In this work, a reliable and low computational õ complexity frame synchronization method for quantum key distribution (QKD) systems is proposed. The proposed method is based on the computation of the quantum bit error rate (QBER) and requires no more hardware than the one used for qubit coding and decoding. In a initial stage, pre-shared and received sequences are shifted until a minimum QBER value is reached. In a second stage, to confirm that the frames are correctly positioned, and to ensure that during key transmission a correct association of each received bit is made with the corresponding bit sent by the transmitter, a pre-determined number of inverted frames is sent, rising the QBER value to maximum values. As the number of inverted frames is a pre-agreed number, after detecting the first inverted frame it is possible to determine exactly when the secret key transmission will start. We use a polarization-encoding QKD testbed to assess the overall performance of the method. With the proposed frame synchronization method, the synchronization is achieved after roughly 104clock cycles, despite the concerns imposed by the low probability detection inherent to the QKD systems.
In this work, a reliable and low computational õ complexity frame synchronization method for quantum key distribution (QKD) systems is proposed. The proposed method is based on the computation of the quantum bit error rate (QBER) and requires no more hardware than the one used for qubit coding and decoding. In a initial stage, pre-shared and received sequences are shifted until a minimum QBER value is reached. In a second stage, to confirm that the frames are correctly positioned, and to ensure that during key transmission a correct association of each received bit is made with the corresponding bit sent by the transmitter, a pre-determined number of inverted frames is sent, rising the QBER value to maximum values. As the number of inverted frames is a pre-agreed number, after detecting the first inverted frame it is possible to determine exactly when the secret key transmission will start. We use a polarization-encoding QKD testbed to assess the overall performance of the method. With the proposed frame synchronization method, the synchronization is achieved after roughly 104clock cycles, despite the concerns imposed by the low probability detection inherent to the QKD systems.
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.
Abstract We present the design and implementation of a state-of-polarization (SOP) management technique and two efficient synchronizing methods for quantum key distribution (QKD) systems. This is achieved following a wavelength-division multiplexing approach, where the classical synchronization signal and the quantum states are propagated in the same optical fiber. The employed frame synchronization method is based on the monitoring of the quantum bit error ratio (QBER) of the quantum channel, thus avoiding additional hardware and high computational resources. We evaluate the operation of SOP generation method through the assessment of the individual response of the waveplates that comprise the employed electronic polarization controller. Finally, the performance was assessed by computing the overall QBER and the QBER contributions of each of the four polarization states associated with the different qubits. The measurements, obtained during six hours, show a slight variation of the QBER values associated with the individual contributions, reaching an overall QBER of 0.75%. This demonstrates the capability of the presented methods to operate, stably, with very low QBER values, making its application in practical QKD systems reliable.
Quantum key distribution (QKD) is a cryptographic technique that uses quantum mechanical principles to enable secure key distribution, offering information-theoretic security guaranteed by physical laws. Practical deployment of QKD requires robust, cost-effective systems that can operate in challenging field environments. A major challenge is achieving reliable clock synchronization without adding hardware complexity. Conventional approaches often use separate classical light signals, which increase costs and introduce noise that degrades quantum channel performance. To address this limitation, we demonstrate a QKD system incorporating a recently proposed qubit-based distributed frame synchronization method, deployed over a metropolitan fiber network in Nanning, China. Using the polarization-encoded one-decoy-state BB84 protocol and the recently proposed qubit-based distributed frame synchronization method, our system achieves synchronization directly from the quantum signal, eliminating the need for dedicated synchronization hardware. Furthermore, to counteract dynamic polarization disturbances in urban fibers, the system integrates qubit-based polarization feedback control, enabling real-time polarization compensation through an automated polarization controller using data recovered from the qubit-based synchronization signals. During 12 hours of continuous operation, the system maintained a low average quantum bit error rate of 1.12 ± 0.48%, achieving a secure key rate of 26.6 kbit/s under 18 dB channel loss. Even under a high channel loss of 40 dB, a finite-key secure rate of 115 bit/s was achieved. This study represents a successful long-term validation of a frame-synchronization-based QKD scheme in a real urban environment, demonstrating exceptional stability and high-loss tolerance, and offering an alternative for building practical, scalable, and cost-efficient quantum-secure communication networks.
In this work, a fully functional polarization-encoding quantum key distribution system (QKD) is demonstrated. At the transmitter side, the polarization states are prepared using an algorithm to determine the voltages to be applied on the electronic polarization controller (EPC), to generate up to four polarization states. At the receiver, a second EPC is used to perform the basis alignment. The symbol synchronization is achieved using a reference optical signal that is wavelength division multiplexed with the quantum signal which carries the polarization-encoded qubit. Frame synchronization is achieved at a protocol level. The presented QKD system works at a repetition rate of 500 Hz, with an average quantum bit error rate of 1.35%.
The technological development of QKD (Quantum Key Distribution) is accelerating from a research topic towards commercial deployment. QKD initiatives are increasingly addressing real-world use cases. Besides lots of literature on theory and lab-demos, first high Technology-Readiness Level systems are available. Due to the security guarantees it has to provide, the performance of any QKD implementation is particularly sensitive to any imperfections that may lead to an increase of the QBER (Quantum Bit Error Rate). Achieving high performance, or a low QBER value, constitutes a challenging engineering task, as the operating systems need to be highly optimized in many aspects. In particular, accurate phase modulation is the foundation of high system performance in case of phase-encoding protocols. In most cases, the overall QBER is measured with a QKD receiver, which accumulates errors from transmitter, quantum channel and receiver. This requires an operational transmitter system and additionally prevents the investigation of the pure transmitter output signal’s characteristics. However, for support of transmitter integration and test activities as well as for validation tasks, a more suitable method remains to be developed. In this paper, we propose such a method by making use of a coherent receiver. This device provides extended signal information including the signal amplitude and its current relative phase. Therefore, it is possible to monitor phase-encoding correctness and precision, among other relevant signal properties. We analyze the reasonability of using a coherent receiver, by outlining its capabilites to measure QBER contributors occurring in a given basic reference QKD transmitter system. Our findings indicate that employing a coherent receiver is a simple method to accelerate development and validation of phase-encoding QKD transmitter systems.
The increasing efforts by different organizations and countries regarding quantum key distribution (QKD) for secure communications are likely to result in diverse implementations for distinct users. To address this, the development of a software framework for a multi-encoding QKD system is proposed. Part of this framework are the time synchronization between transmitter and receiver, input generation for the arbitrary waveform generator (AWG) used in the implementation, as well as computing the quantum bit error rate (QBER) and secret key rate (SKR) for the employed protocol. This framework support various protocols, such as decoy-state BB84, differential phase-shift (DPS) or coherent one-way (COW), enabling a transmitter to securely exchange keys with different types of receivers. Our approach involves designing a modular and scalable architecture accommodating multiple QKD protocols to ensure compatibility and flexibility. The framework is a step towards interoperability and future-proofing in case current protocols become obsolete.