## 3 Experimental Setup  
The schematic of the experimental setup used to generate the two-color quantum correlated down-conversion beams at 1.5 \(\upmu\)m and 3.3 \(\upmu\)m from an SRO is shown in Figure 2. The pump source was a home-made low noise CW single-frequency laser with output power of 30 watts at 1064 nm. An optical isolator (OI) was used to eliminate the back-reflection light of the infrared laser. The SRO was composed of two concave mirrors with a radii of 100 mm, two plane mirrors, and a type-I phase-matched MgO:PPLN crystal. Concave mirrors M1 and M2 provided high transmission (HT) for the pump and idler (T\({}_{\text{1064 nm}}\) & 3.3 \(\upmu\)m \(>\) 95%) and high reflection (HR) for the signal (R\({}_{\text{1.5 $\upmu$m}}\)\(>\) 99.8%). Plane mirror M3 was the HR for the signal (R\({}_{\text{1.5 $\upmu$m}}\)\(>\) 99.8%). Plane mirror M4 was the output coupler for the signal, which provided partial transmission at 1.5 \(\upmu\)m (T\({}_{\text{1.5 $\upmu$m}}\)\(=\) 1.5%). Only the signal resonated in the ring cavity. The pump and idler single-passed MgO:PPLN crystal. Both end faces of MgO:PPLN were antireflection coated for 1.5 \(\upmu\)m (R\({}_{\text{1.5 $\upmu$m}}\)\(<\) 0.5%), 3.3 \(\upmu\)m (R\({}_{\text{3.3 $\upmu$m}}\)\(<\) 1%), and 1064 nm (R\({}_{\text{1064 nm}}\)\(<\) 2%). MgO:PPLN crystal, with dimensions of 30 mm (length) \(\times\) 10 mm (width) \(\times\) 1 mm (thickness) and a poling period of 30.6 \(\upmu\)m, was housed in a copper oven and temperature-controlled by a homemade temperature controller with an accuracy of 0.01 \({}^{\circ}\)C. The concave mirror M3 was mounted on a piezo-electric transducer (PZT) to control the length of the SRO cavity. The optical cavity length of the SRO was 569 mm. When the SRO was operated above the pump threshold, twin beams with two colors (1.5 and 3.3 \(\upmu\)m) were generated. A dichroic beam splitter M6, coated for high reflectivity at 1064 nm and high transmissivity at 3.3 \(\upmu\)m, was used to separate the two beams. The noise power of the signal at 1.5 \(\upmu\)m and the idler at 3.3 \(\upmu\)m was measured directly using the photodetectors (PD1 and PD2). The balance homodyne detection cannot be used because there were no suitable half-wave plates and polarization beam splitters for the 3.3 \(\upmu\)m laser due to conventional silica-based glass having a high absorption at 3.3 \(\upmu\)m. PD1 (ETX 500, Epitaxx) with a quantum efficiency of 90% was used to detect the signal at 1.5 \(\upmu\)m. The photodiode works in photoconductive mode. The saturation power of PD1 can be increased by increasing the reverse bias, and the gain can be improved by fine tuning the feedback resistor. The measured saturation power of PD1 was 26 mW, and the clearance between the SNL and the electronic noise level (ENL) was more than 10 dB at a frequency range of 2-20 MHz with the  
Figure 2: Schematic of the experimental setup used to generate two-color quantum correlated twin beams at 1.5 \(\upmu\)m and 3.3 \(\upmu\)m. OI: optical isolator; SRO: singly resonant optical parametric oscillator; PZT: piezo-electric transducer; PD: photodetector; SA: spectrum analyzer; EA: electronic attenuator; +/-: positive/negative power combiner; LIA: lock-in amplifier; PID: proportional–integral–derivative; HV: high voltage amplifier; +: adder. incident power of 4 mW. PD2 (Lms-36 PD-03-R, LED Microsensor NT) with a quantum efficiency of 60% was used to detect the idler at 3.3 \(\upmu\)m. The working mode of the photodiode is photovoltaic mode. In this mode, the photodiode was used under no reverse bias, which means that there is no dark current. A high gain, low noise, and broad bandwidth detector for the 3.3 \(\upmu\)m laser was designed by paralleling the feedback capacitor on the feedback resistor. The measured saturation power of PD2 was 8 mW, and the clearance between the SNL and ENL was more than 8 dB at a frequency range of 5-20 MHz with the incident power of 2 mW.