  We report on the generation of a stable continuous-wave low-frequency
squeezed vacuum field with a squeezing level of $3.8\pm0.1$ dB at 1064 nm, the
wavelength at which laser interferometers for gravitational wave (GW) detection
operate, using periodically poled KTiOPO$_4$ (PPKTP) in a sub-threshold optical
parametric oscillator. PPKTP has the advantages of higher nonlinearity, smaller
intra-crystal and pump-induced seed absorption, user-specified parametric
down-conversion temperature, wider temperature tuning range, and lower
susceptibility to thermal lensing over alternative nonlinear materials such as
MgO doped or periodically poled LiNbO$_3$, and is, therefore, an excellent
material for generation of squeezed vacuum fields for application to laser
interferometers for GW detection.
