# In-Field Quantum-Protected Control-Based Key Distribution with a Lossy Urban Fiber Link
## 6 Discussion  
In this study, we have experimentally realized QCKD using the real-world fiber infrastructure. The security of the protocol is ensured by the appropriate system configuration based on the information theoretic analysis. The presented experimental results show that the precision of the loss control and the dynamics of losses within a real-life telecommunication line allow for the secret key rate of 490 bit/s on average. With that, the presented statistical analysis of the resulting key underscores its high quality for secure communication. Our results show the practical way of deploying quantum-resistant communications through adopting the existing fiber infrastructure.  
The study shows that QCKD remains secure even in the presence of high-loss connectors within the transmission line. Thus, we expect that the system would tolerate the introduction of additional users through imperfect optical switches, as proposed in Ref. [22]. To this end, we further plan to demonstrate a multi-node implementation of the QCKD within a single telecommunication network. For the next development, we are working on several hard- and software enhancements for improving key rates. The input data rate will be increased by expanding FPGA bandwidth, by increasing the bit density in the transmitted packet, and by implementing several QRNGs in parallel on Alice's side. Improving the bit shape will result in the lower BLR. Furthermore, the new software architecture will enable postprocessing to be performed in parallel with signal transmission, and updated postselection and error correction algorithms will speed up the system. In addition, implementing our own high-frequency OTDR monitoring with modulated pulses will make the system even more robust against interceptors. There is also existing another approach to quantum communication such as quantum secure direct communication (QSDC) [35]. The QSDC differs from the QKD by the directly transmitting information encoded in quantum states, rather than the exchanging quantum-encoded key for later encryption. The possible developments will be addressed in our subsequent publications.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Statistical Test** & _p-Value_ & **Proportion** & **Result** \\ \hline Frequency & 0.64 & 1.0 & SUCCESS \\ Block frequency & 0.83 & 1.0 & SUCCESS \\ Cumulative sums & 0.16 & 1.0 & SUCCESS \\ Runs & 0.44 & 1.0 & SUCCESS \\ Longest run & 0.28 & 0.9 & SUCCESS \\ Rank & 0.83 & 1.0 & SUCCESS \\ FFT & 0.44 & 1.0 & SUCCESS \\ Non-overlapping template & 0.06 \(\div\) 0.98 & 0.9 \(\div\) 1.0 & SUCCESS \\ Overlapping template & 0.28 & 0.9 & SUCCESS \\ Universal & 0.16 & 1.0 & SUCCESS \\ Approximate entropy & 0.16 & 1.0 & SUCCESS \\ Random excursions & 0.14 \(\div\) 0.99 & 1.0 & SUCCESS \\ Random excursions variant & 0.06 \(\div\) 0.98 & 1.0 & SUCCESS \\ Serial & 0.83 \(\div\) 0.96 & 1.0 & SUCCESS \\ Linear complexity & 0.96 & 1.0 & SUCCESS \\ \hline \hline \end{tabular}
\end{table}
Table 1: Results of NIST statistical tests for randomness. For each test in the NIST statistical test suite, the table reports the test name, the corresponding _p_-value, the proportion of sequences that passed (i.e., the fraction of sequences with _p_-values at or above the significance level), and the overall pass/fail verdict. The proportion is evaluated against an expected confidence interval—computed using a normal approximation to the binomial distribution for large sample sizes—to determine if the randomness criteria are met.