The polarization-encoded quantum key distribution system uses the polarization property of photons to realize secure key sharing. For fiber-based systems, however, the birefringence effect in single-mode fibers significantly affects the preservation of photon polarization states. Therefore, it is essential to carry out polarization compensation for polarization coding system. The current mainstream polarization compensation systems utilize Field Programmable Gate Array for data acquisition, calculation, and control. This method offers rapid and efficient performance but comes with high costs and complex designs. To address this problem, a polarization compensation system based on hardware equipment is proposed in this paper. The system not only reduces the cost of polarization compensation, but also is simple and easy to operate by using computer for data processing. To further demonstrate the feasibility of the system, a polarization compensation procedure based on a gradient algorithm is designed, with the error count rate of a single-photon detector serving as an evaluation parameter. In addition, this algorithm is applied to the aforementioned system for long-term and long-distance polarization monitoring. Over a period of approximately 24 hours, the average error count rate for a 20km transmission fiber is 0.59% and the average error rate for a 50km transmission fiber is 0.55%. These experimental results demonstrate that the system exhibits good polarization compensation performance and long running ability.
The polarization-encoded quantum key distribution system uses the polarization property of photons to realize secure key sharing. For fiber-based systems, however, the birefringence effect in single-mode fibers significantly affects the preservation of photon polarization states. Therefore, it is essential to carry out polarization compensation for polarization coding system. The current mainstream polarization compensation systems utilize Field Programmable Gate Array for data acquisition, calculation, and control. This method offers rapid and efficient performance but comes with high costs and complex designs. To address this problem, a polarization compensation system based on hardware equipment is proposed in this paper. The system not only reduces the cost of polarization compensation, but also is simple and easy to operate by using computer for data processing. To further demonstrate the feasibility of the system, a polarization compensation procedure based on a gradient algorithm is designed, with the error count rate of a single-photon detector serving as an evaluation parameter. In addition, this algorithm is applied to the aforementioned system for long-term and long-distance polarization monitoring. Over a period of approximately 24 hours, the average error count rate for a 20km transmission fiber is 0.59% and the average error rate for a 50km transmission fiber is 0.55%. These experimental results demonstrate that the system exhibits good polarization compensation performance and long running ability.
The physical effects such as random birefringence of fiber optic channels due to environmental influences make the optical signals transmitted in them have sensitive polarization variations, which seriously affects the performance of polarization biased code quantum key distribution systems. In this paper, a low-noise fiber channel wavelength division multiplexing real-time polarization compensation system is presented, where single photon counting is used as a feedback signal. The system can acquire the fiber channel polarization change information by detecting the photon counting of the conjugate reference light. In the system, the compensation algorithm is designed to control the electric polarization controller to calibrate the polarization state of the quantum signal light under the corresponding polarization base in real time, and the stable fiber channel polarization compensation is successfully achieved. In order to verify the effectiveness of the compensation system, a quantum key distribution test based on BB84 protocol with a transmission distance of 25.2 km is conducted, and stable test results of up to 8 hours are obtained in the laboratory environment and the simulated metropolitan area network buried fiber environment, with the average quantum bit error rate being 0.52% and 1.25%, respectively. The experimental results show that this system can guarantee the stable operation of polarization-encoded quantum key distribution in the buried fiber environment in urban areas.
In quantum key distribution (QKD) systems using polarization entanglement, channel polarization perturbations significantly compromise both efficiency and security. To address the slow response and instability of current entanglement-based polarization compensation techniques for entanglement-based quantum key distribution, we propose a polarization compensation scheme based on an adaptive gradient descent algorithm. The system's performance is tested over a 10 km single-sided fiber channel. Results show that this scheme maintains long-term stability of the polarization state under both random and periodic perturbations. The average compensation time is 248 ms, with an entanglement source average visibility of 83.43% and an average quantum error rate of 8.28%. This method effectively mitigates polarization perturbation in entanglement-based quantum key distribution (QKD). These results are pivotal for the development of long-distance, multi-node quantum communication networks.
Polarizations of single-photon pulses have been controlled with long-term stability of more than 10 hours by using an active feedback technique for auto-compensation of unpredictable polarization scrambling in long-distance fiber. Experimental tests of long-term operations in 50, 75 and 100 km fibers demonstrated that such a single-photon polarization control supported stable polarization encoding in long-distance fibers to facilitate stable one-way fiber system for polarization-encoded quantum key distribution, providing quantum bit error rates below the absolute security threshold.
Due to the fast-paced advancements of the quantum computing field, current public key cryptographic protocols need to be rethought. One promising solution is Quantum Key Distribution (QKD), whose security is robust to any computational advancement. The security of QKD relies on the laws of quantum mechanics, ensuring that any eavesdropper attempting to access the data will be detected. In particular, polarization-encoding QKD systems allow to generate secret keys in two remote locations to encode/decode information, using the State of Polarization (SOP) of single photons. Although its security has been theoretically proven, its practical implementation poses some challenges, part of which are the main focus of this PhD thesis. One relevant process in QKD systems is the synchronization between the transmitter and receiver, namely, the frame synchronization. This way, in this thesis, a frame synchronization method is developed and implemented, showing its potential for the use in prepare-and-measure QKD systems. The developed method has been shown to be effective, even with a low number of photons reaching the receiver. Furthermore, its architecture allows it to be adjusted for different attenuation regimes of the QKD system. The ability to encode data on the physical properties of single photons, in this case, polarization, highly depends on the capacity to accurately generate the required polarization states on demand. This way, an automatic SOP generation method, comprising an optical polarization monitoring system and a dedicated algorithm was developed and implemented, capable of generating four SOPs. This method reached an average Quantum Bit Error Rate (QBER) of 0.75%, which is well below the security threshold. After the SOP generation, and their transmission through the quantum channel, polarization alignment algorithms are required to compensate for the random rotations induced by birefringence in the optical fiber. In this thesis, a non-iterative polarization compensation method was implemented. This method requires only three QBER estimations to re-align the polarization, reaching average QBERs of 1.8%. Additionally, multiple iterative methods were implemented for comparison purposes, with the results clearly showing a better efficiency of the deterministic method. Beyond fiber-based transmissions, free-space transmissions were analysed due to their ability to reach sites without installed fiber and due to its potential for satellite-based quantum communications. This thesis demonstrates the feasibility of polarization-encoding QKD over free-space channels, by subjecting quantum signals to atmospheric turbulence to study its impact. In that sense, polarization variations over a 1.8 km outdoor free-space channel were monitored, to understand the effect of atmospheric turbulence on polarization. These results have shown that SOP variations in free-space links are slow, allowing their compensation. Furthermore, it was identified that, under conditions tested, the Secret Key Rate (SKR) could reach values up to 8.7×10ˉ⁴ bits per second, allowing QKD over free space for distances up to 2.31 km.
Secure communications have become a requirement for virtually all kind of applications. Currently, two distant parties can generate shared random secret keys by using public key cryptography. However, quantum computing represents one of the greatest threats for the finite complexity of the mathematics behind public key cryptography. In contrast, Quantum Key Distribution (QKD) relies on properties of quantum mechanics, which enables eavesdropping detection and guarantees the security of the key. Among QKD systems, polarization encoded QKD has been successfully tested in laboratory experiments and recently demonstrated in closed environments. The main drawback of QKD is its high cost, which comes, among others, from:i) the requirements for the quantum transmitters and receivers; andii) the need of carefully selecting the fibers supporting the quantum channel to minimize the environmental effects that could dramatically change the polarization state of photons. In this paper, we propose a Machine Learning (ML) -based polarization tracking and compensation that is able to keep shared secret key exchange to high rates even under large fiber stressing events. Exhaustive results using both synthetic and experimental data show remarkable performance, which can simplify the design of both quantum transmitter and receiver, as well as enable the use of aerial optical cables, thus reducing total QKD system cost.