Progress Report
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Quantum Cyberspace with Networked Quantum Computer[3] Semiconductor networking technology
Progress until FY2025
1. Outline of the project
We will develop the technologies of quantum interface between photons and silicon qubits based on spin in semiconductor quantum dots (QDs) and couplings between silicon qubits for constructing networks of semiconductor quantum computers. We also develop the connections of semiconductor qubits to large-scale optical networks. By accomplishing these subjects, we will contribute to the semiconductor networking technologies for fault-tolerant quantum computers as a Moonshot project goal.
Toward this goal, we develop quantum interfaces including germanium hole qubits, the intermediate range couplings between silicon qubits, and entanglement generation between remote spin qubits pair from photon pairs.
2. Outcome so far
① Development of a Photon-Spin Quantum Interface Using Germanium Quantum Dots
We have been developing germanium (Ge) QDs as a photon-spin quantum interface that operates at telecom wavelengths. Ge hole qubits have recently attracted attention as a new semiconductor qubit system that exhibits performance comparable to silicon qubits. In this FY, we have proposed the quantum state conversion from photon polarization to hole spin and demonstrated Ge QDs.
For the quantum state conversion, it is crucial to control the g-factors of electrons and holes. In collaboration with Tokura PI of the Koashi Project, we studied that the electron g-factor of a strained Ge at Γ point is approximately -2.8 using the k·p method [A. Imakire, A. Oiwa, and Y. Tokura, Jpn. J. Appl. Phys. 64, 071001 (2025).].
Finally, with a reported hole g-factor of ~0.3, we proposed an optical transition at Γ point that enables quantum state conversion from photon polarization to hole spin.
Meanwhile, in the development of Ge QDs, we achieved operation as QDs through collaborative research between Kodera PI of the Tarucha Project and a group at CEA in France (Figure 2).
② Cryogenic Optical Systems and Entangled Photon Pair Sources
Towards a networked semiconductor quantum computer, we are tackling generating entanglement between distant semiconductor qubits with an entangled photon pair source. We have developed a cryogenic optical system combining optical fiber delivery and piezoelectric stages for precise position controls at low temperature and installed it in the dilution refrigerator. We have also constructed a standard spontaneous parametric down-conversion system, making it possible to deliver correlated photons to the dilution refrigerators via optical fiber.
3. Future plans
We will promote research aimed at demonstrating fundamental technologies for realizing a networked quantum computer of semiconductor qubits by combining Ge spin qubits with newly developed bullseye optical resonator for improving conversion efficiency.