## 2 Experimental Setup
PMF: polarization-maintaining fiber. SMF: single mode fiber. FC: fiber collimator. the pumping light by >25 dB. To block the non-converted input light at 795 nm and the light at 780 nm created by SHG, we use a band-pass filter BF2 (72% transmission, bandwidth of 12 nm), its optical density is \(\sim\)6 in the wavelength range of 780-795 nm. The SRS noise typically covers over a region of several hundreds of nanometers, which seriously degrades the fidelity of quantum frequency conversion [23]. We design a three-stage filtering system to reduce the noise level by narrowing its bandwidth. After BF2 the converted light passes through two etalons E1 and E2, a F-P cavity and a FBG in turn. The FBG (1 nm bandwidth) works together with a fiber circulator, as shown in Figure 1. The BF2 and FBG form the first-stage filter, in which BF2 narrows the noise bandwidth to 12 nm, and FBG further narrows it to 1 nm. Considering the coupling efficiency of the input fiber and the insertion loss of the circulator, the measured whole efficiency of FBG is 0.43, that is, the ratio of output power of the circulator to the input power in front of the input fiber collimator. The efficiency of the first-stage filter is 31%, which is the product of the transmission of BF2 and FBG. The second-stage filter consists of the two etalons E1 and E2 with finesse of 30. E1 and E2, made of fused quartz with refractive index of 1.44, are 11.7 mm and 4.3 mm long, respectively. For E1 (E2), the free spectral range is 8.9 GHz (24.2 GHz) and the measured linewidth is 284 MHz (770 MHz). They are both resonant with the down-converted light field by adjusting their temperatures. Figure 2a shows the theoretical transmission spectra of E1 and E2 as a function of the frequency detuning from down-converted field. In the wavelength range of 1 nm which corresponds to the frequency detuning from \(-\)55 GHz to 55 GHz, their transmission peaks only coincide at the converted frequency, while other transmission peaks are all staggered. The spectral function of the second-stage filter depends on the product of the transmission spectra of E1 and E2, as shown by the green solid line in Figure 2a. It is obvious that the noise is suppressed at all frequencies except for a narrow region centered at the converted frequency. After the first- and second-stage filters, the noise bandwidth (FWHM) is narrowed to 254 MHz. The transmission of a single etalon is 0.92, and the efficiency of the second-stage filter is 0.84. Figure 2: **(a)** The theoretical transmission spectrum of etalons E1 (blue dotted) and E2 (green dash). The red solid line represents the spectral function of the second-stage filter. **(b)** The spectral function of the F-P cavity (blue dotted) and the second-stage filter (green dash). The red solid line represents the spectral function of the three-stage filtering system. The third-stage filter is the confocal F-P cavity with standing wave configuration. Its free spectral range and linewidth are measured to be 336 MHz and 11.7 MHz, respectively. Therefore, the finesse of the cavity is 28.7. A locking beam at 1560 nm is injected into the F-P cavity from polarization beam splitter PBS1 to lock the cavity length to be resonant with the converted frequency. The 1560 nm locking light and the pumping light are derived from the same ECDL master oscillator. We use an electro-optic modulator (EOM) to produce phase-modulation sidebands and lock the cavity by Pound-Drever-Hall technology [45]. The transmitted locking beam from the cavity is steered to detector D2 by PBS2. To make sure that the cavity resonates with the 1621 nm converted light and the 1560 nm locking beam at the same time, two acousto-optic modulators AOM2 and AOM3 are employed to adjust the frequency of the locking beam. Figure 2b shows the transmission spectra of the F-P cavity (blue dotted line) and the second-stage filter (red dashed line).