A Mach–Zehnder modulator (MZM) is necessary for implementing a decoy-state protocol in a practical quantum key distribution (QKD) system. However, an MZM bias control method optimized for QKD systems has been missing to date. In this study, we propose an MZM bias control method using N (≥2) diagnostic pulses. The proposed method can be efficiently applied to a QKD system without any additional hardware such as light sources or detectors. Furthermore, it does not reduce the key rate significantly because it uses time slots allocated to existing decoy pulses. We conducted an experimental demonstration of the proposed method in a field-deployed 1 × 3 QKD network and a laboratory test. It is shown that our method can maintain the MZM extinction ratio stably over 20 dB (bit error rate ≤1%), even in an actual network environment for a significant period. Consequently, we achieved successful QKD performances.
A Mach–Zehnder modulator (MZM) is necessary for implementing a decoy-state protocol in a practical quantum key distribution (QKD) system. However, an MZM bias control method optimized for QKD systems has been missing to date. In this study, we propose an MZM bias control method using N (≥2) diagnostic pulses. The proposed method can be efficiently applied to a QKD system without any additional hardware such as light sources or detectors. Furthermore, it does not reduce the key rate significantly because it uses time slots allocated to existing decoy pulses. We conducted an experimental demonstration of the proposed method in a field-deployed 1 × 3 QKD network and a laboratory test. It is shown that our method can maintain the MZM extinction ratio stably over 20 dB (bit error rate ≤1%), even in an actual network environment for a significant period. Consequently, we achieved successful QKD performances.
Mach-Zehnder modulators (MZMs) are the most commonly used external modulators for optical communication links. The bias point of MZMs may drift due to environmental changes in practical applications, leading to system performance degradation. The latest decoy-state twin-field quantum key distribution (TF-QKD) system requires multiple modulators to generate multi-level light intensities and maintain an ultra-high extinction ratio between the signal-state light and vacuum-state light. This is essential to ensure the QKD security and extend the transmission distance. To meet this tight requirement, this paper presents an efficient bias control technique of multistage MZMs based on the dither signal method. Considering the complex signal processing algorithms involved in the dither signal method, this work designs a high-throughput hardware architecture based on field programmable gate array (FPGA) platform to realize real-time feedback control. It is comprised of three modules: the dither generation module, the fast Fourier transform (FFT) module, and the bias control module. Both static and dynamic extinction ratio tests were conducted to verify the performance of the system. Especially, this paper proposes an innovative and dynamic measurement method based on the highly sensitive single-photon detector to support the direct extinction ratio test of high-speed signals. The experimental results show that the system can stably maintain an ultra-high extinction ratio of 66.92 dB, demonstrating itself suitable for the majority of optical communication systems with very tight extinction ratio requirements. The feedback control time is about only 0.5 seconds per stage, which is fast enough to compensate for the usual environmental drift. In the temperature stability experiments, when the temperature varies between 26-64°C, the proposed system achieves a 74% reduction of the extinction ratio fluctuation compared to the result without using the feedback system.
Abstract Quantum key distribution (QKD) can help distant agents to share unconditional secret keys, andthe achievable secret key rate can be enhanced with the help of decoy-state protocol. To implementQKD experimentally, the agents are supposed to accurately transmit a number of different intensitypulses with the LiNbO3 based Mach-Zehnder (LNMZ) intensity modulator. However, the bias driftof LNMZ intensity modulator may affect the performance of a QKD system. In this letter, we reveala simple RC circuit model to demonstrate the bias drift in the LNMZ intensity modulator. Andbased on the model, we propose a multi-step bias stable scheme to control the bias working point.Experimental result shows that our scheme can eliminate the bias drift of at arbitrary working pointwithin a long time range. Besides, there is no need of any feedback mechanisms in the scheme. Thismeans our scheme will not lead to any increasement in system complexity, making it more suitablefor a QKD system.
In today’s world, a vast amount of sensitive information - from personal communications to financial transactions and government data - is constantly moving across platforms and networks. As a result, ensuring the security and privacy of this information has become a priority for individuals, organizations and governments alike. Quantum key distribution (QKD) offers a promising solution for secure communications, allowing the generation of identical keys between two entities separated in space. Although QKD systems have evolved considerably in recent years, there is still a need for optimization in various subsystems to make these technologies more efficient. One such area in need of improvement is the synchronization process, which is critical to the proper functioning of QKD systems. Taking this into account, this thesis investigates an effective synchronization technique with low computational complexity for QKD systems, aiming to strike a balance between efficiency and simplicity/cost, using the Qubit4Sync method as a model. The technique studied eliminates the need to prepare reference signals, as is the case with current systems operating in gated mode, and recovers the period using only the qubits similar to those exchanged during the QKD protocol. In this context, simulations were carried out using the Qubit4Sync method and the setup was adapted to work without the reference signal, with the detectors operating in free-running mode. The data from the setup used was processed and the simulation confirmed that once the dark counts are greater than the quibts detected, it becomes more difficult to recover the correct period. In the laboratory, the method was tested with various conditions of efficiency and dead-time of the single photon detectors. Due to the high optical noise of the channel, the recovery of the period was unsuccessful, as it was always equal to the value of the dead-time. After comparing the simulation with the practical results, it turned out that the noise in practice was higher than predicted. When investigating the setup, it was found that the extinction rate imposed by the Mach-Zehnder Modulator (MZM) was lower than expected. This may have contributed to the transmitter generating more optical noise than initially anticipated. Consequently, this factor may explain the practical results, since the detection rate of the detector is saturated at its dead-time value.
Quantum key distribution (QKD) is a technology that allows two users to exchange cryptographic keys securely. The decoy state technique enhances the technology, ensuring keys can be shared at high bit rates over long distances with information theoretic security. However, imperfections in the implementation, known as side-channels, threaten the perfect security of practical QKD protocols. Intensity modulators are required for high-rate decoy-state QKD systems, although these are unstable and can display a side channel where the intensity of a pulse is dependent on the previous pulse. Here we demonstrate the superior practicality of a tunable extinction ratio Sagnac-based intensity modulator (IM) for practical QKD systems. The ability to select low extinction ratios, alongside the immunity of Sagnac interferometers to DC drifts, ensures that random decoy state QKD patterns can be faithfully reproduced with the patterning effects mitigated. The inherent stability of Sagnac interferometers also ensures that the modulator output does not wander over time.
The performance of a practical quantum key distribution (QKD) system is often limited by the multi-photon state emission of its source and the dark counts of its detectors. Here, we present two methods to improve its performance. The first method is decoy state QKD: the sender randomly sends out weak coherent states with various average photon numbers (which are named as signal state and decoy states). In [14,15], we have performed the first experimental implementation of decoy state QKD over 15km and 60km respectively, thus dramatically increasing the distance and secure key generation rate of practical QKD systems. Our work has been followed up by many research groups worldwide [16-18]. The second scheme is QKD with "dual detectors" [19]: the legitimate receiver randomly uses either a fast (but noisy) detector or a quiet (but slow) detector to measure the incoming quantum signals. The measurement results from the quiet detector can be used to upper bound the eavesdropper's information, while the measurement results from the fast detector are used to generate a secure key. We applied this idea to various QKD protocols. Simulation results demonstrated significant improvements in both BB84 protocol with ideal single photon source and Gaussian-modulated coherent states protocol.
Passive decoy-state quantum key distribution is more desirable than the active one in some scenarios. It is also affected by the imperfections of the devices. In this paper, the influence of modulator attenuation on the passive decoy-state method is considered. We introduce and analyze the unbalanced Mach–Zehnder interferometer, briefly, and combining with the virtual source and imaginary unitary transformation, we characterize the passive decoy-state method using a weak coherent photon source with modulator attenuation. According to the attenuation parameter δ , the pass efficiencies are given. Then, the key generation rate can be acquired. From numerical simulations, it can be seen that modulator attenuation has a nonnegligible influence on the performance of passive-state QKD protocol. Based on the research, the analysis method of virtual source and imaginary unitary transformation are preferred in analyzing passive decoy state protocol, and the passive decoy-state method is better than the active one and is close to the active vacuum + weak decoy state under the condition of having the same modulator attenuation.