# Optical Link Design for Quantum Key Distribution-Integrated Optical Access Networks
## 3 Case Study on QKD in P2 MP Topology  
### Time-Bin-Encoded Discrete-Variable (DV)-QKD with GPON  
Figure 3a shows the schematic diagram of a US DV-QKD system in a P2 MP topology, designed to reduce the cost associated with QKD receivers [14]. Each QKD transmitter sends its key information over designated time slots, while the QKD receiver detects time-interleaved QKD signals. The configuration of QKD transmitters and receivers varies depending on the specific QKD protocol implemented. For instance, Figure 3b and Figure 3c illustrate the BB84 [34] and coherent one-way (COW) [35] protocols, respectively. The feasibility of US QKD was experimentally demonstrated in a 20 km P2 MP network comprising 4.4 km of feeder fiber and either 15.5 km or 11.8 km of drop fiber under dark fiber conditions [14]. In this experiment, the time-bin-encoded BB84 protocol was adopted. Although the OAN was assumed to accommodate eight subscribers using a 1 \(\times\) 8 power splitter, only two QKD signals were transmitted to an OLT in the experiment. When two time slots out of eight were used, approximately 1% quantum bit error rates (QBERs) were achieved. However, an increase in QBERs was observed as crosstalk increased due to the reduced time slot interval between the two allocated subscribers. Consequently, it needs to be further demonstrated whether a QBER with an acceptable secret key rate can be achieved when all eight subscribers transmit QKD signals simultaneously (i.e., when the time slot interval is minimized among all subscribers). The feasibility of the time-bin-encoded BB84-based QKD was investigated in a GPON environment [36]. Since US and DS GPON signals operate in the O-band and S-band, respectively, the QKD signal was transmitted at 1550 nm (i.e., within the C-band region). The optical spectrum was then measured at the OLT side (by tapping part of the power) to identify the major noise source [36]. The launched powers of the US and DS GPON signals were 4 dBm and 1 dBm, respectively. The feeder and drop fiber lengths were 15.5 km and 4.4 km, respectively. The results indicated that the noise was primarily influenced by the DS GPON signal [36], likely due to the smaller wavelength difference between the QKD and the DS GPON signal compared to the US GPON signal [9]. The feasibility of the QKD-integrated GPON system was evaluated based on the secret key rates, which were calculated from the experimental parameters. The following conditions were assumed:  
* Total fiber length (feeder + drop fiber): 20 km;
* Launched DS GPON signal: \(-\)3 dBm;
* Number of ONUs: 8;
* Use of single photon detector (SPD) operating at 1 GHz with 26% efficiency. Under these conditions, the feeder fiber length needed to remain below 8 km, as longer feeder fibers resulted in increased Raman backscattering from the DS GPON signals. Although reducing DS signal power could mitigate this issue [36,37], this approach is not viable for 10G or beyond-10G-class PON systems, which require higher launched power  
Figure 3: (**a**) Systematic configuration of US DV-QKD in P2 MP topology. Setup for time-bin-encoded (**b**) BB84 and (**c**) COW protocols (ILC: intensity level controller, IM: intensity modulator, PM: phase modulator, QRNG: quantum random noise generator). levels. The inherent splitting loss in TDM-PONs poses additional challenges in scaling the number of ONUs. To mitigate Raman scattering, a dual feeder fiber scheme was proposed, wherein the \(1\times N\) power splitter at the cabinet side was replaced with a \(2\times N\) power splitter to separate the DS GPON signal path from the QKD signal reception path. However, this scheme required additional fiber for QKD, leading to increased costs. Recently, the integration of QKD with the COW protocol and GPON was experimentally demonstrated, where US QKD was implemented, as shown in Figure 4a[9].