  We achieved continuous-wave 8.3-dB squeezed light generation using a
terahertz-order-broadband waveguide optical parametric amplifier (OPA) by
improving a measurement setup from our previous work [T. Kashiwazaki, et al.,
Appl. Phys. Lett. 119, 251104 (2021)], where a low-loss periodically poled
lithium niobate (PPLN) waveguide had shown 6.3-dB squeezing at a 6-THz
frequency. First, to improve efficiency of the squeezed light detection, we
reduced effective optical loss to about 12% by removing extra optics and
changing the detection method into a low-loss balanced homodyne measurement.
Second, to minimize phase-locking fluctuation, we constructed a
frequency-optimized phase-locking system by comprehending its frequency
responses. Lastly, we found optimal experimental parameters of a measurement
frequency and a pump power from their dependences for the squeezing levels. The
measurement frequency was decided as 11 MHz to maximize a clearance between
shot and circuit noises. Furthermore, pump power was optimized as 660 mW to get
higher squeezing level while suppressing anti-squeezed-noise contamination due
to an imperfection of phase locking. To our knowledge, this is the first
achievement of over-8-dB squeezing by waveguide OPAs without any
loss-correction and circuit-noise correction. Moreover, it is shown that the
squeezing level soon after our PPLN waveguide is estimated at over 10 dB, which
is thought to be mainly restricted by the waveguide loss. This broadband
highly-squeezed light opens the possibility to realize fault-tolerant
ultra-fast optical quantum computers.
