Fig. 4

Experimental setup to implement the RRDPS protocol with L = 3. CW Continuous Wave, IM Intensity Modulator, PM Phase Modulator, VA Variable Attenuator. At Alice’s site, a pulse train with a repetition rate of 1 GHz is generated by modulating a 1550.12 nm CW laser using IM₁. Every 3 pulses (L = 3) are defined as one packet. The intensity of each packet is randomly modulated by IM₂ to prepare signal, decoy and vacuum states. PM₁ adds phase − π/2 or π/2 on each pulse to encode the key bits, and PM₂ adds a random global phase on each packet. The VA is used to adjust the average photon-number per pulse. At Bob’s site, through a 1 × 2 beam-splitter, the incoming signal is randomly coupled into one of two unbalanced Faraday–Michelson interferometers (FMIs) with 1 and 2 ns temporal delays. Each output of the FMIs is led to an SPD. Finally, the detection events are recorded to extract key bits