Alice set-up. Own figure and presented in . Bob set-up. Mirrors and attenuators are used in order to calibrate the optical power received in each photodetector implemented in each BHDs due to the different optical paths. Therefore, the complete Bob scheme is based on coherent detection technique, where an opto-electronic Costal loop permits the improvement of the complete performance reducing the phase noise due to different optical paths.
The final secret key rate measurements were 20 and 40 Kbps for minimum and maximum throughput in the LAN. Finally, although common elements i. QBER, final secret key rate. In general, the QKD protocols describe the particular tasks or steps i. In particular, the protocols and their performance depend on the statistical information discrete and continuous variables, DV and CV, respectively regarding the quantum state used.
The BB84 protocol is the first protocol proposed to guarantee unconditional security because it is based on orthogonal states when transmitting the cryptographic key that gives access to the information of a message. It has been considered as the ideal protocol, at least in theory, since it is based on the transmission of the quantum state of a single photon to represent a bit of information and provide immediate information through the characteristics of the received signal, about a possible attack by a spy [ 19 ].
In addition, if we add that the key is used in a single occasion One Time Pad it provides better protection in case that at some point an intruder manages to obtain the key. Next, B92 protocol was proposed based on BB84 protocol. While BB84 protocol uses 4 orthogonal states, B92 only uses 2 non-orthogonal states.
Therefore, the different quantum states i. Since the BB84 protocol is extremely vulnerable to Photon Number Splitting attacks, the SARG04 protocol was proposed, which uses 4 non-orthogonal quantum states; however, the final secret key rate is also affected [ 21 ].
Additionally, there exists the E91 protocol based on Einstein, Podolsky and Rosen EPR paradox that uses entangled quantum states generated either by Alice, Bob or a trusted third party [ 22 ]. Until now, the protocols mentioned are based on State of Polarization SOP , DV framework and general stages such as: raw key exchange, key sifting and privacy amplification, that is, all the protocols have the same stages in order to generate the final quantum key. On the other hand, QKD protocols based on CV variables are also suitable, such as COW protocol Coherent One-Way , which is based on an amplitude encoded sequence of weak coherent pulse with the same phase for each particular time slot.
In particular, different time slots have several optical pulses related to an optical power average and, occasionally, decoy sequences are sent in order to hinder the eavesdropped process [ 24 ]. Due to the different quantum states and encoding scheme used, this protocol is so-called distributed-phase-reference DPR , in fact, there are many protocols in the same category such as the differential-phase-shift DPS , which uses different phases but the amplitude remains constant.
Therefore, interferometric techniques are required in the receiver [ 25 ]. All the DPR protocols perform joint measurements on subsequent signals. Actually, GG02 protocol is present in many commercial equipment. Finally, each protocol mentioned has a particular security principle, be it the Heisenberg uncertainty or quantum entanglement. Although there exist novel protocols that change the security principle in order to improve the performance of particular QKD systems. In particular, although each protocol uses different security principle and quantum states, the important issues are increasing the security level, secret key rate and distance link between Alice and Bob in presence of Eve system.
In fact, while a particular protocol presents a high security level and particular secret key rate for short distance links, other protocol presents the same security level and secret key rate for long distance links. However, as was mentioned, a QKD protocol requires the other subsystems, thus, a hypothetically complicated protocol imposes a strict and detailed design, that is, the experimental set-up is not simple.
Therefore, the tendency of the protocols refers to proposing novel QKD protocols that allow to easily implement them in optical commercial networks, while the performance parameters remain constant or improved. In addition, a high dimension protocol is proposed in order to increase the photon information capacity when the photon rate is restrained. This protocol is based on entangled photon pairs that allow information to be transmitted using an extremely large alphabet [ 27 ]. Now, each QKD protocol has been theoretically described, however, free space and atmospheric channels impose important trade-off that determines the suitable protocol.
However, BB84 protocol remains unchanged while other subsystems are modified. In fact, many QKD protocols have been implemented in FSO links in order to demonstrate their performance under particular conditions. The techniques and structures used in QKD context involve the different set-ups, operation rules and devices that perform a particular protocol.
Therefore, the first step is choosing the quantum protocol and next, the general structure can be proposed and implemented. In particular, the structure consists of optical source, optical detector, digital processing unit the challenges and trends that have already been mentioned among other specific devices connected together in order to perform a complete QKD system.
Making quantum encryption practical
On the other hand, the techniques are the novel operational rules in order to enhance the complete performance of the QKD system. Each protocol mentioned was proofed, first, using a particular technique and structure, these can be can be found and analyzed in the references listed. In order to mitigate the degraded performance of the secret key rate for QKD systems based on BB84 protocol, an optimization technique was proposed based on an adaptive optical power transmission considering the random irradiance fluctuation [ 28 ].
In addition, many structures and techniques used in conventional classical optical communication systems have been adapted to QKD-FSO systems.
Among the structures and techniques necessary to implement a QKD-FSO system are the subsystems used in order to pointing, acquisition and tracking the two parties Alice and Bob, represented by satellites and ground stations. In this case, pointing systems used in satellites have reached from 0. Therefore, the design of techniques and high-end structures allows to support in a better way the actual QKD system proposals.
In fact, the principal challenges are related with the optimization and improving of the secondary subsystems of a QKD-FSO systems i. Finally, the QKD-FSO system trends related with the structures and techniques are: maximize the channel capacity, increase the distance link and secret key rate, increase the power consumption efficiency in order to support long-time missions, improve the thermal control and isolation, among others.
In addition, novel encoding technique for classical channel has been proposed in order to increase the secret key rate at QKD-FSO links. Figure 4 shows a diagram proposed based on an adaptive LDPC Low-Density Parity-Check Codes encoder in order to countermeasure the effect caused by the dynamical atmospheric turbulence [ 29 ].
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Block diagram of the simulation-experimental set-up of the QKD system proposed with dynamical encoder for different atmospheric turbulence levels. Own figure and presented in . Basically, Alice and Bob monitored the dynamical atmospheric turbulence calculating the error probabilities and modifying LDPC encoder capacity used by them. The proper understanding of the high-end hardware, protocols, techniques and schemes used in FSO-QKD systems allow to improve the performance parameters such as secret key rate, distance link, security level, among others.
In particular, although there are wide suitable options for the subsystems required for FSO-QKD systems, it is necessary that the high-end subsystems are more accessible and compact in order to increase their uses in traditional optical networks. Many thanks to CETYS University for the administrative and technical support in the development of diverse projects related to the subject mentioned in this chapter. Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3. Help us write another book on this subject and reach those readers.
Login to your personal dashboard for more detailed statistics on your publications. Edited by Oleg Morozov. Edited by Asma Salman. We are IntechOpen, the world's leading publisher of Open Access books. Built by scientists, for scientists. Our readership spans scientists, professors, researchers, librarians, and students, as well as business professionals. Downloaded: Abstract Nowadays, high security levels are required to transmit critical information for government, private and personal sectors. Keywords performance parameters secret key challenges trends Quantum Key Distribution.
Introduction Currently, crucial information is shared between two parties located either near or far, in the quantum cryptographic context, the transmitter and receiver side are called Alice and Bob, respectively.
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High-end hardware: challenges and trends As mentioned above, although QKD systems are unconditionally secure based on laws of quantum mechanics, it is necessary to understand the technological limit of high-end hardware to increase the research, innovation and development activities in order to reach the theoretical performance of a QKD system step by step. Techniques and structure in QKD: challenges and trends The techniques and structures used in QKD context involve the different set-ups, operation rules and devices that perform a particular protocol.
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