Coexistence of continuous variable QKD with intense DWDM classical channels (1412.1403v2)

Abstract: We demonstrate experimentally the feasibility of continuous variable quantum key distribution (CV-QKD) in dense-wavelength-division multiplexing networks (DWDM), where QKD will typically have to coexist with several co- propagating (forward or backward) C-band classical channels whose launch power is around 0dBm. We have conducted experimental tests of the coexistence of CV-QKD multiplexed with an intense classical channel, for different input powers and different DWDM wavelengths. Over a 25km fiber, a CV-QKD operated over the 1530.12nm channel can tolerate the noise arising from up to 11.5dBm classical channel at 1550.12nm in forward direction (9.7dBm in backward). A positive key rate (0.49kb/s) can be obtained at 75km with classical channel power of respectively -3dBm and -9dBm in forward and backward. Based on these measurements, we have also simulated the excess noise and optimized channel allocation for the integration of CV-QKD in some access networks. We have, for example, shown that CV-QKD could coexist with 5 pairs of channels (with nominal input powers: 2dBm forward and 1dBm backward) over a 25km WDM-PON network. The obtained results demonstrate the outstanding capacity of CV-QKD to coexist with classical signals of realistic intensity in optical networks.

Overview of CV-QKD Coexistence with DWDM Classical Channels

The paper presents a comprehensive investigation into the coexistence of Continuous Variable Quantum Key Distribution (CV-QKD) with Dense Wavelength Division Multiplexing (DWDM) classical channels. This paper is pivotal for the integration of quantum communications into existing optical network infrastructures. The authors provide both experimental and theoretical insights into the interaction between CV-QKD protocols and classical channels, highlighting the capability of CV-QKD systems to tolerate substantial noise from classical channels.

Experimental Investigation and Results

The research demonstrates the feasibility of CV-QKD over DWDM networks through a series of experimental tests. A substantive portion of the research focuses on the ability of a CV-QKD channel, operating at 1530.12 nm, to coexist alongside a classical DWDM channel at 1550.12 nm. The paper reveals that the CV-QKD system can endure noise from classical channels with input power up to 11.5 dBm over a fiber distance of 25 km. Moreover, at an expanded range of 75 km, positive key rates of 0.49 kb/s were maintained with classical channel power levels of -3 dBm and -9 dBm in forward and backward directions respectively.

The paper elucidates the superior tolerance of CV-QKD systems to DWDM-induced noise due to the coherent detection methodology that selectively filters out unmatched noise photons. This characteristic gives CV-QKD systems a robust operational advantage in realistic network settings where multiple intense classical channels coexist.

Noise Sources Analysis

A thorough noise analysis is conducted, identifying Spontaneous Anti-Stokes Raman Scattering (SASRS) as the primary source of excess noise in CV-QKD systems deployed in DWDM environments. The authors quantify the amount of noise induced by SASRS and demonstrate its impact through experimental calibration. This noise analysis underpins the understanding of CV-QKD viability in the presence of high-intensity classical signals.

Additionally, the paper evaluates other potential sources of noise, such as Rayleigh scattering, Stimulated Brillouin Scattering (SBS), and Four Wave Mixing (FWM), and concludes their negligible impact under the specific experimental setups used.

Comparison with DV-QKD and Implications

The paper compares the performance of CV-QKD with that of Discrete Variable QKD (DV-QKD) in similar environments, demonstrating CV-QKD's superior coexisting capability with higher classical input power levels. This finding is critical for practical deployments, implying that CV-QKD could be more effectively integrated into large-scale optical networks, especially those employing DWDM technology.

The implications of these findings are profound for both access network configurations and core network deployments. CV-QKD's ability to coexist with standard optical power levels suggests more cost-effective and widespread deployment capabilities compared to DV-QKD, especially in modern Gigabit PON and WDM-PON environments.

Theoretical and Practical Contributions

The research contributes significantly to the theoretical modeling of excess noise and provides practical guidelines for channel allocation optimization in WDM-PON networks to minimize quantum channel noise. Through simulation and experimental tests, the authors propose strategies for integrating CV-QKD with multiple classical channel pairs, balancing power levels to achieve optimal performance over varying fiber lengths.

Conclusion

Overall, the paper presents strong evidence supporting the practicality of deploying CV-QKD alongside DWDM classical channels, with critical implications for the future of secure communications in commercial optical networks. The demonstrated resilience of CV-QKD to DWDM-induced noise, coupled with inherent filtering within coherent detection systems, marks a pivotal step towards real-world quantum communication applications. Future developments in this area could focus on extending these findings to even longer transmission distances and higher data rates, fostering further integration of quantum technologies into existing network infrastructures.