Experimental setup. In contrast with the SLAZ2013 protocol, an iterative module in the dashed box is introduced to replace the original inner cycle. Here, BS_i stands for a beam splitter, and D_3(i) denotes a photon detector for i = 1, 2, …, N. Bob uses a switch (SW) to carry out the blocking operation. SPR and SM stand for switchable polarization rotator and switchable mirror, respectively. Other notations: C is a circulator; D stands for the single-photon detector; S is the single-photon source; PBS stands for the polarizing beam splitter which only reflects vertically polarized photons; OD and MR stand for the optical delay and Mirror, respectively. In real implementations, the MR could be replaced by the Faraday mirror (FM) to offset the side effects induced by the noises and birefringence. The settings of PBS₀, PBS₁, SPR, and SM are the same as those in Ref. [17]. For instance, we have the rotating angle of SPR as β_M = θ/2 = π/4M.