Asymmetric 4+2 protocol for quantum key distribution with finite resources

In this paper, we present an asymmetric "4+2" protocol for quantum key distribution with finite photon pulses. The main work of this paper focuses on the composable security proof for this protocol in a finite-key scenario. Based on the essence security basis of the original "4+2" protocol proposed by Huttner et al. (Phys Rev A 51(3):1863---1869, 1995), we first develop the squashing model for this protocol with the quantum non-demolition measure theory. From this model, against the collective photon-number-splitting attack, we then provide the security proof (formulas of finite-key security bounds) for this protocol. The expected performance of this protocol are also evaluated on a priori reasonable expected values of parameters. Our work shows that the performance we derived is the lower one and it can cover long distances in the lossy channel.