Fig. 1. (a) Components inside the energy-time entangled biphoton source; (b) integrated all-in-one machine and appearance dimensions; (c) setup for monitoring the CW pump laser coupled into the PPLN waveguide; (d) setup for coincidence measurement and evaluating the indistinguishability of the biphotons with HOM interference. DBR, distributed Bragg reflector laser; VOA, variable optical attenuator; PPLN, periodically polarized lithium niobate waveguide; WDM, wavelength division multiplexer; BP, bandpass filter; PBS, polarization beam splitter; HWP, half-wave plate; FC, fiber coupler; OPM, optical power meter; MDL, motorized optical delay line; ODL, manual optical delay line; BS, 50/50 beam splitter; PC, polarization controller; SNSPD, superconducting nanowire single-photon detector; PH, PicoHarp 300.

Fig. 2. Comparison of coupled optical power into the PPLN waveguide with two different coupling structures of freespace to fiber (black circles) and all fiber (red triangles).

Fig. 3. (a) Detected photon count rates for different pump powers; (b) CAR for different pump powers; inset, typical coincidence counts at a pump power of 150 µW; (c) calculated generation rate of photon pairs for different pump powers.

Fig. 4. (a) Measurement of HOM interferograms of unfiltered photons (red dots) and photons with 5 nm BP (black dots); (b) measured single-photon spectral distributions of the signal (red dots) and the idler photons (black dots) that were obtained by scanning the center wavelength of the programmable optical BP (Finisar WaveShaper 4000, 0.08 nm bandwidth) placed in corresponding path, respectively.

Fig. 5. (a) Long-term stability of CAR and (b) HOM interference visibility in the period of more than 10 h at the pump power of 60 µW.