Qubit4Sync is a synchronization method for quantum key distribution which uses the same qubits exchanged during the protocol; in the same spirit of classical communication protocols reling on clock recovery methods that uses the data stream itself.
We propose Qubit4Sync, a synchronization method for quantum-key-distribution (QKD) setups, based on the same qubits exchanged during the protocol and without requiring additional hardware other than that necessary to prepare and measure the quantum states, in a similar fashion to the clock recovery used in classical communications. Our approach introduces a cross-correlation algorithm with low computational complexity for high channel losses. We test the robustness of our scheme in a real QKD implementation, and we believe it may find application in other quantum communication protocols.
Quantum key distribution (QKD) is a method that enables two remote parties to share a secure key string. Clock synchronization between two parties is a crucial step in the normal operation of QKD. Qubit-based synchronization can achieve clock synchronization by transmitting quantum states between two remote parties, eliminating the necessity for hardware synchronization and thereby greatly reducing the hardware requirements of a QKD system. Nonetheless, classical qubit-based synchronization exhibits poor performance in continuous and high-loss systems, hindering its wide applicability in various scenarios. Here, we propose a qubit-based distributed frame synchronization method that can achieve time recovery in a continuously running system and resist higher losses. Experimental results show that the proposed method outperforms the advanced qubit-based synchronization method Qubit4Sync in a continuously running system. Particularly, the results demonstrate that our method surpasses all previous works in key parameters, including frequency and the synchronization length. We believe our method is applicable to a broad range of QKD scenarios, including drone-based QKD and quantum network construction.
Clock synchronization serves as a foundational subsystem in quantum key distribution (QKD). The recently proposed Qubit-based synchronization (Qubit4Sync) has opportunities in eliminating additional cost, noise, and potential side channels. It offers a promising alternative to dedicated synchronization hardware. However, the current frequency recovery process in Qubit4Sync requires high data throughput and computational speed, limiting practical use. To overcome these issues, we developed a fast frequency recovery algorithm that increases the recovery rate by orders of magnitude and remains robust under bad signal-to-noise ratio (SNR). This enables Qubit4Sync to operate effectively in mainstream gated-mode QKD systems. We further establish a theoretical model for frequency recovery, showing that our algorithm is robust against disturbances like dead time, jitter, and afterpulse. A frequency-domain SNR calculation method is also provided to guide parameter design for specific experimental conditions. This work opens the door to practical Qubit4Sync deployment in general QKD systems.
Quantum key distribution (QKD) systems provide a method for two users to exchange a provably secure key. Synchronizing the users' clocks is an essential step before a secure key can be distilled. Qubit-based synchronization protocols directly use the transmitted quantum states to achieve synchronization and thus avoid the need for additional classical synchronization hardware. Previous qubit-based synchronization protocols sacrifice secure key either directly or indirectly, and all known qubit-based synchronization protocols do not efficiently use all publicly available information published by the users. Here, we introduce a Bayesian probabilistic algorithm that incorporates all published information to efficiently find the clock offset without sacrificing any secure key. Additionally, the output of the algorithm is a probability, which allows us to quantify our confidence in the synchronization. For demonstration purposes, we present a model system with accompanying simulations of an efficient three-state BB84 prepare-and-measure protocol with decoy states. We use our algorithm to exploit the correlations between Alice's published basis and mean photon number choices and Bob's measurement outcomes to probabilistically determine the most likely clock offset. We find that we can achieve a 95 percent synchronization confidence in only 4140 communication bin widths, meaning we can tolerate clock drift approaching 1 part in 4140 in this example when simulating this system with a dark count probability per communication bin width of 8×10-4 and a received mean photon number of 0.01.
Quantum key distribution (QKD) enables the generation of information-theoretic secure keys between two remote parties. A critical component of successful QKD implementation is the effective synchronization of clocks between these users. Traditional methods of locking to a reference clock or transmitting a synchronization signal rely on auxiliary equipment, which increases the complexity and cost, thus hindering the widespread deployment of QKD. Here, we propose a method to achieve clock and pulse synchronization by exploiting a subset of the transmitted qubits. We demonstrate our scheme with a time-bin phase-coding BB84 QKD system and verify the realization of long-term stable key generation over distances of 50 km and 100 km. Our method simplifies the implementation of QKD systems and introduces new possibilities for synchronization algorithms in other quantum communication protocols.
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.