automatic quantum dot tuning technique using model-based reinforcement learning, which minimizes time-consuming learning processes, unlike other methods. [C. Kondo, et al., APL Mach. Learn. 3, 016114 (2025)] Additionally, we applied machine-learning to readout signal encoding to enable multiplexed readout for large-scale systems and developed a state discrimination technique for encoded multi-qubit states [D. Zhang et al., Jpn. J. Appl. Phys., 65, 05SP22 (2026)]. We also evaluated the temperature dependence of charge noise, a dominant factor limiting qubit performance, in electron and hole systems on the same device and channel, confirming minimal noise increase up to 300 mK and no significant disparity. Furthermore, we are advancing the development of integration techniques for qubits and their control with cryogenic classical circuits. Through collaboration with Kobe University, we confirmed the expected characteristics at 4K for a silicon qubit chip flip-mounted on an interposer. Additionally, we demonstrated selective control using selector circuits implemented on the same interposer. [K. Higashimomo et al., Jpn. J. Appl. Phys., 65, 03SP19 (2026)]
④Outreach activities: The realization of silicon quantum computers requires the development and integration of individual elemental technologies and techniques. Therefore, it is important to disseminate these technologies to researchers across various fields. For this purpose, we summarized and presented the research trends and prospects of semiconductor qubits, as well as individual elemental technologies and challenges. [T. Kodera, JSAP Rev. 2024, 240101 (2024), etc.] We are also committed to nurturing future quantum human resources who will lead future research and development. We cooperated in holding events such as the QIH-MS6 Co-hosted Quantum Technology Spring School.