# Optical Link Design for Quantum Key Distribution-Integrated Optical Access Networks
## 3 Case Study on QKD in P2 MP Topology
### CV-QKD with PONs  
While US QKD offers advantages in terms of reducing the number of QKD receivers, it introduces challenges in time-interleaving QKD signals from different ONUs, which can lead to crosstalk [36; 38]. In contrast, CV-QKD leverages commercially available coherent receivers, enabling DS QKD and thereby reducing crosstalk-related performance degradation. By assigning a wavelength distinct from PON signals, a TDM-based DS QKD system can be implemented, as depicted in Figure 5a. Even though an OLT transmits QKD signals to all connected ONUs in this architecture, the security among ONUs is maintained [38]. An example of a CV-QKD system is illustrated in Figure 5b [39]. In this setup, a CV-QKD signal is transmitted according to the GG02 protocol [22] over a 202.81 km SSMF with a total loss of 32.45 dB [39]. Given that PONs typically require a power budget exceeding 29 dB for a 20 km transmission distance (i.e., a total loss of >33 dB) [5], this suggests that a TDM-based QKD access network can feasibly be implemented. In this 202.81 km transmission experiment, the QKD receiver was implemented using homodyne detection (which detects a single quadrature via a balanced photodetector) [22] and a transmitted local oscillator (TLO) scheme [40].  
The experiments in [38] were conducted without the presence of PON signals, meaning that excess noise induced by crosstalk and Raman scattering was not considered [38]. However, excess noise increases when QKD is multiplexed with PON signals. The tolerable excess noise was analyzed in [38], showing that for a 10 km link, the acceptable excess noise decreased from 0.14 shot noise units (SNU) to 0.04 SNU, as shown in Figure 6. For a 20 km link, the tolerable excess noise was slightly reduced to 0.11 SNU. A group at Telecom ParisTech successfully demonstrated a CV-QKD system operating alongside a classical communication system over a 25 km link [41]. This demonstration is particularly relevant, as it falls within the typical PON reach. In their experiment, classical dense-WDM (DWDM)  
Figure 5: (**a**) Configuration of DS CV-QKD integrated with PON. (**b**) Exemplary setup for CV-QKD with GG02 protocol (BPD: balanced photodiode, DPC: dynamic polarization controller, EDFA: erbium-doped fiber amplifier, IQM: in-phase and quadrature modulator, LD: laser diode, OBPF: optical bandpass filter).  
signals and CV-QKD signals were multiplexed within the C-band, with a wavelength separation of just 20 nm. Their results indicated that the sum of excess noise and the photodetector's thermal noise remained below 0.11 SNU, which is within the acceptable range for a 20 km TDM-based quantum access network supporting four ONUs, provided that the launched power of the DS and US signals was kept below 4.7 dBm.  
Since the spectral separation between the QKD and classical signals in this experiment was only 20 nm, it can be inferred that QKD signals can also be transmitted in the L-band, coexisting with legacy PON signals operating in the O-, S-, and L-bands. Moreover, if the spectral separation between QKD and PON signals is increased, the launched power of PON signals can also be raised without significantly affecting QKD performance. Further improvements in scalability can be achieved by multiplexing QKD signals in the wavelength domain [42]. In one experiment, 194 CV-QKD signals were multiplexed using 25 GHz channel spacing, resulting in an excess noise per channel of less than 0.02 SNU. The crosstalk-induced excess noise was also inferred to be significantly below 0.02 SNU, indicating that WDM CV-QKD is a promising approach for future large-scale QKD-PONs.