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Bulletin of the Russian Academy of Sciences Physics

Experimental Demonstration of the Coexistence of Phase-Time Coding QKD with Intense C-Band Signals

A.V. Borisova, Konstantin V. Stepanov, Andrey N. Klimov, A. S. Timoshenko

Published Dec 1, 2024• 2 citations
Publisher

Abstract

Abstract Integration of quantum key distribution (QKD) into existing optical transport network is one of the important problems that should be solved to expand the applicability of QKD systems and to develop quantum networks. When multiplexing a quantum channel with classical ones, researchers and engineers are faced with a high background count rate caused by spontaneous Raman scattering. This paper presents the results of Raman noise measurement in the O-band at different distances and launched power of the C-Band channels. Based on this data, the 1310 nm QKD system capable to integrate into DWDM optical network was developed. Its performance has been experimentally confirmed in testing on coexistence with intense C-band signals. The experiments were carried out on the propagation of classical and quantum signals in the same fiber as well as in the opposite directions. As a result, the stable QKD over a distance up to 60 km with a total launched power of classical channels up to 20 mW has been demonstrated.

DOI
10.1134/s106287382470936x

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Abstract Integration of quantum key distribution (QKD) into existing optical transport network is one of the important problems that should be solved to expand the applicability of QKD systems and to develop quantum networks. When multiplexing a quantum channel with classical ones, researchers and engineers are faced with a high background count rate caused by spontaneous Raman scattering. This paper presents the results of Raman noise measurement in the O-band at different distances and launched power of the C-Band channels. Based on this data, the 1310 nm QKD system capable to integrate into DWDM optical network was developed. Its performance has been experimentally confirmed in testing on coexistence with intense C-band signals. The experiments were carried out on the propagation of classical and quantum signals in the same fiber as well as in the opposite directions. As a result, the stable QKD over a distance up to 60 km with a total launched power of classical channels up to 20 mW has been demonstrated.
A successful commercial deployment of quantum key distribution (QKD) technologies requires integrating QKD links into existing fibers and sharing the same fiber networks with classical data traffic. To mitigate the spontaneous Raman scattering (SpRS) noise from classical data channels, several quantum/classical coexistence strategies have been developed. O-band solutions place the QKD channel in the O-band for lower SpRS noise but with the penalty of higher fiber loss and can rarely reach beyond 80 km of fiber; another method is C-band coexistence with attenuated classical channels, which sacrifices the performance of classical channels for lower SpRS noise. In this work, a time-interleaving technique is demonstrated to enable the co- propagation of quantum and classical channels in the C-band without sacrificing either performance. By embedding QKD pulses in the gaps between classical data frames, the quantum channel is isolated from SpRS noise in both wavelength and time domains. C-band co- propagation of a polarization-encoding decoy-state BB84 QKD channel with a 100 Gb/s QPSK channel is experimentally demonstrated with quantum bit error rate (QBER) of 1.12%, 2.04%, and 3.81% and secure key rates (SKR) of 39.5 kb/s, 6.35 kb/s, and 128 b/s over 20, 50, and 100 km fibers, respectively. These results were achieved with the presence of classical launch power up to 10 dBm, which is at least one order of magnitude higher than reported works. We also demonstrated the co- propagation of a QKD channel with eight classical channels with total launch power up to 18-dBm (9-dBm per channel), which is the highest power of classical channels reported in C-band coexistence works.
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 0 dBm. 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 25 km fiber, a CV-QKD operated over the 1530.12 nm channel can tolerate the noise arising from up to 11.5 dBm classical channel at 1550.12 nm in the forward direction (9.7 dBm in backward). A positive key rate (0.49 kbits s −1 ) can be obtained at 75 km with classical channel power of respectively −3 and −9 dBm 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 five pairs of channels (with nominal input powers: 2 dBm forward and 1 dBm backward) over a 25 km WDM-PON network. The obtained results demonstrate the outstanding capacity of CV-QKD to coexist with classical signals of realistic intensity in optical networks.
Abstract Integrating quantum key distribution (QKD) into optical networks is a crucial step toward the adoption of quantum technologies in existing telecommunication fiber infrastructures. However, state-of-the-art solutions face significant challenges, including sensitivity to classical noise, particularly spontaneous Raman scattering, limited transmission distances, and varying network conditions. In this work, we present a novel discrete-variable QKD system running a time-bin BB84 protocol operating in the O-band (1295.56 nm), where the impact of dominant noise sources is effectively reduced. Combined with passive narrow-spectral filtering at the receiver, our system demonstrates robustness across diverse dense wavelength-division multiplexing scenarios, making it practical for real-world deployments. We validate its performance through extensive testing, showing stable key generation coexisting with different classical traffic conditions up to 17 dBm of total launch power. These results represent a significant advancement toward the integration of QKD into existing fiber networks, paving the way for secure quantum communication on a large scale.
Integrating quantum key distribution (QKD) with classical optical communication is a deployment-friendly and cost-effective approach to advancing QKD network implementation. However, the noise introduced by intense classical signals into the quantum channel severely impacts the performance of the QKD system, presenting challenges for long-distance coexistence transmission. In this paper, we successfully demonstrate the simultaneous propagation of QKD and optical transport network (OTN) in the C-band over distances exceeding 100 km. We establish a coexistence transmission system based on a commercial OTN equipment and QKD device equipped with the Fabry-Perot filters, demonstrating the feasibility of combining QKD with 11 Tbps (110×100 Gbps) classical data over a 101.86 km fiber. To investigate longer transmission distances, we employ an optimized wavelength allocation method and conduct the co-propagation of QKD and 1 Tbps classical data over a 152.34 km fiber. To the best of our knowledge, these results are the first time that report the integration of QKD and large-capacity classical communication over hundred-kilometer scale fiber. Our works represent a significant advancement in developing OTN networks incorporating QKD systems.
Theoretical research and numerical simulation of the noise influence caused by spontaneous Raman scattering, four-wave mixing, and linear channel cross talk on the performances of quantum key distribution (QKD) systems were conducted. Three types of QKD systems were considered: a coherent one-way QKD protocol, subcarrier-wave QKD system, and continuous-variable QKD integrated with classical dense wavelength division multiplexing (DWDM) channels. We calculate the secure key generation rate for the systems mentioned addressing different channel allocation schemes (i.e., configurations). A uniform DWDM grid is considered with a quantum channel located in the C-band and O-band (at 1310 nm) of a telecommunication window. The systems’ performances are analyzed in terms of the maximal achievable distance values. Configurations for further analysis and investigation are chosen optimally, i.e., their maximal achievable distances are the best.