The results of the study on the correlation of polarization distortions in a multi-core fiber (MCF) under external influences are presented. The influence of mechanical effects and the effect of a magnetic field on the cores of the MCF are investigated. A high correlation of polarization distortions in different cores of a multi-core fiber under physical influence is observed. These results are useful for developing methods to mitigate distortions in the polarization states of single photons in wire quantum channels for quantum key distribution systems.
The results of the study on the correlation of polarization distortions in a multi-core fiber (MCF) under external influences are presented. The influence of mechanical effects and the effect of a magnetic field on the cores of the MCF are investigated. A high correlation of polarization distortions in different cores of a multi-core fiber under physical influence is observed. These results are useful for developing methods to mitigate distortions in the polarization states of single photons in wire quantum channels for quantum key distribution systems.
Abstract The results of examination of the effect of ac magnetic fields on a multi-core fiber and the induced polarization distortions of signals propagating through different fiber cores are reported. A strong cross correlation of polarization distortions in different cores of a multi-core fiber is established. An approach is proposed for compensating rapid and chaotic changes in the polarization states of single photons induced by electromagnetic fields on a multi-core fiber telecommunication cable. The proposed approach is based on measuring polarization distortions of a classical communication channel in one of the multi-core fiber cores and compensating for these distortions in a quantum channel within the time interval established using a delay line through which the multi-core fiber cores with implemented quantum communication channels are connected. The need for careful stabilization of the characteristics of a polarization distortion compensation system, which will be connected immediately after mating the multi-core fiber with a conventional single-mode fiber (at constant temperature and without strong external fields) is demonstrated.
The article studies the influence of electromagnetic influence on a fiber-optic communication system with quantum cryptographic equipment. The developed test benches and methods of experimental study of the influence of electromagnetic effects on a communication system with quantum cryptographic equipment with fiber-optic communication line are presented. The results of experimental study of electromagnetic influence on the communication system with quantum key distribution and an optical cable are discussed. The results of studying showed that the influence of magnetic fields on the transmission of an optical signal through an optical fiber leads to a significant rotation of the plane of polarization, and therefore to errors and failures in the system with quantum key distribution. At the same time, the experiment showed that the influence of electric fields is insignificant. The obtained data emphasize the need to take into account the electromagnetic environment when designing and operating communication systems with a fiber-optic communication line
In this paper, the influence of noise induced by classical signals on independent core transmitting quantum key distribution (QKD) is studied over multicore fiber (MCF). We propose a theoretical model of the cascaded noise of spontaneous Raman scattering and inter-core crosstalk (SpRS-ICXT), and the cascaded noise of four-wave mixing and inter-core crosstalk (FWM-ICXT) considering modulation frequency over MCF. To analyze the effect of modulation frequency on QKD, we define and simulate the average power difference and power fluctuation difference of SpRS-ICXT and FWM-ICXT. Then, SpRS-ICXT and FWM-ICXT noises are simulated in two cases: (1) 1 or 3 classical channels and 1 km MCF; (2) 8 or 16 classical channels and 45 km MCF. For the latter case, SpRS-ICXT and FWM-ICXT noises have a serious impact on QKD. Therefore, the impact of noise on QKD is studied. A higher modulation frequency can improve QKD performance, and the secure transmission distance can be further extended by 15.2% when the modulation frequency is 10 GHz relative to ignoring the modulation frequency. However, inter-core crosstalk (ICXT), SpRS-ICXT and FWM-ICXT noises will shorten 52.2% secure transmission distance compared to only transmitting QKD. Finally, the noise of quantum channel is experimentally measured at different symbol rates and modulation formats of classical signals, and it is shown that increasing the symbol rate of data signals at the same optical power degrades the secure key rate of QKD system.
Polarization variations in the installed fibers are complex and volatile, and would severely affect the performances of polarization-sensitive quantum key distribution (QKD) systems. Based on the recorded data about polarization variations of different installed fibers, we establish an analytical methodology to quantitatively evaluate the influence of polarization variations on polarization-sensitive QKD systems. Using the increased quantum bit error rate induced by polarization variations as a key criteria, we propose two parameters - polarization drift time and required tracking speed - to characterize polarization variations. For field buried and aerial fibers with different length, we quantitatively evaluate the influence of polarization variations, and also provide requirements and suggestions for polarization basis alignment modules of QKD systems deployed in different kind of fibers.
In contemporary society, communication security has become increasingly important. The security of the current cryptosystems, based on mathematical assumptions, will not be guaranteed when quantum computers become available [1]. This has spurred investigations into new security technologies based on quantum physics. In order to exchange secure information between users, quantum key distribution (QKD), a branch of Quantum Communications (QCs), provides good prospects for ultimate security based on the laws of quantum mechanics [2–7]. Most of QKD systems are implemented in a point-to-point link using bulky, discrete and expensive devices. Consequently, a large scale deployment of this technology has not been achieved. In a future scenario, where QCs will become standard technology, and where infrastructures like banks and government buildings, will be connected through a quantum network, different requirements in terms of key generation are needed. A solution may be represented by new technologies applied to quantum world. In particular multicore fiber (MCF) open a new scenario for quantum communications, from high-dimensional (HD) spatial entanglement generation, to HD QKD and multi-user key generations, to HD-entanglement distribution. Furthermore, MCFs are expected as a good candidate for overcoming the capacity limit of a current optical communication system, as example the record capacity of 661 Tbits/s was obtained last year with a 30-cores fiber [8]. Proof of concept experiment has already proved the coexistence of classical and quantum communications transmitted into different cores of MCF [9]. On the other hand, photonic integration has played a critical role in recent quantum information revolution by integrating functionalities of traditional discrete bulky components into ultra-compact chips [10, 11]. In fact, integrated photonic circuits provides excellent performances (compacts, good optical phase stability, access to new degrees of freedom), and are particularly suitable for the manipulation of quantum states. Some recent experiments have already demonstrated conventional binary QKD systems, using polarization and phase reference degrees of freedom [12, 13]. Moreover, by using integrated solution new high-dimensional quantum states can be generated and propagated. Based on compact silicon photonic integrated circuits, we here show how a MCF can be used for quantum communications protocols by proving decoy-state HD-QKD and multi-users quantum key generations.
Inter-core phase control is a prerequisite for interferometry in multi-core fiber (MCF), but no compact inline element for this purpose is presently available, and existing approaches require fanout to single-mode fiber or free-space optics. We show that a commercial inline polarization controller, a low-loss and widely available component, can be operated as an inline inter-core phase shifter. Two-core interferometry on a jacketed four-core patchcord gives a reversible inter-core phase of 0.95±0.02~rad from a single device, with an operating-point-independent lower bound of 0.38~rad, driven through repeated load-unload cycles at a rate set by the actuator. A diametrically compressed disk model is used to predict the operating condition: with the input polarization aligned to a principal stress axis the device imparts a relative phase between cores without rotating the polarization state of either, which we verify by polarimetry on all four cores. The response is polarization dependent, falling to less than 0.58~rad for the orthogonal input. The result is a practical, inline route to inter-core phase control and stabilization in all-MCF interferometers, requiring neither fanout to single-mode fiber nor free-space optics.