## 2 Experimental Setup  
The experimental setup is shown in Figure 1. The 795 nm input light comes from an external cavity diode laser (ECDL). Its frequency is locked to the saturated absorption line of rubidium atom. A seed laser beam from another ECDL at 1560 nm is amplified by an Erbium-doped fiber amplifier, and then it is used as the pumping field for the DFG. The frequency of the 1560 nm ECDL is stabilized by locking it to a reference cavity made of ultra-low expansion (ULE) glass. A band-pass filter BF1 centered at 1560 nm with a bandwidth of 12 nm is used to reduce the amplified spontaneous emission noise generated by the amplifier. The light pulse at 795 nm is produced by an acousto-optic modulator AOM1 (Gooch & Housego 3200-124, rise time 15 ns) which is arranged in double passage configuration. AOM1 is driven by an RF source (MODA200-B4-33). The output power of the RF source can be modulated by external analog signal, which drives AOM1 to generate analog light pulse. This configuration allows the generation of Gaussian light pulses with a FWHM of 30 ns. A set of neutral density filters are employed to acquire the light pulse containing several photons. The total transmittance of the set of neutral density filters \(T_{att}=4.3\times 10^{-6}\). The power distribution of Gaussian pulse generated by AOM1 can be expressed as \(f(t)\)\(=A\)Exp\([-\frac{4\ln 2}{\Delta_{\text{FWHM}}^{2}}t^{2}]\), where \(A\) is the peak power, \(\Delta_{\text{FWHM}}\) is the FWHM of light pulse. After the set of neutral density filters, the power distribution of light pulse is attenuated to \(T_{att}f(t)\). By integrating the power distribution, we can obtain the energy contained in the pulse \(\frac{A\tau_{att}\Delta_{\text{FWHM}}}{2}\sqrt{\frac{\pi}{\ln 2}}\). The average photon number contained in the pulse \(\mu_{in}=\frac{A\tau_{att}\Delta_{\text{FWHM}}}{2\ln\
u}\sqrt{\frac{\pi}{\ln 2}}\), where \(\hbar\
u\) is the energy of a photon, and \(\
u\) is the photon frequency corresponding to 795 nm. For a fixed \(\Delta_{\text{FWHM}}\), A specific \(\mu_{in}\) can be obtained by adjusting the peak power of light pulse. We use a fiber-coupled PPLN waveguide module (HP Photonics) for DFG process, which includes a 2 \(\times\) 1 combiner and a waveguide module. The input light at 795 nm and the pumping light at 1560 nm are set to vertical polarization by half wave-plates HWP1 and HWP2, respectively, and then are guided to the 2 \(\times\) 1 combiner through polarization-maintaining fibers (PMF). The 2 \(\times\) 1 combiner couples the two light beams into a single output fiber which is fused to the input of the waveguide. The transmission of the 2 \(\times\) 1 combiner for 795 nm and 1560 nm beams are 67% and 60%, respectively. The input coupling efficiency of the waveguide is 55% for both beams. The waveguide is 19 mm long and heated to 58 \({}^{\circ}\)C to produce effective nonlinear process. The 1621 nm converted light from the waveguide is outputted through a PMF with coupling efficiency of 65%. For quantum frequency conversion, the converted field at 1621 nm needs to be isolated from the noise. The noise of the down-conversion process mainly comes from: leakage of the strong pumping light at 1560 nm, non-converted 795 nm input field, SRS noise by strong pump, and a light at 780 nm created by weakly phase matched second harmonic generation (SHG) of the strong pump. After the waveguide module, a dichroic mirror DM1 (82% transmission) is used to remove the leaked 1560 nm pumping light, which suppresses  
Figure 1: Experimental setup for frequency down conversion. ECDL: external cavity diode laser. AMP: fiber amplifier. SAS: saturated absorption spectroscopy. AOM1-3: acousto-optic modulators. ND: neutral density filter. HWP: half waveplate. BF1-3: band-pass filters. DM1-2: dichroic mirrors. E1-2: etalons. PBS1-2: polarization beam splitters. Circ. : fiber circulator. FBG: fiber Bragg grating. SPD: single photon detector. EOM: electro-optic modulator. D1-2: photodetectors. BS: beam splitter. HR: high reflectivity mirror.