Preface

Quantum cryptography (QC) is a special powerful tool for the wide spectrum of security applications based on both fundamental and applied principles of quantum mechanics, which allows two subscribers to generate, exchange, and process perfectly unique keys via a potentially insecure and intercepting quantum channel. Proposed 35 years ago, quantum key distribution (QKD), as a process to realize the advantages of QC, attracts more and more attention. Significant progress has been made in both its theory and practice from many points of view. The present book has four exclusive chapters. All chapters are focused on the most important provable developments in this critically important area of humankind wide spectra communications, because transferring information in a secure and private manner is a key ingredient to many aspects of society.

QKD systems with carrier modulation coding are prominent representatives of classical fiber telecommunication systems based on the principles of microwave photonics, with their inherent advantages and disadvantages. If the former can be attributed to the universality of modulation schemes, and the undoubted provision of a high signal-to-noise ratio in both quantum and synchronization channels of almost any length, the latter will begin to manifest with a constant increase in the range of quantum communication channels and the duration of the connection. First of all, they should include polarization distortion and chromatic dispersion. The introductory chapter focuses on monitoring the chromatic dispersion of the synchronization channel, which is controlled by the same generators as Alice and Bob's quantum channels, but in the absence of phase switching corresponding to the polarization state of the photons. In particular, the monitoring of chromatic dispersion in the synchronization channel is considered with the possibility of eliminating the influence of polarization distortions and using a clock frequency equal to the phase switching frequency, with its separation in a fiber Bragg grating filter.

Optical network security is attracting increasing research interest. Currently, a software-defined optical network (SDON) has been proposed to increase network intelligence (e.g., flexibility and programmability), which is gradually moving towards industrialization. However, varieties of new threats are emerging in SDONs. Data encryption is an effective way to secure communications in SDONs. In the first chapter of the book "Quantum Key Distribution over Software Defined Optical Networks," an architecture of QKD over SDONs, based on the QKD enabling technologies, is presented. The resource allocation problem is elaborated in detail and is classified into wavelength allocation, time-slot allocation, and secret-key allocation problems in QKD over SDONs. Finally, several open issues and challenges are discussed.

Free-Space Optical Quantum Key Distribution (FSO-QKD) systems present an innovative way for sharing secure information between two parties located at ground stations, spacecraft, or aircraft. However, these scenarios present several challenges regarding the hardware, protocols, and techniques used that must be solved to enhance the performance parameters (security level, distance link,

final secret key rate, among others) for any QKD system; although, in particular, a high transmission performance is required for both the classical and quantum channels. These issues impose the roadmap and trends in the research, academic, and manufacturing sectors around the world, which are presented and discussed in the second chapter of the book "Free-Space-Optical Quantum Key Distribution Systems: Challenges and Trends."

Chapter 1

Coding

1. Introduction

of the modulating signals of Alice and Bob.

constant monitoring.

1

Oleg G. Morozov

Introductory Chapter:

Chromatic Dispersion Monitoring

Systems with Carrier Modulation

The method of quantum key distribution (QKD) with carrier modulation coding (CMC) was proposed in [1, 2] and developed in [3–8]. Its advantages are the ease of input and coordination of the optical phase, the high data transfer rate, the fundamental possibility of frequency multiplexing of the signal, as well as the simplicity of constructing a consistent scheme. The main difference lies in the fact that in the QKD-CMC systems, the quantum signal is not generated directly by the source but is carried to side frequencies as a result of phase, amplitude modulation, or a combination of these. In the latter works, to each state of the photons, instead of the amplitude or phase of the modulating signal at a certain frequency, one or more lateral component frequencies either photon optical carrier [9, 10] are put into line. We in [11–13] present a universal system capable of realizing all the mentioned types of modulation transformation and the new one based on tandem amplitude modulation and phase commutation with partially or full suppressed carrier.

Alice and Bob's synchronization channel provides frequency and phase matching of the modulating signals. The deviation of the phase of the modulating signal from the base position during the re-modulation of the signal spectrum at Bob leads to a change in the radiation power at the side frequencies. A phase setting error, at the same time, reduces the visibility of the interference pattern and increases the level of quantum bit errors in the signal. The values of the optical signal-to-noise ratio (OSNR) in the system are also determined by the accuracy of setting the amplitude

The changing ambient temperature and chromatic dispersion (CD), in turn, depending on temperature, introduce an additional time-varying phase delay of the synchronization signal, which leads to a phase mismatch. In [14] it is shown that the correction of the clock phase should be carried out every 2–3 seconds to eliminate the influence of temperature and 2–3 hours to correct the effect of CD. If we take into account that CD itself can change both with a change in the temperature of the fiber and with a change in its configuration, we should speak about the need for its

in Synchronization Channel of

Quantum Key Distribution

The third chapter "Coherence Proprieties of Entangled Bi-photon Fields and Its Application in Holography and Communication" is dedicated to the problem of coherence that appears not only between quanta, but between groups of photons, generated in the process of nonlinear interaction of an electromagnetic field with radiators. Two-photon interactions of light are today one of the main areas of research in quantum optics. The encrypted information, using the coherence of multi-mode bimodal field in quantum holography, opens a new perspective, in which the coherence proprieties between bi-photons are used together with nonlocal states of entangled photon pairs. This method of recording of information affords the new perspectives in QC, QKD, and quantum theory of information and opens new possibilities in the coding and decoding of data generally.

The editor is grateful to all the authors of the chapters for participating in the preparation of the book and hopes that its original material will undoubtedly benefit researchers, engineers, graduates, and doctoral students working in quantum cryptography and information security-related areas.

> **Oleg G. Morozov** Professor, Radiophotonics and Microwave Technologies Department, Institute of Radio Electronics and Telecommunications, Kazan National Research Technical University, named after A.N. Tupolev-KAI (KNRTU-KAI), Kazan, Russia
