## 4 Results and Discussion  
The intracavity round trip loss included the residual reflection, absorption, and the scattering from the mirrors and the crystal. Another non-negligible loss of the nonlinear crystal was induced by green light-induced infrared absorption (GLIIRA) [31; 32; 33]. In our experiment, we tested two types of nonlinear crystals, MgO: PPLN (Periodically Poled Lithium Niobate) and PPKTP, and compared their GLIIRA effects. Compared with the PPKTP, the MgO: PPLN crystal had a higher nonlinear coefficient; however, the thermal effect due to GLIIRA was more severe, which made the locking of the cavity length very hard. Therefore, we finally chose the PPKTP crystal. Theoretically, the highest entanglement is achieved at the  
Figure 3: Schematic of the experimental setup. NOPO: nondegenerate optical parametric oscillator; NOPA: nondegenerate optical parametric amplifier; PZT: piezoelectric transducers; PD: photodetectors; APD: avalanche photodetector; HD: balanced homodyne detector, NDF: neutral density filter; LO: local oscillator; DBS: dichroic beam splitter; PID: proportional-integral-derivative controller; HV: high voltage amplifier; SA: spectrum analyzer. Black lines denote the electrical channels for measurement and control. Green line, orange lines, and blue lines represent the pump laser, 1.5 \(\upmu\)m, and 0.8 \(\upmu\)m optical fields, respectively. threshold pump power. However, when considering the effect of GLIIRA, there exists an optimal pump power level for the best entanglement. This is because higher pump power results in a stronger GLIIRA effect, which will spoil the entanglement. In our experiment, the optimal pump power for the NOPO and NOPA are 1.1 times and 0.6 times the threshold power, respectively. Figure 4 plots the measured quantum correlation between the signal and idler beams at 1.5 \(\upmu\)m and 0.8 \(\upmu\)m with a spectrum analyzer. The electronic noise of the homodyne detector was 28 dB below the quantum noise limit at 1 MHz, and was not subtracted from the data. The quantum correlation noise of the NOPA output optical fields was recorded at \(\pi\)/2 relative phase between the 1.5 \(\upmu\)m quantum field and its LO, whereas the relative phase between the 0.8 \(\upmu\)m quantum field and its LO was scanned. The observed squeezing and anti-squeezing levels were \(-\)6 dB and 16.4 dB, respectively. Figure 5 records the quantum correlation noise spectrum between the signal and idler beams at 1.5 \(\upmu\)m and 0.8 \(\upmu\)m in a broadband frequency range from 1 MHz to 5 MHz. The observed phase-quadrature sum and the amplitude-quadrature difference quantum correlation were \(-\)6 dB at 1 MHz, and \(-\)5 dB at 5 MHz. The decrease of the quantum correlation at an analysis frequency of 5 MHz was due to the finite linewidth (17 MHz) of the NOPA cavity. Figure 4: Quantum correlation noise of the phase quadrature sum. The measurement was recorded at an analysis frequency of 1 MHz, with a RBW (Resolution Band Width) of 200 kHz and a VBW (Video Band Width) of 100 Hz. The observed squeezing and anti-squeezing levels were \(-\)6 dB and 16.4 dB, respectively. Figure 5: Measured broadband noise spectrum between the two-color down-converted optical fields from 1 MHz to 5 MHz. (**a**) phase quadrature sum; (**b**) amplitude quadrature difference. The RBW and VBW of the SA were 300 kHz and 100 Hz, respectively. Table 1 lists the relevant parameters of both the former system and the current improved system. In the previous work, the measured quantum correlation of the amplitude quadrature difference, and the phase quadrature sum was \(\sim\)3.3 dB, which subtracts the electronic dark noise; namely, \(\eta_{\text{en}}\) is 100%. For the current system, we improved the escape efficiency of the NOPA, the stability of the phase-locking servo system, and the detection efficiency. These approaches boosted the entanglement significantly from 3.3 dB to 6 dB.