# Practical Real-Time Phase Drift Compensation Scheme for Quantum Communication Systems
## 1 Introduction  
Quantum communication [1, 2] is a field dedicated to achieving unconditional security between two legitimate parties, namely, [PERSON] and [PERSON]. Over the years, significant advancements have been made in both theoretical and experimental aspects [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16], garnering considerable attention from diverse disciplines. Notably, the introduction of commercial single-photon level applications of quantum physics [17] has underscored the growing significance of practical quantum communication systems in real-world scenarios. However, it is crucial to acknowledge that real-world application environments are considerably more intricate and diverse compared to controlled laboratory settings. These complexities have the potential to impact system operations and even lead to disruptions.  
Coupling with environment will adversely influence the performance of quantum systems [17, 18]. Unlike laboratory environments where temperature control is commonly achieved through air conditioning and optical fiber channels are carefully arranged, the field environment lacks specialized environmental control measures and most fiber channels are installed on-site. The impact of field environments on quantum communication systems can be broadly categorized into two main aspects: polarization and phase drift. Polarization issues primarily arise from channel disturbances in installed fiber channels, while phase drift is often caused by interferometer drift. In phase-encoding quantum communication systems, polarization-related challenges can be effectively mitigated through ingenious system architecture design [4, 19, 20], in which long-term stability can be achieved without the need for polarization compensations. However, addressing phase drift requires careful consideration of operational conditions. Environmental isolation is one possible countermeasure, which reduces the speed of phase drift but increases systemcomplexity [21]. Another popular approach is the active feedback scheme, which has received significant attention. These schemes involve techniques such as using additional reference light for active alignment or performing pre-transmission phase drift parameter scanning [10]. However, such schemes often lead to a reduction in transmission efficiency. Machine learning techniques [22] have also been employed to predict phase drift. However, a substantial amount of training data must be collected beforehand for accurate predictions. More recently, efficient schemes utilizing mismatched data [23] for calibration have emerged, although they still require additional data transmission.  
In conclusion, phase drift in field environments poses significant challenges for phase-coding quantum communication systems. While innovative design approaches and active feedback schemes have been explored, further research is needed to enhance transmission efficiency and develop robust solutions for real-world applications. Here, we propose a practical real-time calibration scheme for phase tracking. In our scheme, the system can continuously run without any additional transmission efficiency reduction or information exchange process by a finely designed closed-loop control algorithm, which is validated in our subsequent experiment. Experimental results show that the phase-coding quantum communication system with our phase tracking scheme can be operated stably and continuously for 16 h with an average quantum bit error rate (QBER) of 1.60% and a standard deviation of 0.0583%.