4. Numerical analysis and discussions

### 4.1. Point-to-point QKD network

responsivity and noise equivalent power (NEP) of the receiver at 1550 nm are 4 K.V/W, 0.8 A/ W and 22 pW/pHz, respectively. For our analysis, we have kept the high-power laser at the receiver, that is, integral part of Bob in order to avoid any eavesdropping or hacking on the reference signal. That is why, it is termed as local local oscillator (LLO). The LLO photon level is considered as 1 <sup>10</sup><sup>8</sup> photon per pulse. A phase noise cancelation (PNC) [25] based algorithm is implemented to minimize the excess noise as shown in Figure 2(c). The PNC stage has two square operators for in phase and quadrature operators of the light signal, and a digital DC cancelation stage, assisted by a down-converter. The comprehensive implementa-

As a first step, we investigated the coherent receiver to detect the m-PSK signals as it is well known that the specific modulation formats require a very particular optical signal-to-noise ratio (OSNR) in order to be detected at a bit error rate (BER) threshold. After the modulation stage, the 4-PSK and 8-PSK signals, back-to-back signals are detected at the coherent receiver and normalized signal-to-noise ratio (Eb/N0, the energy per bit to noise power spectral density ratio) is plotted against BER. The results are plotted in Figure 3(a). The BER threshold is set to be 3.8 103 (Q-factor of <sup>≈</sup> 8.6 dB), corresponding to a 7% overhead, that is, hard-decision forward error correction (HD-FEC). While soft-decision FEC (SD-FEC) level of BER 2.1 102 (Q-factor of ≈ 6.6 dB) can also be used corresponding to 20% overheard. From the results, we can depict that the minimum of 10 dB and 6 dB Eb/N0 values is required for the 8-PSK and 4- PSK signals at HD-FEC. While this limit can further be reduced to smaller values but at the cost

We have summarized the ADC requirements to detect the m-PSK signals. The results are as shown in Figure 3(b). The ADC resolution (bits) is analyzed with respect to the SNR penalty for 1- and 4 GBaud m-PSK modulated signals. From the results, it is clear that 6–8 bit ADC can be installed in the network to recover the noisy m-PSK signals at diverse baud rates while keeping the SNR penalty ≈ 1 dB. Despite the well-known fact that high-resolution ADC can

tion of the PNC module is described in Section 2.2 [25].

3.4. Characterization of coherent receiver

Table 2. Physical characteristics of the fiber at 1550 nm.

28 Telecommunication Networks - Trends and Developments

of 20% overheard in data rates, that is, SD-FEC.

Since the noise equivalent power (NEP) determines electronic noise of the coherent receiver and digital post processing unit, it is important to choose a TIA and ADC with lower NEP values for low aggregate electronic noise to shot noise ratio (ESR). Furthermore, as the NEP of the TIA is amplified by the TIA itself (gain amplifiers), it governs the total electronic noise. However, the ESR negligibly changes as the bandwidth of the detector is increased. This is because of the fact that both electronic and shot noise variances linearly increase with the bandwidth, so it is advantageous to use the receivers having 1–20 GHz bandwidth. Since, 20 GHz receivers are easily commercially available so we have modeled them for our investigations. Furthermore, the quantum channel includes the standard SMF and VOA to model the channel loss. The variance of the excess noise is largely due to the bias fluctuation of the I/Q modulator and timing jitter of the Bob, that is, receiver modules. It is estimated that the excess noise can be limited to be as small as 0.01 [25] below the zero key rate threshold. After optimizing the transmission model: (1) the corresponding power is approximately 70 dBm (approximately 7.8 106 photons per pulse) [26], (2) the detector efficiency is 60% and (3) reconciliation efficiency is 95%.

spaced system shows loss in SKR due to the impact of intersymbol interference between the

Recent Progress in the Quantum-to-the-Home Networks http://dx.doi.org/10.5772/intechopen.80396 31

The best available resource to mitigate the artifacts from the low-quality signal is to use raisedcosine filters for the pulse shaping at the transmitter. We can infer from the analysis that the quantum signals are compatible and ideal with classical optical add-drop multiplexers (OADMs) but the insertion loss from these modules can impact the SKR. A comparison of distance and secure key generation rate between CV-QKD using 20 GHz receiver and stateof-the-art DV-QKD systems based on T12 protocol [27, 28] is shown in Figure 5. The transmission distance of CV-QKD systems is limited than for DV-QKD demonstrations. However, comparative analysis of DV-QKD and CV-QKD shows that CV-QKD has the required performance to offer higher speed secure key transmission within an access network area (100 m to 50 km). Especially from 0 to 20 km range, that is, typical FTTH network, the SKR generated by using the traditional telecommunication components are 10s of magnitude higher than that of

Most of the efforts on the QKD system design and experimental demonstrations are limited to laboratory environments and point-to-point transmissions. While actual FTTH networks have in-line optical devices including but not limited to routers, switches, passive splitters, adddrop multiplexers, erbium-doped fiber amplifiers (EDFA), as envisioned in Figure 6(a). This restricts the deployment of QKD networks along with the classical data channels. However, in this chapter, we have investigated the compatibility of optical network components and their impact on the secure key rates. We have emulated the scenario of a typical quantum access

Figure 5. Performance comparison of CV-QKD vs. DV-QKD for access and metro networks.

adjacent neighboring channels.

DV systems.

4.2. QTTH network

network as shown in Figure 6(b).

Based on the abovementioned values, we extended our studies to calculate the secure key rates (SKR) at different transmission distances, that is, transmittance values. The input power is the same for every evaluation. Furthermore, SKR for both the 4-PSK and 8-PSK modulation formats under collective attack [22] are shown in Figure 4(a). The maximum of 100 Mbits/s SKR can be attained with this simple configuration by using the commercially available modules for transmittance (T) =1 for 4-PSK modulation. While SKR of 25 Mbits/s and 1 Mbit/s at T = 0.8 and 0.6, respectively. From the results, it can also be concluded that the maximum transmission distance for CV-QKD-based network is 60 km. Hence, it is recommended that this QKD protocol can effectively be used for access network, that is, QTTH. We have also investigated the performance of 8-PSK modulation and the results are plotted in Figure 4(a). The transmission performance is affected as compared to 4-PSK modulation, and this is due to the PNC algorithm that is executed to post-process the received quantum signal. This concept of generating seamless quantum keys can further be improved for multichannel networks that will help to generate high aggregate SKR via diversely multiplexing the neighboring quantum channels either by time, space, wavelength or polarization. In this chapter, we have multiplexed 12 WDM quantum channels to generate the aggregate SKR with the minimum channel spacing of 25- and 50 GHz. The WDM-QKD results, based on 4-PSK modulation, are shown as in Figure 4(b).

The results depict that the classical multiplexing techniques can efficiently be used to multiplex quantum signals without any degradation in the SKR. We have multiplexed the signals by using 25- and 50 GHz channel spacing. Also, aggregating secure key rate can reach upto 1.2 Gbits/s for a 12 WDM quantum system at T = 1. The importance of these results are due to the fact that next-generation PON services are already aiming at Gbits/s data rates, so QKD can match the data rates. The 50 GHz channel spaced system depicts insignificant performance degradation as compared to single wavelength transmission. However, the 25 GHz channel

Figure 4. Calculated QKD with respect to transmission distance for: (a) 4-PSK and 8-PSK modulation and (b) single channel (1-Ch) 4-PSK modulation, 12 channel WDM 4-PSK modulation with 25 and 50 GHz channel spacing. (Note: Simulations are performed by assuming 60% detector efficiency and 95% reconciliation efficiency).

spaced system shows loss in SKR due to the impact of intersymbol interference between the adjacent neighboring channels.

The best available resource to mitigate the artifacts from the low-quality signal is to use raisedcosine filters for the pulse shaping at the transmitter. We can infer from the analysis that the quantum signals are compatible and ideal with classical optical add-drop multiplexers (OADMs) but the insertion loss from these modules can impact the SKR. A comparison of distance and secure key generation rate between CV-QKD using 20 GHz receiver and stateof-the-art DV-QKD systems based on T12 protocol [27, 28] is shown in Figure 5. The transmission distance of CV-QKD systems is limited than for DV-QKD demonstrations. However, comparative analysis of DV-QKD and CV-QKD shows that CV-QKD has the required performance to offer higher speed secure key transmission within an access network area (100 m to 50 km). Especially from 0 to 20 km range, that is, typical FTTH network, the SKR generated by using the traditional telecommunication components are 10s of magnitude higher than that of DV systems.

## 4.2. QTTH network

optimizing the transmission model: (1) the corresponding power is approximately 70 dBm (approximately 7.8 106 photons per pulse) [26], (2) the detector efficiency is 60% and (3)

Based on the abovementioned values, we extended our studies to calculate the secure key rates (SKR) at different transmission distances, that is, transmittance values. The input power is the same for every evaluation. Furthermore, SKR for both the 4-PSK and 8-PSK modulation formats under collective attack [22] are shown in Figure 4(a). The maximum of 100 Mbits/s SKR can be attained with this simple configuration by using the commercially available modules for transmittance (T) =1 for 4-PSK modulation. While SKR of 25 Mbits/s and 1 Mbit/s at T = 0.8 and 0.6, respectively. From the results, it can also be concluded that the maximum transmission distance for CV-QKD-based network is 60 km. Hence, it is recommended that this QKD protocol can effectively be used for access network, that is, QTTH. We have also investigated the performance of 8-PSK modulation and the results are plotted in Figure 4(a). The transmission performance is affected as compared to 4-PSK modulation, and this is due to the PNC algorithm that is executed to post-process the received quantum signal. This concept of generating seamless quantum keys can further be improved for multichannel networks that will help to generate high aggregate SKR via diversely multiplexing the neighboring quantum channels either by time, space, wavelength or polarization. In this chapter, we have multiplexed 12 WDM quantum channels to generate the aggregate SKR with the minimum channel spacing of 25- and 50 GHz. The WDM-QKD results, based on 4-PSK modulation, are

The results depict that the classical multiplexing techniques can efficiently be used to multiplex quantum signals without any degradation in the SKR. We have multiplexed the signals by using 25- and 50 GHz channel spacing. Also, aggregating secure key rate can reach upto 1.2 Gbits/s for a 12 WDM quantum system at T = 1. The importance of these results are due to the fact that next-generation PON services are already aiming at Gbits/s data rates, so QKD can match the data rates. The 50 GHz channel spaced system depicts insignificant performance degradation as compared to single wavelength transmission. However, the 25 GHz channel

Figure 4. Calculated QKD with respect to transmission distance for: (a) 4-PSK and 8-PSK modulation and (b) single channel (1-Ch) 4-PSK modulation, 12 channel WDM 4-PSK modulation with 25 and 50 GHz channel spacing. (Note:

Simulations are performed by assuming 60% detector efficiency and 95% reconciliation efficiency).

reconciliation efficiency is 95%.

30 Telecommunication Networks - Trends and Developments

shown as in Figure 4(b).

Most of the efforts on the QKD system design and experimental demonstrations are limited to laboratory environments and point-to-point transmissions. While actual FTTH networks have in-line optical devices including but not limited to routers, switches, passive splitters, adddrop multiplexers, erbium-doped fiber amplifiers (EDFA), as envisioned in Figure 6(a). This restricts the deployment of QKD networks along with the classical data channels. However, in this chapter, we have investigated the compatibility of optical network components and their impact on the secure key rates. We have emulated the scenario of a typical quantum access network as shown in Figure 6(b).

Figure 5. Performance comparison of CV-QKD vs. DV-QKD for access and metro networks.

Figure 6. (a) Deployment of FTTH network with classical optical components; (b) downstream and upstream quantum access network and (c) hybrid classical-quantum traffic in access networks.

The optical line terminal (OLT) consists of a QKD transmitter, that is, in this chapter a m-PSK modulated transmitter is modeled. The optical distribution comprises as follows: (a) standard single mode fiber of 5 km length and (b) passive optical splitter with different split ratios. The commercially available splitters have insertion loss that is listed in Table 4.

4.3. Hybrid classical-quantum traffic in access networks

signals as broadband lasers are readily available commercially.

For the commercial compatibility of quantum signals with the existing optical networks, the wavelength and optimum power assignment to the signals are very much important. Different wavelength assignment [29–31] techniques have been investigated to avoid possible intersymbol interference between the classical and quantum signals. The best possible solution is to place the classical channels at 200 GHz channel spacing [31] in order to avoid any interference with the weakly powered quantum signals. Most importantly, we have implemented the concept of LLO, hence local oscillator signal is not generated from transmitter by using 90:10 coupler [18]. So apparently with LLO and 200 GHz channel spacing, there is no cross-talk among the hybrid classical quantum signals in the quantum channel. This is very much ideal for commercially available telecommunication components in the C-band (1530–1565 nm). Furthermore, with 200 GHz channel spacing, the classical channels can be encoded up to 400 Gbits/s line rate with advanced modulation formats, that is, dual-polarization m-QAM (m = 16, 32, 64 ….). But allimportant thing is, high data rate classical channels need sophisticated high bandwidth receivers that inherently have high electronic noise. For this reason, they are not suitable for quantum multiplexed signals as shown in Figure 6(c). As we are investigating a 20 GHz coherent receiver, so we have kept the data rate at 2.5 Gbits/s/polarization of quadrature phase-shift keying (QPSK) signals for classical data. The power of the classical data channels is optimized below 15 dBm. The quantum channel loss in this analysis corresponds to the 20 km of the optical fiber. The results for quantum signals at diverse wavelengths are depicted in Figure 8. The wavelength windows that are not occupied with the quantum channels are used for classical data transmission of QPSK signals. These signals are efficiently detected below the HD-FEC level. While the SKR of the quantum signals is 10 Mbits/s. We can conclude from the results the compatibility of quantum signals with the classical telecommunication components. Furthermore, L-band (1565– 1625 nm, extended DWDM band) can also be used to generate the hybrid classical-quantum

Recent Progress in the Quantum-to-the-Home Networks http://dx.doi.org/10.5772/intechopen.80396 33

Figure 7. Performance comparison of QTTH network with diverse passive split ratios as a function of achieved.

The variable splitting ratio is vital for the secure key rates as it will contribute to the attenuation and excess noise of the system. To test the simulation model under realistic conditions, we have also added 0.15 dB splicing loss for every connection with the passive optical splitter. The results are depicted in Figure 7 where we have plotted the SKR with respect to the splitting ratio of the system. It can be deduced from the graph that for a 1 2 splitting ratio, the SKR drops down to 10 Mbits/s per user while the SKR of 1 Mbits/s can be achieved with the splitting ratio of 1 4. Moreover, the classical telecommunication components can be used to design a seamless QTTH network and for short-range transmission as well as for data center applications it can perform better as compared to the much expensive DV-QKD systems [10].


Table 4. Summary of the average attenuation (dB) associated with the standard passive optical splitters.

Figure 7. Performance comparison of QTTH network with diverse passive split ratios as a function of achieved.

#### 4.3. Hybrid classical-quantum traffic in access networks

The optical line terminal (OLT) consists of a QKD transmitter, that is, in this chapter a m-PSK modulated transmitter is modeled. The optical distribution comprises as follows: (a) standard single mode fiber of 5 km length and (b) passive optical splitter with different split ratios. The

Figure 6. (a) Deployment of FTTH network with classical optical components; (b) downstream and upstream quantum

The variable splitting ratio is vital for the secure key rates as it will contribute to the attenuation and excess noise of the system. To test the simulation model under realistic conditions, we have also added 0.15 dB splicing loss for every connection with the passive optical splitter. The results are depicted in Figure 7 where we have plotted the SKR with respect to the splitting ratio of the system. It can be deduced from the graph that for a 1 2 splitting ratio, the SKR drops down to 10 Mbits/s per user while the SKR of 1 Mbits/s can be achieved with the splitting ratio of 1 4. Moreover, the classical telecommunication components can be used to design a seamless QTTH network and for short-range transmission as well as for data center applications it can perform better as compared to the much expensive DV-QKD systems [10].

commercially available splitters have insertion loss that is listed in Table 4.

Table 4. Summary of the average attenuation (dB) associated with the standard passive optical splitters.

access network and (c) hybrid classical-quantum traffic in access networks.

32 Telecommunication Networks - Trends and Developments

For the commercial compatibility of quantum signals with the existing optical networks, the wavelength and optimum power assignment to the signals are very much important. Different wavelength assignment [29–31] techniques have been investigated to avoid possible intersymbol interference between the classical and quantum signals. The best possible solution is to place the classical channels at 200 GHz channel spacing [31] in order to avoid any interference with the weakly powered quantum signals. Most importantly, we have implemented the concept of LLO, hence local oscillator signal is not generated from transmitter by using 90:10 coupler [18]. So apparently with LLO and 200 GHz channel spacing, there is no cross-talk among the hybrid classical quantum signals in the quantum channel. This is very much ideal for commercially available telecommunication components in the C-band (1530–1565 nm). Furthermore, with 200 GHz channel spacing, the classical channels can be encoded up to 400 Gbits/s line rate with advanced modulation formats, that is, dual-polarization m-QAM (m = 16, 32, 64 ….). But allimportant thing is, high data rate classical channels need sophisticated high bandwidth receivers that inherently have high electronic noise. For this reason, they are not suitable for quantum multiplexed signals as shown in Figure 6(c). As we are investigating a 20 GHz coherent receiver, so we have kept the data rate at 2.5 Gbits/s/polarization of quadrature phase-shift keying (QPSK) signals for classical data. The power of the classical data channels is optimized below 15 dBm. The quantum channel loss in this analysis corresponds to the 20 km of the optical fiber. The results for quantum signals at diverse wavelengths are depicted in Figure 8. The wavelength windows that are not occupied with the quantum channels are used for classical data transmission of QPSK signals. These signals are efficiently detected below the HD-FEC level. While the SKR of the quantum signals is 10 Mbits/s. We can conclude from the results the compatibility of quantum signals with the classical telecommunication components. Furthermore, L-band (1565– 1625 nm, extended DWDM band) can also be used to generate the hybrid classical-quantum signals as broadband lasers are readily available commercially.

frequency (RF) signals are modulated via an electro-optical I/Q modulator, where RF frequency is kept at 2 GHz. The modulation variance (VA) of the generated quantum signal is optimized by a tunable optical attenuator (TOA). As it is a hybrid classical quantum network, therefore classical 10 Gbits/s QPSK channels are multiplexed at 1531.2, 1571.4, 1591.1 and 1611.2 nm wavelengths. All the classical data channels are optimized at 0.5 mW input power. Whereas, multiplexers and de-multiplexers have 45 dB of isolation between the two adjacent channels, 80 dB isolation between nonadjacent channels and 0.85 dB of insertion loss at 1550 nm. The quantum channel comprises pure silica core fiber (PSCF) with different transmission lengths (maximum = 35 km) and the physical parameters of the fiber under test are enlisted in Section 3.2. The system performance of QKD network is also compared with the SSMF fiber in terms of secure key rates and transmission distance, while Alice and Bob

Recent Progress in the Quantum-to-the-Home Networks http://dx.doi.org/10.5772/intechopen.80396 35

The coherent receiver (Bob) consists of a 90 optical hybrid, a high optical power handling balanced photo-diodes with 20 GHz bandwidth and a real-time oscilloscope with a 100 GSa/s sample rate and 50 GHz analog bandwidth. We have kept the high power, narrow line width local oscillator at the receiver. It is termed as local local oscillator (LLO). The mean LLO photon level is 1 108 photon per pulse. The line width of the LLO is <10 kHz. After the system calibration at room temperature, the detector efficiency is measured as 0.6, while the electrical noise Vel is 0.85 (in shot noise units). The shot noise variance No is determined with sufficient LLO power to set the balanced detector in the linear detection regime. Shot noise calibration can be performed by shutting down all sources of incoming light or by ceasing the signal optical port on Bob side. The measured No for our setup is ≈ 17 0 mV2. The output signal is processed by the off-line digital signal processing module comprises phase noise cancelation (PNC) algorithm as depicted in Figure 9. The PNC stage has two square operators for in phase and quadrature operators, one addition operator and a digital DC cancelation block assisted by a down-converter. While all the secure key rate measurements are concluded with reconciliation efficiency of 90% for diverse modulation variances and transmission distances [30].

The extended experimental set-up for multiuser optically switched QKD network is shown in Figure 10. A MEMS-based 2 2 switch is incorporated after the quantum channel and demultiplexing, to implement inserted- and by-pass operations. The insertion loss of the switch is measured as 0.8 dB, while the cross talk (XT) is 52 dB, that is, negligible. In a bypass state, the input and output ports are connected to each other and in inserted state, the input and drop ports are connected to each other. In order to recover the classical 10 Gbits/s signals, we have used a standard coherent receiver with built-in DSP module of finite-impulse response filters (FIR) to compensate chromatic dispersion (CD). The forward error correction

We evaluated the average SKRs for diverse modulation variances and transmission distances. While for this investigation, we use two types of quantum channel, that is, SSMF and ultra-low loss PSCF. The results are as shown in Figure 11(a) and (b), respectively. Modulation variance is considered as 0.2, 0.3 and 0.4, while the length of quantum channel is considered upto 35 km

architectures are the same in both the cases.

(FEC) threshold is kept at 3.8 <sup>10</sup><sup>3</sup> BER (bit error ratio).

5.2. Results and discussions

Figure 8. Optimum system performance and wavelength assignment for hybrid classical-quantum transmission.
