# Free-Space QKD with Modulating Retroreflectors Based on the B92 Protocol
## 4 Discussion  
In the previous sections, we analyzed our MRR-QKD scheme with the B92 protocol and compared it with RM2018. The results are listed in Table 2. Our scheme requires only three MQW modulators, which is significantly less than the eight in RM2018. Two of them are used for polarization modulation and the other is used for decoy state generation. Based on the superposition principle of perpendicular polarization states, we make full use of each beam to solve the problem of extra QBER on account of the finite extinction ratio. This could effectively improve the performance of the MRR-QKD system. Although the protocol efficiency was only half that of RM2018, our scheme could reach a much higher SKR, while having a high optical misalignment error or low extinction ratio.  
These two systems both modulate the polarization of the beam by means of intensity modulation. The modulation rate of the MQW intensity modulator can be up to 100 MHz [16]. The low extinction ratio of the MQW will cause higher QBER in RM2018. The protocol efficiency in our system is half that in RM2018 because of the principle of the modulation method. In the ideal circumstance of the infinit  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Items** & **RM2018** & **Our Scheme** & **Condition** \\ \hline Complexity & high & low & \\ \hline Multiple quantum wells & 8 & 3 & \\ \hline Protocol & BB84 & B92 & \\ \hline \multirow{4}{*}{SKR 1  } & \(3.21\times 10^{-5}\) & \(2.65\times 10^{-5}\) & \(r=22.5\) dB \\  & \(1.71\times 10^{-5}\) & \(2.65\times 10^{-5}\) & \(r=20\) dB \\  & \(0\) & \(2.64\times 10^{-5}\) & \(r=17.5\) dB \\  & \(0\) & \(2.61\times 10^{-5}\) & \(r=15\) dB \\ \hline \multirow{2}{*}{SKR 2  } & \(8.05\times 10^{-5}\) & \(5.44\times 10^{-5}\) & \(e_{detect}=1\)\% \\  & \(3.79\times 10^{-5}\) & \(3.00\times 10^{-5}\) & \(e_{detect}=3\)\% \\  & \(1.50\times 10^{-6}\) & \(9.69\times 10^{-6}\) & \(e_{detect}=5\)\% \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of our scheme and RM2018 [16].  
Figure 5: The relationship between the extinction ratio and the maximal tolerance of misalignment-error probability \(e_{detect}^{\text{II}}\) in transmission probability of 100%.  
performance of RM2018 is better than ours. But if the MQW intensity modulator for polarization modulation are replaced by phase modulators, it is possible to apply the BB84 protocol in our system [30]. Thus, we can achieve the same performance as RM2018 with smaller cost. The decoding system of the B92 protocol in our system is the same as that of RM2018 and very close to the classical free-space QKD system based on the BB84 protocol [6; 14; 31]. This means that it is convenient to fit our system into the current free-space QKD network [31; 32].  
The imperfection of the device, such as misalignments and detection efficiency mismatch, will allow eavesdroppers to attack in side channels. These flaws have been proved not affecting the security of the system within several counter-measures(temporal, spatial and frequency filtering and so on) [33]. The signal in the MRR-QKD system makes a round-trip through atmospheric channels. The transmission process is similar to the \"plug-play\" system in fiber channels [34]. The source of such two-way QKD systems is untrusted; its security has been proven [35; 36]. Unlike the \"plug-play\" system, the MRR-QKD system based on free-space channels will be subject to the off-axis attack. This attack can be resisted by monitoring the incident angle of the interrogation beam and using modulators with pixel units to code beams with authentication. Therefore, those modulators incapable of pixelated modulation are not suitable to be used in MRR-QKD system, like AOMs modulators with comparable modulation rate and higher extinction ratio [37]. Other attacks, like photon number splitting, intercept-resend and on-axis attacks, are also explicated [16].