Progress Report

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Development of Large-scale Fault-tolerant Universal Optical Quantum Computers[3] R&D of waveguide optical parametric amplifiers and optical quantum waveguide circuits

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Progress until FY2025

1. Outline of the project

In this project, we advanced squeezed-light generation and optical quantum waveguide circuit technologies essential for large-scale, high-speed universal optical quantum computers. For PPLN waveguide optical parametric amplifiers (OPAs), improvements to the waveguide structure, fabrication process, and phase-locking method achieved the final target of 10-dB squeezed-light generation. We also developed broadband measurement technology that uses an OPA as a preamplifier for quantum-signal detection, demonstrating THz-class observation and a basis for 40-GHz-class real-time quantum-signal acquisition. For silica-based planar light-wave circuits (PLCs), we studied elementary circuits such as EPR-generation circuits and module packaging, clarifying loss factors and issues for practical implementation.

Fig.1
Fig. 1 Devices for Optical Quantum Computers

2. Outcome so far

(1) 10-dB Squeezed-Light Generation with Waveguide OPAs

We worked on high-performance PPLN waveguide OPAs, broadband measurement technology, and stable fabrication for practical systems. By reviewing the PPLN waveguide fabrication process, we realized low-loss waveguides and improved the squeezing level to the 8-dB class. We then introduced a new phase-locking technique that suppresses both optical loss caused by control-light injection and phase error, achieving the final target of 10-dB squeezed-light generation. The developed OPAs can be manufactured stably for practical systems and have contributed to stable operation of RIKEN's optical quantum computer.

Fig.2
Fig. 2 Demonstration of a 10-dB squeezing level

Broadband quantum measurement technology was also created. Conventional homodyne measurements are limited by electrical circuit bandwidth. We proposed a quantum measurement method using an OPA as a preamplifier before detection and demonstrated squeezed-light observation over up to 6 THz by direct optical detection. Combined with high-speed detectors for optical communications, the method realized real-time amplitude measurement over a 43-GHz bandwidth, establishing a measurement basis for ultrafast and massively parallel optical quantum computing.

Fig.3
Fig. 3 High-speed real-time quantum-noise
(2) EPR-Generation Circuits and Modularization Using PLCs

Starting from verification of universal squeezer circuits, we compared different type-PLC platforms and fabricated and evaluated EPR-generation circuits. In the EPR-generation circuit, the total loss along the squeezed-light paths was reduced to 0.9 dB, and an output squeezing level of about 5.7 dB is expected for a 10-dB input. Module packaging with fiber arrays increased connection loss from 0.2 dB to 1.2 dB, identifying polishing angle, mode-field diameter mismatch, and alignment shift during bonding as key issues.

3. Future plans

We will focus on realizing high-performance PPLN-based quantum light sources and plan to:

  • Further enhance high-level OPA performance and stable manufacturing to improve the reliability of practical device supply.
  • Reduce PLC circuit loss and excess loss from modularization, including fiber connections.
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