Passive state BB84 for last mile quantum key distribution networks

The complexity of QKD devices is one of the barriers to the scalability of quantum networks. One of the factors is the preparation of random states, which involves quantum state measurement in the quantum random number generator and then the use of active light modulation to prepare QKD states. The alternative approach is passive state preparation. It was proposed in 2010 [1] and recently investigated for security aspects [2]. The idea is to use the natural phase randomness of the laser pulses to prepare random states. This approach can help to solve the security problem of correlating the state modulation voltage. Originally, the focus was on the preparation of the polarization state. In this work, we use a laser that generates random phase pairs of consecutive pulses as a ready-to-use qubit. This allows us to simplify the Alice device. To know the sent state we slit signal and perform tomography where we post-select four BB84 states. We experimentally demonstrate passive state QKD on 10 km deployed fiber. [1] M Curty, et al. Physical Review A 82.5 (2010): 052325 [2] W Wang, et al. arxiv.org/abs/2207.05916