Researches are progressing in various physical systems towards the implementation of practical quantum computers. In most systems, major challenges lie in the requirement of highly complex quantum processors necessary for practical quantum computing. On the other hand, optical system can perform practical quantum computing with compact quantum processors. As such quantum processors have already been demonstrated, the main focus of developments is set 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 enable to generate squeezed light (over 8 dB) in the range of several THz. Putting comb-shaped spectrum local oscillator lights into a several THz widband 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 units 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.

Using optical parametric amplifiers jointly developed within this research project, we have achieved the world's highest generation rate of optical quantum entanglement. The generation rate at 60 GHz is higher than the conventional rates by a factor more than 1000. Real-time measurement of optical quantum entanglement has been successfully achieved even at this high rate, which itself is essential for quantum technologies that involve real-time information processing, such as quantum computing and quantum communication.
The precision fabrication technology of the OPA and a new phase synchronization method for multiple high-speed measurement systems made it possible to have achieved the world's first high-speed real-time measurement of a quantum entanglement state between two parties.
Applying this technology, we are trying to generate GKP quantum bits from the multiple quantum states with strong non-classicality, called Schrödinger's cat states. In conventional optical systems, the generation rate of Schrödinger's cat states was on the order of kHz, resulting in very low generation rates of GKP quantum bits.

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 error-tolerant optical quantum computers, and furthermore that it will provide an approach to realizing "all-optical" optical quantum computers that do not require conversion to electrical signals.