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A continuous-wave (CW) broadband high-level optical quadrature squeezer isessential for high-speed large-scale fault-tolerant quantum computing on atime-domain-multiplexed continuous-variable optical cluster state. CWTHz-bandwidth squeezed light can be obtained with a waveguide opticalparametric amplifier (OPA); however, the squeezing level have been insufficientfor applications of fault-tolerant quantum computation because of degradationof the squeezing level due to their optical losses caused by the structuralperturbation and pump-induced phenomena. Here, by using mechanical polishingprocesses, we fabricated a low-loss quasi-single-mode periodically-poled LiNbO3(PPLN) waveguide, which shows 7% optical propagation loss with a waveguidelength of 45 mm. Using the waveguide, we assembled a low-loss fiber-pigtailedOPA module with a total insertion loss of 21%. Thanks to its directly bondedcore on a LiTaO3 substrate, the waveguide does not show pump-induced opticalloss even under a condition of hundreds of milliwatts pumping. Furthermore, thequasi-single-mode structure prohibits excitation of higher-order spatial modes,and enables us to obtain larger squeezing level. Even with including opticalcoupling loss of the modularization, we observe 6.3-dB squeezed light from theDC 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 anddetection, the squeezing level at the output of the PPLN waveguide is estimatedto be over 10 dB. Our waveguide squeezer is a promising quantum light sourcefor high-speed large-scale fault-tolerant quantum computing.

Fabrication of low-loss quasi-single-mode PPLN waveguide and its application to a modularized broadband high-level squeezer