Progress Report

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 FY2024

1. Outline of the project

In FY2024, we worked to improve squeezed light sources using optical parametric amplifiers (OPAs) and waveguide technology. These light sources are essential for advancing future quantum technologies. We focused on making them more efficient and stable by testing new designs and improving the quality of the light output. We also studied how to build generation circuits for fully entangled photon twins(EPR pairs named after Einstein–Podolsky–Rosen gedanken experiment) using planar lightwave circuits (PLCs), a key step toward integrating quantum optical systems in future. This helped us identify technical challenges for future development.

Fig.1
Fig. 1 Devices for Optical Quantum Computers

2. Outcome so far

(1) Squeezed Light Source Development with Waveguide OPAs

We focused on improving squeezed light generation, which plays a vital role in quantum communication and sensing. Using a compact optical parametric amplifier (OPA), we tested a new method that uses an optical spectrum analyzer to measure how much "quantum noise" can be reduced (Fig.2). As the amplification of OPAs is limited in practical systems, we constructed a simulation model to predict how the gain would affect measurement accuracy. To validate this, we connected two OPAs in series and measured the output. The results closely matched our theoretical predictions, confirming the reliability of our method (Fig. 3).

Fig.2
Fig. 2 All-optical squeezing level measurement
Fig.3
Fig. 3 intensity ratio of excitation light intensity and amplified squeezed light.
(a) Calculated (b) Measuredd

In addition, we worked to improve the spatial quality of the squeezed light. The spatial mode, referring to the shape and symmetry of the light beam, is critical for ensuring consistent performance in practical devices. We built a custom measurement setup and identified key design parameters that affect the symmetry. Based on these insights, we fabricated 15 devices, all of which achieved over 99% symmetry, making them suitable for integration into the quantum computer currently under development at RIKEN.

(2) Evaluation of EPR Circuits with PLCs

We developed an EPR pair generation circuit using planar lightwave circuit (PLC) technology and modularized it by attaching fiber arrays. The core chip exhibited low loss of 0.2 dB; however, after modularization, an additional 1 dB loss was observed. Analysis pointed to small misalignments during assembly, such as fiber core height and angle variations. These results suggest that improving assembly precision can reduce losses, providing valuable insights into the development of scalable quantum optical modules.

3. Future plans

We plan to:

  • Further improve light mode quality and manufacturing precision.
  • Reduce losses in optical connections.
  • Build a reliable system to supply quantum devices for future quantum computers.
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