Fig. 1. Spectrum of squeezing from OPO (red), and transmission spectra of filter cavity (green) and interference filter (blue). Horizontal axis is difference of optical frequency and central frequency of squeezing, the unit is Δν_FSR,OPO.

Fig. 2. Experimental setup of Schrödinger cat. OPO, optical parametric oscillator; FC, filter cavity; PD, photon detector; SPD, single-photon detector; PID, proportional-integral-differential amplifier.

Fig. 3. Cavity transmission (green line) and the corresponding scanning voltage (blue line): (a) Three FSR is scanned; (b) expansion of the second transmission peak.

Fig. 4. Fitting of Piezo response (circles correspond to experimental data, solid line is fitting curve).