Progress Report

Quantum Cyberspace with Networked Quantum Computer[1] Atom networking technology

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Progress until FY2024

1. Outline of the project

In this research and development (R&D) item, we will develop a quantum interface for networking atom-based quantum computers consisting of neutral atoms that process quantum information as qubits and the necessary photon detection technology. The goal of this R&D item will establish the elemental technology of quantum networking for scaling up various quantum computers, including atom-based quantum computers, and the realization of a fault-tolerant universal quantum computer, which is the final target of Moonshot Goal 6.
To achieve this goal, we are working on creating quantum entanglement among atoms and photons on a large scale as a challenging theme. Beyond the realization of one-to-one qubit connections demonstrated already, we are working on an attempt at multiplexing technologies for atoms, optical circuits, and photon detectors (Fig. 1). These developed elemental technologies useful in other R&D items and projects will be actively provided and contribute to the final target of Moonshot Goal 6.

Fig.1
Fig. 1: Concept of a networked atom quantum computer.

2. Outcome so far

  1. Demonstration of Rb atom array and construction of photon detection system, and Proposal of Bell state distillation between logical qubits (Fig. 2).
  2. Proposal and demonstration of “Optical frequency tweezers” for multiplexed network connections. Quantum frequency conversion development.
  3. Superconducting nanowire photon detector (SNSPD) device development for 710 nm, 780 nm, and 850 nm wavelength bands, achieving a detection efficiency of more than 90% and a dark count rate of less than one count per second. For 1550 nm, both high efficiency (>90%) and low dark count rate (<1cps) are achieved (Fig. 3). Development of a novel superconducting wide strip photon detector (SWSPD). 14 patent applications have been made.
  4. Refrigerator system development with 32 channels of SNSPD for cooling below 2.3 K. 32 channel SNSPD refrigerator system has been successfully demonstrated, achieving a high detection efficiency over 90% for each channel. As a recent development, 32-ch SNSPD system is installed into a rack together with power supply and amplifier units, vacuum gauge, compressor and so on, which operates as an all-in-one system (Fig. 4).
Fig.2
Fig. 2: (a) CCD image of atom array. (b) Concept of logical Bell-state distillation.
Fig.3
Fig. 3: (a) Achievement of both high efficiency (>90%) and low dark count rate (<1cps). (b) SWSPD device.
Fig.4
Fig. 4: 32-ch SNSPD system rack

3. Future plans

Our research achievements so far include elemental technologies for quantum processors of atom arrays and a proposal of quantum protocols, demonstration of the working principle of routing photons from multiplexed photon sources, expansion of the wavelength of SNSPD and performance enhancement, and demonstration of a refrigerator system capable of multiplexing SNSPDs to the scale of 32 channels (world competitive scale). In the future,

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