## 5 Limitations and Challenges
### Future Challenges  
Although we successfully optimized the system's noise resistance, numerous practical challenges remain that demand careful attention and resolution:  
\(\bullet\)**High-performance requirements for components:** The stringent performance requirements for both the transmitter and receiver components are particularly noteworthy. To ensure signal integrity, modulators and photodetectors must meet the substantial bandwidth demands of 100 Gbaud operation. In our simulations, we investigated commercially available devices that can meet our parameter settings, such as the thin-film lithium niobate (TFLN) modulators reported in [36] and a 100 GHz balanced photodetector from Coherent [37]. However, beyond bandwidth considerations, additional critical specifications, including modulation rate and phase accuracy, impose rigorous constraints on compo  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Input OSNR** & **0** & **2** & **4** & **6** & **8** & **10** & **12** & **14** \\ \hline \(Q\) factor (dB) & 161.12 & 166.36 & 165.27 & 168.26 & 169.27 & 170.08 & 172.26 & 175.16 \\ \hline \(\log_{10}BER\) & \(-\infty\) & \(-\infty\) & \(-\infty\) & \(-\infty\) & \(-\infty\) & \(-\infty\) & \(-\infty\) & \(-\infty\) \\ \hline
**Input OSNR** & **16** & **18** & **20** & **22** & **24** & **26** & **28** & **30** \\ \hline \(Q\) factor (dB) & 177.66 & 180.15 & 182.45 & 184.25 & 185.31 & 185.76 & 185.91 & 185.94 \\ \hline \(\log_{10}BER\) & \(-\infty\) & \(-\infty\) & \(-\infty\) & \(-\infty\) & \(-\infty\) & \(-\infty\) & \(-\infty\) & \(-\infty\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Q factor and \(\log BER\) at different input OSNRs.  
nent selection. These demanding requirements inevitably lead to significantly elevated experimental costs.  
\(\bullet\)**High-precision requirements for measurement equipment:** In experiments, it is necessary to monitor signals at various nodes to optimize the parameters of system components. Since key elements such as the PSA module, frequency converter, AND gate, and noise-suppression structure are all implemented based on the four-wave mixing (FWM) effect, a high-precision optical spectrum analyzer (OSA) is essential for observing nonlinear effects. This imposes stringent requirements on the OSA's resolution and dynamic range. For example, the Yokogawa AQ6370E Telecom OSA [38] can be employed, offering a wavelength accuracy of 0.1 nm and a typical dynamic range of 78 dB. For the receiver, the oscilloscope must have a bandwidth of at least 70% of the symbol rate of the signal to fully capture signal details. Our investigation confirms that the Keysight UXR1104B Infiniium Oscilloscope [39], with a bandwidth of 110 GHz, could meet this requirement.  
\(\bullet\)**Design of Temporal and Phase Synchronization:** In practical implementations, precise temporal and phase synchronization of signals must be ensured. The PSA module imposes stringent phase-matching requirements between the input signal and the pump light. Additionally, the output signal from the PSA module and the local target sequence optical signal must maintain both temporal and phase synchronization to yield correct interference results. To address this, [PERSON] et al. proposed a phase-locked pumping method based on degenerate four-wave mixing (DFWM), ensuring phase synchronization for all PSA inputs [14]. To achieve phase synchronization between the input QPSK optical signal and the local target sequence optical signal, an optical phase-locked loop (OPLL), such as the Costas-loop-based OPLL proposed in [40] and the OPLL system implemented using an unbalanced delay interferometer in [41], is required. For the recirculating loop, achieving precise switching and delay control presents a critical challenge. Given that 100 Gbaud signals have a symbol duration of only 10 ps, picosecond-level timing accuracy in switching and delay alignment becomes a nontrivial engineering problem.