Fig. 1. The experimental setup of the HOM interference measurement system. (a) Two independent MRRs are fabricated on a silicon nitride chip and generate DKS combs simultaneously. (b) Two sets of 10 comb lines are filtered using DWDM fiber filters and circulators. (c) Each comb line possesses an encoder consisting of an IM that generates a short-time-domain pulse. The following VOA is used to attenuate the pulses to the single-photon level, and the two-photon HOM interference is measured by SNSPDs. (d) Setup of the SSB modulator section: A voltage source, which is constantly loaded with 12 V, is used as the power supply of the modulator. In order to achieve high carrier extinction, we use a single-frequency laser with a central wavelength of CH19 as an auxiliary light source to lock the working point of the modulator. The microwave source is connected to one of the radio freqency (RF) ports (RF-up) to load the frequency-shifted signal, and the other terminal (RF-down) is connected to a 50 Ω load to prevent reflection. After sideband modulation, the signal light is divided by a CH19 WDM filter to isolate the interference of the auxiliary light. The generation and monitoring parts of DKS combs have been set in Lab 1, and the HOM interference part is set in Lab 2. FPC, fiber polarization controller; EDFA, erbium-doped optical amplifier; WDM, 200 GHz wavelength division multiplexing filter; PM, power meter; PD, power detector; CIR, three-port circulator; OSA, optical spectral analyzer; AWG, arrayed waveguide grating; IM, intensity modulator; VOA, variable optical attenuator; FPBS, fiber polarization beam splitter; PMBS, polarizaztion-maintaining beam splitter; SSB, single-sideband modulator; SNSPD, superconducting nanowire single-photon detector.

Fig. 2. (a) Light coupling and packaging using 32-channel ultra-high numerical aperture (UHNA7) fiber arrays. (b) Silicon nitride chip under optical microscope. (c) The transmission spectra of two independent microcavities. The red circles represent the loaded quality factors derived from each near-cold resonances, and the green hollow circles represent the location of the main pump laser, respectively. The middle illustration represents the intrinsic quality factor data for the main pump of two different microcavities. For Cavity 1 (Cavity 2), the intrinsic quality factor is about 4.1 (4.8) × 10⁶. (d) The simultaneous generation of two DKS combs. The comb power of two DKS combs is recorded with the high-precision frequency detuning of the pump light. Two DKS combs can be produced when the frequency of the main pump reaches the pink region, respectively.

Fig. 3. HOM interference visibility diagram of signal light after alignment. (a) The two-photon HOM interference data of the CH44. The visibility is estimated to be 46.72% ± 0.63%. (b) and (c) The spectra of the aligned 10 comb lines of the two independent DKS combs. (d) The HOM interference visibility data of 10 alignment comb lines from CH38 to CH47.