  Terahertz (THz)-bandwidth continuous-wave (CW) squeezed light is essential
for integrating quantum processors with time-domain multiplexing (TDM) by using
optical delay line interferometers. Here, we utilize a single-pass optical
parametric amplifier (OPA) based on a single-spatial-mode periodically poled
ZnO:LiNbO3 waveguide, which is directly bonded onto a LiTaO3 substrate. The
single-pass OPA allows THz bandwidth, and the absence of higher-order spatial
modes in the single-spatial-mode structure helps to avoid degradation of
squeezing. In addition, the directly bonded ZnO-doped waveguide has durability
for high-power pump and shows small photorefractive damage. Using this
waveguide, we observe CW 6.3-dB squeezing at 20-MHz sideband by balanced
homodyne detection. This is the first realization of CW squeezing with a
single-pass OPA at a level exceeding 4.5 dB, which is required for the
generation of a two-dimensional cluster state. Furthermore, the squeezed light
shows 2.5-THz spectral bandwidth. The squeezed light will lead to the
development of a high-speed on-chip quantum processor using TDM with a
centimeter-order optical delay line.
