## 3 Experimental Setup
When the modulated frequency of sinusoidal signal is 680 MHz, the auxiliary beam can be simultaneous resonant in the OPO with the \(s\)-polarized squeezed vacuum field. The transmission of auxiliary beam from the OPO was spatially separated with the squeezed vacuum field by a polarization beam splitter (PBS) and detected by a photodetector (PD1). The subthreshold OPO cavity was locked by PDH technique with a radio frequency of 45 MHz via the \(p\)-polarized auxiliary beam. The error signal was fed back to PZT1 to lock the subthreshold OPO cavity. Another portion of 1064 nm laser from MC1 passed through an acousto-optic modulator (AOM1) with a shifted frequency of 200 MHz and an AOM2 with a shifted frequency of \(-175\) MHz, respectively. The output beam is used as the CCF with a frequency shift of \(\Omega\) = 25 MHz. The CCF was injected  
Figure 2: Schematic of the experimental setup used to generate squeezed vacuum states. OI, Optical isolator; F-P, Fabry–Perot cavity; MC, mode cleaner; FC, fiber coupler; EOM, electro–optical modulator; FCEOM, fiber coupled EOM; AOM, acousto-optic modulator; OPO, optical parametric oscillator; DBS, dichroic beam splitter; PBS, polarizing beam splitter; \(\lambda/2\), half wave plate; \(\lambda/4\), quarter wave plate; PZT, piezo-electric transducer; PD, photodetector; SA, spectrum analyzer; PID, proportional-integral-derivative; LPF, low pass filter; 4, high voltage amplifier; \(\otimes\), mixer; \(+/-\), positive/negative power combiner; \(\otimes\), signal generator. to the subthreshold OPO and the outgoing CCF from the subthreshold OPO was detected by PD2. After the photocurrent from PD2 was demodulated by a sinusoidal signal with frequency of 50 MHz and then filtered by a low-pass filter (LPF) with bandwidth of 1.9 MHz, the demodulated and filtered current was used as the error signal and fed back to the PZT to lock the relative phase between the pump field and CCF. The noise power of down-conversion fields from the subthreshold OPO was measured by the BHD system. The down-conversion fields and LO were combined at a 50/50 beam splitter and detected by a pair of InGaAs photodiodes. By the way, the laser intensity noise of the LO was measured by the BHD when the pump and CCF were blocked. The sum of two photocurrents from PD3 and PD4 was the LO intensity noise using a positive power combiner, the subtraction of two photocurrents was the SNL using a negative power combiner. When the pump and CCF were injected, squeezed vacuum states were generated. The generated squeezed vacuum states were measured by the BHD and the alternating current (AC) part of photocurrent subtraction was recorded by the SA. Another AC part of photocurrent subtraction was demodulated by a radio frequency of 25 MHz, and then filtered by an LPF with bandwidth of 1.9 MHz. This demodulated and filtered current was used as the error signal and fed back to PZT3 to lock the relative phase between the CCF and LO, i.e., to lock the relative phase between the LO beam and the squeezed vacuum states. Thus, a stable measurement result can be achieved. For improving the CMRR of two photodetectors, a self-subtraction photodetector scheme was firstly designed on a single electronic board to reduce the influence of the discrepancy of electronics components. The photocurrents of both photodiodes are directly subtracted before any further electronic signal processing, as shown in Figure 3a. Furthermore, an equivalent circuit of photodiode, as shown in Figure 3b, was designed to compensate for the asymmetry of the parameters of two photodiodes (PD1 and PD2), due to it is almost impossible to choose two photodiodes with identical parameters. The differential fine-tuning circuit between two photodiodes was built and the differential photocurrent was obtained by differential operation of two optical signals. The adjustable bias voltage (BV) was set on each photodiode, which were supplied by two stable sources with adjustable ranges of from 3 to 15 V and from \(-\)15 to \(-\)3 V, respectively.