**Abstract**

Nowadays, high security levels are required to transmit critical information for government, private and personal sectors. As a countermeasure, the Quantum Key Distribution systems are the best option in order to protect this information because it provides unconditional security. In addition, increasing the transmission distance is a highlight. Therefore, the Free-Space Optical Quantum Key Distribution systems (FSO-QKD) 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 in order 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.

**Keywords:** performance parameters, secret key, challenges, trends, Quantum Key Distribution

## **1. 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. Therefore, Alice transmits to Bob important information that requires a high secrecy level based on different kind of cryptography systems against a spy system called Eve. In particular, the most secure systems in the practice and theoretically secure are the Quantum Key Distribution (QKD) systems implemented on fiber optical networks and/or Free Space Optics (FSO) links using both Continuous-Variable (CV) and Discrete-Variable (DV) due to they are based on the physics laws [1]. In general, any QKD system requires on the Alice side different "subsystems" such as optical source, quantum state preparation (QSP), modulation scheme (Mod-Sch) and a Digital Processor & Communication (DP&Comm), among other possible subsystems that can improve the overall performance (**Figure 1**). In particular, the optical source has some important physical and technical

requirements that affects the security level; these parameters are the linewidth, quantum optical state generated by the source, wavelength stability, among others. The QSP subsystem is probably the most important subsystem because it intends to prepare a true and well-knowledge quantum state that will be used for encoding the key, that is, to ensure the generation and fidelity features of the quantum state, although some QKD systems impose these requirements to an optimum optical source selection. Regarding the DP&Comm subsystem, many classical technologies are used for digital processing (e.g. central processing unit (CPU), graphical processing unit (GPU), field programmable gate array (FPGA)) in order to implement the algorithm for controlling Mod-Sch subsystem and perform a distillation algorithm for each particular protocol used in QKD systems. On the other hand, the DP&Comm also uses classical telecommunication techniques (e.g. Radio frequency, fiber optics link, FSO links, copper transmission lines) based on a classical and public transmission channel. In addition, the Mod-Sch subsystem uses the output signal of the QSP in order to modify some characteristics (e.g. polarization, amplitude, phase, among others) according to the driving output signal of the DP&Comm. After that, the quantum state that carries information is transmitted through a quantum private channel (fiber optics or free space). At the Bob's side, an optical receiver with support of many optical passive devices receives the quantum optical state and, thus, generates an electrical output signal that will be fed to a demodulation scheme (Demod-Sch). In this case, Bob also has a DP&Comm subsystem with the same characteristics and similar tasks in comparison with the used in Alice.

In addition, Quantum State Determination/Performance Parameters (QSP/PP) subsystem is used for: (a) determining the optical quantum state received by Bob based on optical tomography or calculating the density matrix, or, (b) measuring some important performance parameters of the quantum state received, such as amplitude, phase, polarization, among others, without reconstructing the phase representation state or the density matrix. There exist many subsystems that can improve the performance of QKD systems according to specific conditions, however, this chapter only mentions the most important ones based on authors opinion. On the other hand, Eve system also needs different subsystems in order to "listen" the information from Alice and Bob systems. Therefore, in order to reach the secure level imposed by the physical laws, highend technology is required in each subsystem mentioned concerning hardware (i.e. subsystems mentioned), protocols, novel materials among others highlight topics [2].

### **Figure 1.**

*General block diagram of QKD system emphasizing in the subsystems required for both quantum and classical channel.*

**27**

*Free-Space-Optical Quantum Key Distribution Systems: Challenges and Trends*

Therefore, this chapter explains the state-of-art and actual challenges of each

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. In fact, technical limitations and imperfections in the hardware used gives Eve the opportunity to implement some Side-Channel-Attacks (among other attack kinds)

The most desired optical source for the technical and scientific sector is the single-photon source or on-demand optical source which emit photons at any arbitrary time related to the transmission rate in a deterministic way (not probabilistic), that is, in an ideal case, 100% for emitted a certain photon and 0% for multiple-photons emitted, among others desired features. Thus, many optical sources intend to be a practical single-photon source based on faint laser pulse concept, however, it is not possible to ensure the amount of photons because the probabilistic analysis was made based on the Poisson distribution of the optical signal. On the other hand, there are other type of sources that try the same, but the difference relies in the theory and experiments used in order to generate a single photon. For example: (a) isolated quantum dot systems based on different material such as GaN, CdSe/ZnS, among others. However, these systems are not suitable for C-optical band (i.e. working from ≈340 nm to ≈950 nm) where the conventional telecommunication systems (and QKD systems) work and present a low emission efficiency (from ≈0.02 to ≈0.1) [3, 4]. Although it presents an important feature in single-photon sources, that is the deterministic resolving manner; (b) probabilistic single-photon sources based on Parametric Down-Conversion (PDC) and Four-Wave Mixing implemented in bulk crystals/waveguides and optical fiber, respectively. However, the principal issue is the reduced emission efficiency (from ≈ 0.1 to ≈ 0.85) although they are higher than the systems mentioned in (a). Obviously, this technical option is different compared to the ideal concept of a single-photon source that expects a perfect emission probability for a unique photon; and (c) faint laser is the most useful technique because it relaxes the design and complexity of the implementation of an experiment in both real and laboratory scenarios. This technique presents an emission efficiency of ≈1 and a wide inherent bandwidth suitable for

the immersion of QKD systems in the real optical networks [4].

Thus, for all the optical sources mentioned, the efficiency and non-linear optical elements are an important issue for design and manufacturing. It is also important

subsystem and devices used in QKD systems for both classical and quantum communications involved in this kind of secure systems. The aforementioned information can help to the reader to visualize and establish a general roadmap of the technologies used in QKD systems in order to focus institutional activities to research, development and innovation to contribute to the scientific and technical sector around the world. This chapter are organized as follows: Sections 2.1, 2.2 and 2.3 show the state-of-art regarding optical sources, optical detector and digital processing systems, respectively. Sections 2.1.1, 2.2.1 and 2.3.1 describe the actual challenges in each particular subsystem and the scientific and technological trends,

*DOI: http://dx.doi.org/10.5772/intechopen.81032*

emphasizing the FSO applications.

based on the non-idealities.

**2.1 Optical sources**

**2. High-end hardware: challenges and trends**

*Free-Space-Optical Quantum Key Distribution Systems: Challenges and Trends DOI: http://dx.doi.org/10.5772/intechopen.81032*

Therefore, this chapter explains the state-of-art and actual challenges of each subsystem and devices used in QKD systems for both classical and quantum communications involved in this kind of secure systems. The aforementioned information can help to the reader to visualize and establish a general roadmap of the technologies used in QKD systems in order to focus institutional activities to research, development and innovation to contribute to the scientific and technical sector around the world. This chapter are organized as follows: Sections 2.1, 2.2 and 2.3 show the state-of-art regarding optical sources, optical detector and digital processing systems, respectively. Sections 2.1.1, 2.2.1 and 2.3.1 describe the actual challenges in each particular subsystem and the scientific and technological trends, emphasizing the FSO applications.

## **2. 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. In fact, technical limitations and imperfections in the hardware used gives Eve the opportunity to implement some Side-Channel-Attacks (among other attack kinds) based on the non-idealities.

### **2.1 Optical sources**

*Quantum Cryptography in Advanced Networks*

requirements that affects the security level; these parameters are the linewidth, quantum optical state generated by the source, wavelength stability, among others. The QSP subsystem is probably the most important subsystem because it intends to prepare a true and well-knowledge quantum state that will be used for encoding the key, that is, to ensure the generation and fidelity features of the quantum state, although some QKD systems impose these requirements to an optimum optical source selection. Regarding the DP&Comm subsystem, many classical technologies are used for digital processing (e.g. central processing unit (CPU), graphical processing unit (GPU), field programmable gate array (FPGA)) in order to implement the algorithm for controlling Mod-Sch subsystem and perform a distillation algorithm for each particular protocol used in QKD systems. On the other hand, the DP&Comm also uses classical telecommunication techniques (e.g. Radio frequency, fiber optics link, FSO links, copper transmission lines) based on a classical and public transmission channel. In addition, the Mod-Sch subsystem uses the output signal of the QSP in order to modify some characteristics (e.g. polarization, amplitude, phase, among others) according to the driving output signal of the DP&Comm. After that, the quantum state that carries information is transmitted through a quantum private channel (fiber optics or free space). At the Bob's side, an optical receiver with support of many optical passive devices receives the quantum optical state and, thus, generates an electrical output signal that will be fed to a demodulation scheme (Demod-Sch). In this case, Bob also has a DP&Comm subsystem with the same

characteristics and similar tasks in comparison with the used in Alice.

In addition, Quantum State Determination/Performance Parameters (QSP/PP) subsystem is used for: (a) determining the optical quantum state received by Bob based on optical tomography or calculating the density matrix, or, (b) measuring some important performance parameters of the quantum state received, such as amplitude, phase, polarization, among others, without reconstructing the phase representation state or the density matrix. There exist many subsystems that can improve the performance of QKD systems according to specific conditions, however, this chapter only mentions the most important ones based on authors opinion. On the other hand, Eve system also needs different subsystems in order to "listen" the information from Alice and Bob systems. Therefore, in order to reach the secure level imposed by the physical laws, highend technology is required in each subsystem mentioned concerning hardware (i.e. subsystems mentioned), protocols, novel materials among others highlight topics [2].

*General block diagram of QKD system emphasizing in the subsystems required for both quantum and classical* 

**26**

**Figure 1.**

*channel.*

The most desired optical source for the technical and scientific sector is the single-photon source or on-demand optical source which emit photons at any arbitrary time related to the transmission rate in a deterministic way (not probabilistic), that is, in an ideal case, 100% for emitted a certain photon and 0% for multiple-photons emitted, among others desired features. Thus, many optical sources intend to be a practical single-photon source based on faint laser pulse concept, however, it is not possible to ensure the amount of photons because the probabilistic analysis was made based on the Poisson distribution of the optical signal. On the other hand, there are other type of sources that try the same, but the difference relies in the theory and experiments used in order to generate a single photon. For example: (a) isolated quantum dot systems based on different material such as GaN, CdSe/ZnS, among others. However, these systems are not suitable for C-optical band (i.e. working from ≈340 nm to ≈950 nm) where the conventional telecommunication systems (and QKD systems) work and present a low emission efficiency (from ≈0.02 to ≈0.1) [3, 4]. Although it presents an important feature in single-photon sources, that is the deterministic resolving manner; (b) probabilistic single-photon sources based on Parametric Down-Conversion (PDC) and Four-Wave Mixing implemented in bulk crystals/waveguides and optical fiber, respectively. However, the principal issue is the reduced emission efficiency (from ≈ 0.1 to ≈ 0.85) although they are higher than the systems mentioned in (a). Obviously, this technical option is different compared to the ideal concept of a single-photon source that expects a perfect emission probability for a unique photon; and (c) faint laser is the most useful technique because it relaxes the design and complexity of the implementation of an experiment in both real and laboratory scenarios. This technique presents an emission efficiency of ≈1 and a wide inherent bandwidth suitable for the immersion of QKD systems in the real optical networks [4].

Thus, for all the optical sources mentioned, the efficiency and non-linear optical elements are an important issue for design and manufacturing. It is also important

to remember that the optical sources described have to be suitable for FSO links where a complete QKD system is implemented, that is, the restriction of singlephoton is crucial for support the secure aspect inherent in QKD systems, however, the FSO links imposes trade-off that have to be analyzed. For this reason, the faint pulse is the common technique for FSO applications. Until now, the single-photon source information presented has been analyzed based on certain particular characteristics. However, an important aspect is the quantum state of the single photon generated by the optical source, that is, a photon can be generated with a particular quantum state (related to a quasi-probabilistic density functions) such as coherent, Fock, entanglement, among others. In fact, an ideal single-photon deterministic source should be generating a single photon with Fock distribution. On the other hand, an entanglement "single-photon" (probabilistic way) can be used in some short-distance-FSO-QKD systems and laboratory considering a high efficiency channel and finally, a single-photon source with coherent state (faint laser) is the most useful source and distribution used for long distance free space links.

### *2.1.1 Challenges and trends*

In general, the challenges in the actual optical sources are regarding the band telecommunication of the device, inherent bandwidth, emission efficiency and output spatial mode. Therefore, the important advance imposes a clear trend based on efficient optical sources at common telecom wavelengths (i.e. C-band) [5]. Although sources at O-band are available [6]. Basically, the improved performance of the optical sources is based on the use of novel materials, structures and quantum devices that permits the near-ideal quantum state generation [7].

### **2.2 Optical detector**

An ideal single-photon detector is useful in QKD systems in order to detect and resolve (determinate) an amount of photons per observation time (related to bit), that is, the detector is enabled to detect a single-photon and determine the exact quantity of a single-photon. However, this definition is based on the assumption of an ideal single-photon source. Obviously, ideal single-photon source and detector permits directly assure specific security levels based on the detection of an Eve system that disturbs the amount of photons transmitted by Alice. However, due to physical characteristics of the materials used on the manufacturing, there are deviations between the idealistic and realistic performance parameters. Thus, many realistic single-photon detectors have the ability of distinguish between zero photons per bit and more than zero photons, but they do not resolve the amount of photon. Based on the above, the most common used single-photon detectors are the nonphoton-number-resolving detectors, that is, they have the ability of detecting photon but do not resolving the exact amount of photons. However, there are different modes of operation based on multiple detectors that allow improving the resolving process. Some examples about single-photon detector proposals are: (a) the Photo-Multiplier Tube (PMT) which is a classical single-photon detector that operates from the visible region to the infrared. However, the detection efficiency is considerably reduced, for example, at 500 nm the efficiency is 0.4, while for 1550 nm is 0.02; meaning a major problem for its application in some real optical networks; (b) Single-Photon Avalanche Photodiode (SPAP) category has a wide variety of technical options for detection process, having minimum and maximum efficiencies from 0.40 to 0.74 for 450–780 nm band, respectively (based on Silice). In both cases (i.e. a and b options), the wavelength range is not completely suitable for FSO communications systems, although some beacon systems can use these detectors with previous analysis.

**29**

*2.3.1 Challenges and trends*

*Free-Space-Optical Quantum Key Distribution Systems: Challenges and Trends*

Therefore, SPAP based on InGaAs material is suitable for 1060–1550 nm range with maximum efficiency of ≈0.33 for 1060 nm and ≈0.10 at 1550 nm. Regarding the high-end technology, the superconducting Transition Edge Sensor (TES) is the best option for detecting in FSO-QKD system context based on the detection efficiencywavelength relationship, that is, efficiency of ≈0.95 at 1556 nm. However, the operation temperature is extremely low, ≈0.1°K, whereas the last mentioned detectors work commonly from 240 to 300°K, although there are some exceptions [4].

The principal challenges are related to minimizing the electronic noise and maximizing the gain of the detector maintaining high transmission rates [8, 9]. To do the aforementioned, novel materials and electrical designs are required. In particular, reducing the Noise Equivalent Power (NEP) parameter permits the detection of low optical power with different electrical bandwidth [10]. However, although novel optical detectors have been developed, coherent detection techniques have been helps at Bob side, relaxing the detector selection due to inherent

The DP&Comm subsystem implemented in conventional QKD systems performs particular basic tasks such as: driver for different devices (e.g. phase and amplitude modulators, true random number generator (TRNG), etc.), quantum key data base, perform the algorithm need to distillation, reconciliation and privacy amplification processes between Alice and Bob. In particular, this algorithm requires access to both quantum and classical channels. Therefore, the DP&Comm requires some important technical specifications so as not to degrade the secure level and secrete key rate of the QKD systems. In particular, Field Programmable Gate Arrays (FPGA) have been used in a real-time QKD systems reaching secret key rate at 17 kb/s in an optical fiber link of 20 Km [11]. It is clear that, the FPGA specifications impact the performance of a QKD systems, therefore, improved synchronization and jitter methods based on high speed and precision devices can reduce the Quantum Bit Error Rate (QBER) and increase the final secret key rate [12].

In addition, the secret key rate has an important relation with the performance of the TRNG subsystems, thus, FPGAs have been used for generation and acquisition of true random digital sequences reaching 1.25 Gb/s [13]. An important issue in DP&Comm subsystems is the ability to adapt and generate countermeasures to maintain or improve the specific performance against external dynamic factors such as atmospheric turbulence in FSO links, resizing and adaptive parameters based on an optimization process [14, 15]. In addition, some QKD systems use a Graphics Processing Unit (GPU) as a DP&Comm (although some considerations have to be analyzed to complete all the task of the DP&Comm) because it provides some important technical features such as parallel computing and processing floatingpoint information allowing rates of 1.35 Gb/s [16]. The novel standalone modules for particular stages of the protocol used (e.g. sifting, error correction, and privacy amplification modules) also support the performance of QKD systems, which are based on high-end electronic design. These particular technical innovations in

specific modules permits reaching secret key rate of ≈13.72 Mb/s [17].

Thus, the DP&Comm subsystem depends on the electronic development regarding the high performance related to speed processing and the novel design

amplification and spectrum filtering of the coherent technique.

*DOI: http://dx.doi.org/10.5772/intechopen.81032*

*2.2.1 Challenges and trends*

**2.3 Digital processing systems**

### *Free-Space-Optical Quantum Key Distribution Systems: Challenges and Trends DOI: http://dx.doi.org/10.5772/intechopen.81032*

Therefore, SPAP based on InGaAs material is suitable for 1060–1550 nm range with maximum efficiency of ≈0.33 for 1060 nm and ≈0.10 at 1550 nm. Regarding the high-end technology, the superconducting Transition Edge Sensor (TES) is the best option for detecting in FSO-QKD system context based on the detection efficiencywavelength relationship, that is, efficiency of ≈0.95 at 1556 nm. However, the operation temperature is extremely low, ≈0.1°K, whereas the last mentioned detectors work commonly from 240 to 300°K, although there are some exceptions [4].
