Using the time-domain multiplexing techniques developed by the R&D theme 1, we have generated large-scale cluster states that are currently difficult by other methods. In particular, the generated cluster state has a structure that allows more efficient computation compared to previously generated cluster states.
Thanks to our prior research, we have significantly improved and stabilized the laboratory environment, making it possible to construct the system using a free-space setup. The free-space approach offers lower losses compared to fiber-based approach. As a result, we are now able to generate higher-quality cluster states.
We have successfully built an optical quantum computer and a cloud-based system, making them available to our collaborators, as shown in Figure 1. This prototype operates in a free-space setup at a clock frequency of 100MHz and functions as an analog quantum computer with 101 input modes. The system is capable of performing programmable linear transformations on continuous variables. In addition, thanks to the time-domain multiplexing technique, it is, in principle, capable of executing operations with an arbitrary number of steps. We performed evaluations on basic and multistep operations and verified that the system operates as expected.
Furthermore, we have developed a cloud system including a local server at RIKEN, a framework for remote operation of the hardware, a user authentication system, a compiler that converts quantum circuits into machine parameters, and a software development kit (SDK).
Our quantum computer has been made available to our collaborators, and year-around, long-term operations were tested. This confirmed the stability of our system and that yearly operation is possible with only a few adjustments per year. These developments form the basis for investigating applications of our optical quantum computer.

In this optical quantum computer, the types and precision of measurements directly correspond to the types and precision of possible quantum operations. In the current system, only deterministic addition, subtraction, and scalar multiplication operations are implementable. However, to realize universal quantum computation, the optical quantum computer must be capable of performing “multiplication” operations, which require nonlinear measurements corresponding to deterministic nonlinear operations. We have successfully implemented a basic nonlinear measurement using the setup shown in Figure 2. In this configuration, the target quantum state is superimposed with an auxiliary quantum state, and one of the outputs is measured via homodyne detection. A nonlinear computation is then applied to the measurement result, and based on this result, the phase of the remaining quantum state is rotated before performing a second homodyne measurement. A key component of this system is high-speed digital signal processing, which we have achieved using a field-programmable gate array (FPGA) that incorporates a lookup table for the necessary computations. We are also conducting research to integrate the nonlinear operation into the optical quantum computer, aiming to realize an optical quantum computer capable of performing “multiplication” operations.