Progress Report

Development of Quantum Interfaces for Building Quantum Computer Networks[1] Diamond Quantum Memory

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Progress until FY2024

1. Outline of the project

We are developing quantum interfaces that connect superconducting quantum computers with optical fiber quantum communications to realize a distributed quantum computer system (Fig. 1). The core of this project is the quantum memory and optomechanical crystals, and integrated development from the diamond growth and nanofabrication to 3D mounting will be carried out. In this R&D theme, we have achieved the following outcome.

Fig.1
Fig. 1. Distributed quantum computer system.

2. Outcome so far

Subject 1: Diamond Quantum Memory
Improving Quantum Memory Performance and Optical Interconnection
We developed high-speed and high-fidelity quantum memory devices using nitrogen-vacancy (NV) centers in diamond (Table 1), and demonstrated complete Bell-state measurements with 90% fidelity, as well as error correction with 86% fidelity using these memories. Furthermore, we successfully operated a two-node system in which NV centers were optically connected via photon emission and absorption (Fig. 2).
Localized Creation of Color Centers
We developed techniques for the localized creation of color centers using ion implantation and ultrashort-pulse laser annealing, achieving integration of color centers within nanophotonic cavities and the fabrication of NV center arrays with 1-μm spacing.
Table 1
Table 1. Qubits performance
Fig.2
Fig. 2. Two-node quantum network using NV centers.
Subject 2: Diamond Quantum Structures
Fabrication of diamond nanostructures
We have developed submicron patterning technology using an electron beam lithography system to fabricate nanostructures such as diamond optomechanical cavities and micro comb-shaped electrodes (IDTs).
Fabrication of diamond convergent inter-digital transducer
A convergent inter-digital transducer was fabricated on an aluminum nitride (AlN)/diamond multilayer film including single NV center (Fig. 3) and successfully observed sound waves (acoustic waves) at ~5 GHz.
Fig.3
Fig. 3. (Left) Optical microscope image of diamond convergent inter-digital transducer device, (Right) Acoustic resonance reflection spectrum.
Subject 3: Diamond Quantum Crystals
l Diamond high-purity crystal growth
We have performed high-purity diamond crystal growth and impurity control to stabilize the charge state of NV centers and reduce their spectral diffusion.
Subject 4: Diamond Color Centers
l Development of nano-ion beam for precision implantation
We are developing a system for precise ion implantation into the nanostructure of diamonds. We have installed a sputter-type ion source and an ion acceleration mechanism (Fig. 4). We successfully increased the voltage of the ion trap to +50 kV and accelerated Si++ ions to 100 keV. We plan to focus the ion beam and precisely implant it into the diamond.
Fig.4
Fig. 4. A schematic diagram of the system for precise ion implantation into the nanostructure of diamonds, and the coulomb crystal of Ca+ formed in the Paul trap at an increased voltage of +50kV.

3. Future plans

We will form the quantum memory in a diamond optomechanical cavity and couple it with a piezoelectric microwave cavity.

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