Investigation of Long-Term Stability of Single-Photon Quantum Key Distribution in a Polarization Coding Scheme

Experimental results demonstrating long-term stability of the operation of our atmospheric quantum cryptography setup using the BB84 protocol and polarization coding are presented. It was shown that the “sifted” quantum key distribution rate and the quantum bit error rate in the key remained constant for 1 hour and were equal to 10 kbit/s and 6.5 %, respectively, at a distance between the transmitter and the receiver equal to 20 cm. Theoretical dependences of the secret quantum key generation rate on a quantum channel transmission coefficient for single-photon detectors, which were used in this experiment, and for new detectors with a reduced level of dark pulses are given.

ALGORITHM OF SPATIAL POLARIZATION CODING FOR THE DATA TRANSMISSION SYSTEM OF THE COMBINED HF RADIO CENTER

Рассматриваются вопросы увеличения скорости передачи данных разнесенным радиоцентром декаметрового диапазона (с помощью поляризационного разнесения). Сделан обзор основных исследований и экспериментов, проводимых в этом направлении. Разработан алгоритм поляризационного пространственного кодирования и способ одновременного формирования волн с левой и правой круговыми поляризациями. Приведены результаты математического моделирования, показывающие работоспособность предложенного алгоритма. The article discusses the issues of increasing the data transfer rate by the spaced HF radio center using polarization diversity. A review of the main research and experiments carried out in this direction is made. An algorithm for polarization spatial coding and a method for the simultaneous formation of waves with left and right circular polarizations has been developed. The results of mathematical modeling showing the efficiency of the proposed algorithms are presented.

Design of Bipolar Optical Code-Division Multiple-Access Techniques Using Phase Modulator for Polarization Coding in Wireless Optical Communication

In this study, a bipolar optical code-division multiple-access (Bi-OCDMA) technique based on spectral amplitude coding (SAC) was proposed by using a phase modulator to realize polarization coding through a free-space optical (FSO) channel. Various types of noise, such as amplified spontaneous emission (ASE) noise, thermal noise, and shot noise, were included in the simulation to approach the real application. The first simulation, utilizing a modified M-sequence as signature code, demonstrated that the proposed Bi-OCDMA system could be implemented in FSO communication. The proposed Bi-OCDMA scheme improves the transmission rate and power efficiency compared with the previous scheme. The structure of the proposed system alleviates multiple-access interference (MAI) with a simple and cost-effective design. The second simulation observed the performance of the proposed Bi-OCDMA for two users with several well-known SAC codes, i.e., multi-diagonal (MD) code, modified quadratic congruence (MQC) code, modified maximum length sequence (M-sequence) code, and Walsh–Hadamard code, in extreme weather conditions, both for additive white Gaussian noise (AWGN) and turbulence-induced fading channel. The simulation results indicated that the Walsh–Hadamard code has superior performance compared to other codes. The results show the MD code can be implemented in the proposed Bi-OCDMA scheme for a medium-distance FSO.

Cross-nanofin-based waveplate pixels for broadband hybrid polarization coding in near-field

As an inherent characteristic of light, polarization plays important roles in information storage, display and even encryption. Metasurfaces, composed of specifically designed subwavelength units in a two-dimensional plane, offer a great convenience for polarization manipulation, yet improving their integrability and broadband fidelity remain significant challenges. Here, based on the combination of various subwavelength cross-nanofins (CNs), a new type of metasurface for multichannel hybrid polarization distribution in near-field is proposed. Sub-wavelength CN units with various waveplate (WP) functionalities, such as frequency-division multiplexing WP, half-WP and quarter-WP are implemented with high efficiency in broadband. High-resolution grayscale image encryption, multi-image storage and rapid polarization detection are demonstrated by encoding the WP pixels into single, double and four channels, respectively. All these applications possess good fidelity in an ultrabroad wavelength band from 1.2 to 1.9 µm, and the high degree of integrability, easy fabrication and multifunction make the CN-shaped WP pixels a promising candidate in optical device miniaturization, quantum applications and imaging technologies.

Quantum Cryptography

Cryptography is used for the secure communication in which two parties are involved. The most popular cryptographic issue is the transmission of confidential messages. The privacy is maintained using the cryptographic protocol. The security of quantum cryptography relies more on physics including quantum mechanics and statistics rather than on solving mathematical problems. A well-known application of quantum cryptography is quantum key distribution (QKD) that is used to establish communication by generating cryptographic keys. Moreover, it is based on the Heisenberg uncertainty principle that ensures the security and prevents from eavesdropping. Basically, quantum cryptography with faint laser pulses, polarization coding, phase coding, and frequency coding have been discussed.