Fig. 2: Experimental setup of the four-user fully connected MDI-QN.

a An integrated silicon nitride (Si₃N₄) microring resonator generates the DKS comb via a dual-driven scheme involving two pump lasers and a feedback loop. The insets (b, c) are the optical spectrum and the electrical beat note of the 4-soliton state acquired by an optical spectrum analyzer (OSA) and an electrical spectrum analyzer (ESA), respectively. d Three comb lines separated by WDMs act as light sources for four users. The insets (e, f, g) are the optical spectra of selected comb lines measured by the OSA. A combination of three 1 × 4 couplers and four MUXs is used to route the light of different wavelengths to the users. Each user possesses an encoder consisting of two intensity modulators (IMs) and a phase modulator (PM) that generates time-bin encoded qubits. The following variable optical attenuators (VOAs) are used to attenuate the pulses to the single-photon level and simulate channel loss. h Encoded qubits with different wavelengths are routed to different BSM modules by DEMUXs at the untrusted relay server. All photons are detected by superconducting nanowire single-photon detectors (SNSPDs). See text for details.