## 4 Experimental Results and Discussion  
The measured pump threshold of the SRO was 7.3 W. When the pump power was 7.96 W, the SRO was operated above the threshold. The measured signal output was 10 mW and the idler output was 2 mW. The noise powers of the signal and idler were detected by PD1 and PD2 and recorded by an SA, respectively. The intensity noises of the signal and idler reach the SNL for analysis frequencies above 5 MHz, respectively. When the current output from PD1 was attenuated by an EA so that it was equal to the current output from PD2, the noise power of the intensity difference between two down-conversion beams at an analysis frequency of 5 MHz was measured and recorded by an SA with an resolution bandwidth (RBW) of 100 kHz, a video bandwidth (VBW) of 50 Hz, and the sweep time of 500 ms, as shown in Figure 3, curve (ii). Curve (i) in Figure 3 is the corresponding SNL that was given by the measured noise power of intensity sum of two down-conversion beams. It can be seen that the noise power of the intensity difference between the twin beams is 1.8 dB below the corresponding SNL. It is indicated that the two-color quantum correlation of 1.8 dB between down-conversion beams at 1.5 and 3.3 \(\upmu\)m from the SRO was observed. The normalized noise power spectrum of the intensity difference between two down-conversion beams is shown in Figure 4. Blue diamonds are experimental data. Curve (i) is the normalized SNL  
Figure 3: Measured noise power of the intensity difference between two down-conversion beams. Curve (i): the corresponding shot-noise level (SNL), curve (ii): the noise power of the intensity difference between the twin beams. The analysis frequency is 5 MHz. Curve (ii) is the calculated normalized noise power spectrum of the intensity difference between the signal and idler using Equation (10) with experimental parameters of \(\gamma\) = 0.0135, \(\gamma_{s}\) = 0.0075, \(\tau\) = 1.9 \(\times\) 10\({}^{-9}\), \(\sigma\) = 1.04, \(\varphi\) = 0 and the cavity length of 569 mm. The measured intensity noise of the pump (as shown in the inserted figure in Figure 4) was used in the calculation. Curve (i) and (ii) in the inserted figure in Figure 4 are the normalized SNL and the normalized intensity noise spectrum of the pump, respectively. The intensity noise of the pump reaches the SNL for analysis frequencies higher than 5 MHz. It can be seen that the normalized noise power of the intensity difference between the twin beams is below the normalized SNL at analysis frequencies from 4 to 11 MHz. It means that the two-color quantum correlation between down-conversion beams can be observed at the analysis frequency range. Obviously, the observed quantum correlation between the twin beams degraded when the analysis frequency increases from 5 MHz to 11 MHz, a maximum noise reduction can be observed at an analysis frequency of 5 MHz, and the noise reduction decreases with the decrease of the analysis frequency from 5 to 4 MHz. In principle, the noise reduction should be increased with the decrease of the analysis frequency. The fact of noise reduction decrease at the low analysis frequencies is due to the large excess intensity noise of the pump for analysis frequencies below 5 MHz. The quantum correlation between the twin beams at high frequencies is limited by the bandwidth of the SRO cavity. The measured finesse of SRO was 56 and the bandwidth of the cavity was 9.4 MHz. The discrepancy between the theoretical prediction and the experimental results could be induced by the extra noise caused experimentally by the effect of guided acoustic wave Brillouin scattering within MgO:PPLN crystal [38]. Figure 5 shows the normalized noise power of the intensity difference between the signal and idler at the analysis frequency of 5 MHz with different pump parameters.