Progress Report

Large-scale Silicon Quantum Computer[4] Quantum computing in small qubit systems

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Progress until FY2024

1. Outline of the project

The project aims to identify early-stage challenges in developing a silicon-based large-scale quantum computer by utilizing small-scale experimental circuits as foundational elements for future large-scale silicon qubit array structures.
As shown in Fig. 1, we are using experimental circuits equipped with well-established qubit operations to evaluate and improve fundamental processes such as qubit initialization, readout, and coherent control. By integrating these functions, we demonstrate quantum operations and assess the feasibility of fault-tolerant quantum computation in large systems. Furthermore, this research provides design guidelines for scalable qubit array structures and contributes to the overall project goals.

Fig.1
Fig. 1 Schematic of a Si/SiGe device hosting fully controllable three spin qubits (left) and its scanning electron micrograph image (right). The scale bar indicates 100 nm (one ten-millionth of a meter).

2. Outcome so far

Our outcomes can be categorized into the following six key areas:

  1. Methods for initialization and readout in qubit arrays
  2. Universal control of three spin qubit
  3. High-fidelity two-qubit gate operations for fault tolerance
  4. Balancing qubit control speed and operational errors
  5. Phase-flip error correction code with three spin qubits
  6. Assessment of qubit error correlations

Point 1 involves establishing methods for initializing and reading out qubits within a one-dimensional silicon qubit array–––a process that had been previously challenging.
In Point 2, we demonstrated universal quantum control over three silicon qubits and, for the first time, successfully synthesized a tripartite entangled state with a high fidelity of 88%.
Points 3 and 4 involve achieving high-fidelity controlled-NOT operations between two qubits, overcoming a major bottleneck in this system. A gate fidelity of 99.5% was demonstrated, meeting the threshold for fault-tolerant operations. We derived guidelines for further fidelity improvement by optimizing speed-noise trade-offs.
In Point 5, we integrated high-fidelity control of three qubits to realize the world’s first phase-flip error correction circuit using silicon qubits (Fig. 2), making a major milestone toward fault-tolerant silicon quantum computers.

Fig.2
Fig. 2 Phase-flip error correction experiment using three spin qubits in silicon. (a) Quantum circuit. (b), (c) Results.

In Point 6, we analyzed error correlations between adjacent qubits, which poses a challenge in quantum error correction within array structures. Measurements of phase precession rate fluctuations that cause errors in silicon qubits (Fig. 3) revealed strong local correlations that decay with distance. Additionally, we developed methods to characterize noise sources based on these correlations, including a new technique for evaluating charge noise correlations in qubit devices. These findings will contribute to the future design and performance improvement of silicon quantum computers.

Fig.3
Fig. 3 Correlation spectrum of qubit phase fluctuations.

3. Future plans

To realize fault-tolerant quantum computers, we will continue developing strategies to scale up qubit systems without compromising performance. A deeper understanding of error correlations in silicon qubits will guide this effort. We aim to establish scalable control methods—including initialization, readout, and operation—while taking into account the specific characteristics and constraints of silicon qubits and array structures, with an eye toward applications in large-scale system.

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