Interference of Short Optical Pulses from Independent Gain-Switched Laser Diodes for Quantum Secure Communications

Since the introduction of the decoy-state technique, phase-randomized weak coherent light pulses have been the key to increase the practicality of quantum-based communications. Their ultrafast generation was accomplished via compact gain-switched (GS) lasers, leading to high key rates in quantum key distribution (QKD). Recently, the question arose whether the same laser could be employed to achieve high-speed measurement-device-independent QKD, a scheme that promises long-haul quantum communications immune to all detector attacks. For that, a challenging high-visibility interference between independent picosecond optical pulses is required. Here, we answer the above question in the affirmative by demonstrating high-visibility interference from two independent GS lasers triggered at 1 GHz. The result is obtained through a careful characterization of the laser frequency chirp and time jitter. By relating these quantities to the interference visibility, we obtain a parameter-free verification of the experimental data and a numerical simulation of the achievable key rates. These findings are beneficial to other applications making use of GS lasers, including random number generation and standard QKD.