To establish a time reference frame between two users in quantum key distribution, a synchronization calibration process is usually applied for the case of using gated mode single-photon detectors (SPDs). Traditionally, the synchronization calibration is independently implemented by the line length measurement for each SPD. However, this will leave a loophole which has been experimentally demonstrated by a special attack. Here, we propose an alternative synchronization scheme by fixing the relative delay of the signal time window among all SPDs and jointly performing the line length measurement with multiple SPDs under combining low-precision with high-precision synchronization. The new scheme is not only immune to the vulnerability but also improves the synchronization time from usually a few seconds to tens of milliseconds.
To establish a time reference frame between two users in quantum key distribution, a synchronization calibration process is usually applied for the case of using gated mode single-photon detectors (SPDs). Traditionally, the synchronization calibration is independently implemented by the line length measurement for each SPD. However, this will leave a loophole which has been experimentally demonstrated by a special attack. Here, we propose an alternative synchronization scheme by fixing the relative delay of the signal time window among all SPDs and jointly performing the line length measurement with multiple SPDs under combining low-precision with high-precision synchronization. The new scheme is not only immune to the vulnerability but also improves the synchronization time from usually a few seconds to tens of milliseconds.
A light synchronization scheme to solve the key problem of signal synchronization in quantum key distribution(QKD)system is proposed.At side of the transmitter,the field-programmable gate array (FPGA)chip produces a synchronization pulse sequence of certain pattern.The pulse sequence drives laser to transmit sync light through one fiber along with signal light to the receiver.The sync light reverts to sync pulse sequence through fast discrimination at side of the receiver,and the pulses are provided to the single-photon detector as gate after high-precision delay adjustment.Simultaneously,the receiver uses a high-frequency clock to sample the sync pulse sequence in FPGA for encoding,synchronizing the transmitter and receiver.The scheme has a high-precision,low-cost,high fault tolerance feature,and was successfully applied to metro quantum communication network.
Quantum key distribution allows for a provably secure transmission of cryptographic keys over an optical channel. Encoded polarization states or time-bin degree of freedom have been used for successful demonstrations. However, photon losses in long fibers, slow single photon detectors, and detector dark counts significantly limit the overall bit rate. Improving key throughput and reducing the overhead of key reconciliation remain as major challenges. Methods which utilize multiple time bins allow for multiple key bits to be encoded in a single photon, thus increasing the fidelity of transmitted keys and decreasing the overhead of key reconciliation in real-world conditions. Previous implementations of these methods required that Alice and Bob share a time reference by sharing a dedicated classical channel used for synchronization. This work presents a technique that allows two parties to exchange time-bin encoded photons without the need for synchronized time references. Our technique uses a framing protocol which allows Alice to encode a time reference along with a key which is determined by Alice before transmission. Security can be achieved by monitoring the visibility of a pair of Franson interferometers, using decoy pulses and measuring the round trip time between Alice and Bob. The bit rate of this technique is limited only by the recovery time of the detector and the speed of the modulation electronics. We experimentally demonstrate a raw bit rate of 5Mb/s over an optical channel with 55dB of loss, which is competitive with current research. We also demonstrate absolute timing synchronization with an accuracy of 20ps.
The synchronization a two pass self-compensation quantum key distribution system (QKDS) with phase-encoded photon states are reviewed. A modified algorithm for synchronizing a two pass self-compensation QKDS with phase-encoded photon states was proposed. The first is the emphasis on the implementation in the single-photon mode which improves the synchronization mode security. The second peculiarity is that in case the number of accumulated impulses in two adjacent time windows is equal, it is decided to receive a photon impulse by any of these windows, if the number of accumulated impulses exceeds the number of measured dark current pulses (DCP) in the rest of the windows. In the known algorithm in such a case both windows were taken as noise which was the reason of the signal window skip. An analytical expression for the correct detection probability calculation of the photon impulse receipt in the algorithm proposed for the sync initialization of the QKDS.
In the description of quantum key distribution systems, much attention is paid to the operation of quantum cryptography protocols. The main problem is the insufficient study of the synchronization process of quantum key distribution systems. This paper contains a general description of quantum cryptography principles. A two-line fiber-optic quantum key distribution system with phase coding of photon states in transceiver and coding station synchronization mode was examined. A quantum key distribution system was built on the basis of the scheme with automatic compensation of polarization mode distortions. Single-photon avalanche diodes were used as optical radiation detecting devices. It was estimated how the parameters used in quantum key distribution systems of optical detectors affect the detection of the time frame with attenuated optical pulse in synchronization mode with respect to its probabilistic and time-domain characteristics. A design method was given for the process that detects the time frame that includes an optical pulse during synchronization. This paper describes the main quantum communication channel attack methods by removing a portion of optical emission. This paper describes the developed synchronization algorithm that takes into account the time required to restore the photodetector’s operation state after the photon has been registered during synchronization. The computer simulation results of the developed synchronization algorithm were analyzed. The efficiency of the developed algorithm with respect to synchronization process protection from unauthorized gathering of optical emission is demonstrated herein.
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.
Recently, a new type of attack, which exploits the efficiency mismatch of two single photon detectors (SPD) in a quantum key distribution (QKD) system, has been proposed. In this paper, we propose another "time-shift" attack that exploits the same imperfection. In our attack, Eve shifts the arrival time of either the signal pulse or the synchronization pulse or both between Alice and Bob. In particular, in a QKD system where Bob employs time-multiplexing technique to detect both bit "0" and bit "1" with the same SPD, Eve, in principle, could acquire full information on the final key without introducing any error. Finally, we discuss some counter measures against our and earlier attacks.