Research is progressing in various physical systems toward the implementation of practical quantum computers. In most systems, major challenges lie in the requirement for highly complex quantum processors necessary for practical quantum computing. On the other hand, optical systems can perform practical quantum computing with compact quantum processors. As such quantum processors have already been demonstrated, the main target of development is shifting to the generation of optical qubits. Cluster states are generated using squeezed light. In this project, we have developed a waveguide optical parametric amplifier (OPA) that does not have a resonator structure, which has made it possible to generate wideband squeezed light. By incorporating the waveguide optical parametric amplifier into a fiber-optic system, we have reduced the loss of the optical intensity so that we are able to generate squeezed light (over 8 dB) in the range of several THz. Putting comb-shaped spectral local oscillator lights into a several THz wideband sideband every 100 GHz for about 100 will be equivalent to a super quantum computer with 100 cores operating at a clock frequency of about 40 GHz. We are trying to use this technique as a cloud computer.
As a technology for non-Gaussian manipulation, we are generating quantum entanglement states using an optical waveguide module unit for OPA and beam splitters, and then using a superconducting photon number discriminator, detecting a certain number of photons in a part of the generated quantum entanglement states to create a low-approximation magic state and also a Gottesman-Kitaev-Preskill (GKP) qubit.
We are conducting research to increase the generation rate of the GKP qubits and also that of the highly non-classical quantum states which should yield the better approximation of the magic state.

Therefore, we used OPA as a quantum phase-sensitive amplifier in front of the homodyne detector to speed up the measurement from 100 MHz to 70 GHz, resulting 700 times faster generation of the optical quantum states. This high-speed homodyne measurement technique using OPA as auxiliary device was applied to the generation of non-classical quantum states for the first time. Figure 1 shows the Wigner function (left) and the wave packet profile (right) of the generated Schrödinger cat state. The generation rate of this state reaches 1 MHz, which is about 1000 times higher than the conventional rate.

Increasing the generation rate of GKP quantum bits will lead to the practical application of fault-tolerant optical quantum computers. Furthermore, the technique will provide an approach to realizing much faster "all-optical" optical quantum computers that do not require conversion to electrical signals.