Practical reference-frame-independent quantum key distribution systems against the worst relative rotation of reference frames

Reference-frame-independent quantum key distribution (RFI-QKD) can generate secret keys with the slow drift of reference frames. However, the performance of practical RFI-QKD systems deteriorates with the increasing drift of reference frames. In this paper, we mathematically demonstrate the worst relative rotation of reference frames for practical RFI-QKD systems, and investigate the corresponding performance with optimized system parameters. Simulation results show that practical RFI-QKD systems can achieve quite good performance against the worst relative rotation of reference frames, which exhibit the feasibility of practical QKD systems with free drifting reference frames. Furthermore, we propose a universal estimation method of the secret key rate in practical RFI-QKD systems, which conforms to the nature of RFI-QKD more well than the usual estimation method.