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Quantum key distribution (QKD) provides an attractive solution for secure communication. However, channel disturbance severely limits its application when a QKD system is transfered from the laboratory to the field. Here, a high-speed Faraday-Sagnac-Michelson QKD system is proposed that can automatically compensate for the channel polarization disturbance, which largely avoids the intermittency limitations of environment mutation. Over a 50-km fiber channel with 30-Hz polarization scrambling, the practicality of this phase-coding QKD system was characterized with an interference fringe visibility of 99.35% over 24 hours, and a stable secure key rate of 306k bits/s over 7 days without active polarization alignment.Quantum key distribution (QKD) allows two remote parties (named Alice and Bob) to share secure keys in the presence of an eavesdropper. Its unconditional security is guaranteed by the fundamental laws of quantum physics. As the first commercial application of quantum physics at the single-quantum level [1], the practicality of QKD becomes important when we transfer a QKD system from the laboratory to the field. Contrary to laboratory conditions, field environments are complex and volatile, which would continually interrupt or even terminate the operation of QKD [2-9], and would severely limit its application.In QKD systems deployed over telecom fiber networks, the field environments typically break down into two parts: operating conditions of QKD units, and environments of installed fiber channels that are used to transmit quantum states. By careful design, the operating conditions of QKD units can be well controlled, i.e., isolated from complex environments. While, the environments of installed fiber channels are uncontrollable and ever-changing, quantum states are inevitably disturbed during transmission in fiber channels. More crucially, channel disturbance induced by the external environment accumulates with distance [10]. Due to volatile field environments, variations in optical path length (or time * weich@ustc.edu.cn † yinzq@ustc.edu.cn delay) and fiber channel birefringence [7][8][9][10] are two obstacles preventing long-term continuous operation of QKD systems. However, optical path length variations in fiber channels are relatively slow and could be well tracked [8] or compensated [7] with simple synchronization schemes. Thus, birefringence variations in fiber channels become the most challenging channel disturbance in practical QKD systems. Channel disturbance countermeasures in QKD systems can be classified into active and passive categories. In QKD systems employing active countermeasures, polarization basis alignment modules are added to compensate for birefringence variations in fiber channels [8][9][10] and are essential for low quantum bit error rate (QBER). In QKD systems adopting passive countermeasures, birefringence variations in fiber channels can be automatically compensated, which largely avoid the intermittency limitations of environment mutation. The âĂIJPlug-and-PlayâĂİ system ...

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