Fig. 1. Experimental setup. A mode-locked optical fiber laser generates 500 fs pulses at 1551 nm with a repetition of 59.98 MHz and is used to generate the two chirped broadband single-photon-level coherent states based on FBG1 and FBG2. The two chirped broadband coherent states are combined via a 50∶50 single-mode beam splitter (SBS) and directed to a 5.2 cm long fiber pigtailed Type-0 PPLN-WG chip. The total losses in the WG are 2.2 dB, such as a coupling loss of about 0.7 dB and a total fiber-to-output-facet loss of approximately 1.5 dB. The unconverted photons are deterministically separated from the SFG photons by an IF, and the SFG photons are sent to a single photon detector (silicon APD). The entire experiment is fiber-coupled.

Fig. 2. (a) Pump and signal light spectrums. (b) Spectrums of upconverted light (SFG and SHG): SHG of signal light; SHG of pump light.

Fig. 3. SFG efficiency. SFG efficiency can be tuned by manipulating the PPLN-WG chip’s temperature.

Fig. 4. SFG efficiency and SFG photons that the pump photons are upconverted when interacting with the signal photons inside the PPLN-WG chip, plotted against the number of photons per pulse (equal for signal and pump). Dark counts of 3.8 Hz have been subtracted, and the total losses of about 5.6 dB have been taken into account.