  A continuous-wave (CW) broadband high-level optical quadrature squeezer is
essential for high-speed large-scale fault-tolerant quantum computing on a
time-domain-multiplexed continuous-variable optical cluster state. CW
THz-bandwidth squeezed light can be obtained with a waveguide optical
parametric amplifier (OPA); however, the squeezing level have been insufficient
for applications of fault-tolerant quantum computation because of degradation
of the squeezing level due to their optical losses caused by the structural
perturbation and pump-induced phenomena. Here, by using mechanical polishing
processes, we fabricated a low-loss quasi-single-mode periodically-poled LiNbO3
(PPLN) waveguide, which shows 7% optical propagation loss with a waveguide
length of 45 mm. Using the waveguide, we assembled a low-loss fiber-pigtailed
OPA module with a total insertion loss of 21%. Thanks to its directly bonded
core on a LiTaO3 substrate, the waveguide does not show pump-induced optical
loss even under a condition of hundreds of milliwatts pumping. Furthermore, the
quasi-single-mode structure prohibits excitation of higher-order spatial modes,
and enables us to obtain larger squeezing level. Even with including optical
coupling loss of the modularization, we observe 6.3-dB squeezed light from the
DC component up to a 6.0-THz sideband in a fully fiber-closed optical system.
By excluding the losses due to imperfections of the modularization and
detection, the squeezing level at the output of the PPLN waveguide is estimated
to be over 10 dB. Our waveguide squeezer is a promising quantum light source
for high-speed large-scale fault-tolerant quantum computing.
