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International Conference on Transparent Optical Networks

CV-QKD in Coexistence with 1 Gb/s Classical Channel Using Electrical Frequency Division Multiplexing

Pol Adillon, Samael Sarmiento, Jeison Tabares, E. Llanos, Hugues de Riedmatten, S. Etcheverry

Published Jul 14, 2024• 2 citations
Publisher

Abstract

Quantum key distribution (QKD) leverages the principles of quantum mechanics to enable secure secret key exchange. Continuous-variable QKD (CV-QKD), utilizing coherent states and coherent detection, has emerged as a promising method, easily integrable into existing telecom networks. In this work, we study, through numerical simulation, the coexistence of a CV-QKD channel with a classical channel capable of up to 1 Gb/s. We first analyze the performance of the quantum channel when the classical channel emits a pure tone, by varying the frequency gap between both channels and adjusting their power ratios. Then, by maintaining a fixed electrical separation of 500 MHz, we study QPSK modulation with root-raised-cosine filtering in the classical channel. In this scenario, for several signal-to-noise ratios, we reduce crosstalk by optimizing filter properties such as roll-off factor, symbol rate and number of filter taps.

DOI
10.1109/icton62926.2024.10647700

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Quantum key distribution (QKD) leverages the principles of quantum mechanics to enable secure secret key exchange. Continuous-variable QKD (CV-QKD), utilizing coherent states and coherent detection, has emerged as a promising method, easily integrable into existing telecom networks. In this work, we study, through numerical simulation, the coexistence of a CV-QKD channel with a classical channel capable of up to 1 Gb/s. We first analyze the performance of the quantum channel when the classical channel emits a pure tone, by varying the frequency gap between both channels and adjusting their power ratios. Then, by maintaining a fixed electrical separation of 500 MHz, we study QPSK modulation with root-raised-cosine filtering in the classical channel. In this scenario, for several signal-to-noise ratios, we reduce crosstalk by optimizing filter properties such as roll-off factor, symbol rate and number of filter taps.
Quantum Key Distribution (QKD) offers a secure method for generating cryptographic keys crucial for 6G future networks. It relies on quantum mechanics principles to guarantee protection against potential threats from quantum computers. However, QKD signal is fragile and can be easily degraded by external noise sources. Therefore, integrating QKD into existing networks poses a challenge, as it must seamlessly coexist with classical channels, in optical fiber links utilizing wavelength division multiplexing (WDM). Continuous-variable QKD (CV-QKD), which leverages optical coherent technology, is seen as a promising solution for this integration within current telecom infrastructure. Additionally, Software-Defined Networking (SDN) presents opportunities for enhancing efficiency and facilitating dynamic management in coexistence scenarios. This paper reports key noise sources that degrade CV-QKD performance in coexistence and the parameters contributing to these noise sources such as classical channel (CC) power, separation between CC and quantum channel (QC) and position of QC. We also report strategies for effectively managing coexistence scenarios using SDN. We focus on the coexistence of CVQKD in presence of classical channels spaced according a 100 GHz ITU-T grid over a 10 km link length, providing guidelines to optimize the performance.
We analyze the coexistence of continuous variable quantum key distribution (CV-QKD) and 8×200Gb/s 16-QAM classical channels, evaluating parameters such as quantum channel spectral position, classical channel power, guard-band, and the number of exchanged quantum symbols. Additionally, we assess the performance of classical signals in terms of bit error rate (BER) to jointly analyze the impact of coexistence on both classical and quantum channels. These parameters can be exposed to a Software Defined Network (SDN) controller to dynamically optimize the performance of the CV-QKD system. Our results demonstrate that CV-QKD achieves successful key generation with a 100 GHz guard-band with classical channels at −10 dBm and 200 GHz at 0 dBm. The channel position within the spectrum has minimal impact, and lower key rates for short distances do not necessitate asymptotic convergence in quantum symbols. [−20, −10] dBm per channel with 100 GHz guard-band provides error-free classical signals and positive key generation.
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.
In this paper, we explored various physical-layer parameters affecting the coexistence of continuous-variable quantum key distribution (CV-QKD) and classical channels (CCs) within dense wavelength division multiplexing (DWDM) in the C-band. These parameters include the number of transmitted quantum symbols , power levels of CCs , quantum channel (QC) allocation within the C-band , power distribution among CCs, channel spacing between CCs and the QC, and CC power ranges suitable for both CCs and the QC . These insights provide valuable guidance for a software-defined networking (SDN) control system to manage CV-QKD and CC coexistence effectively. We observed that, for short distances and lower key rates, the number of quantum symbols exchanged does not need to reach the asymptotic convergence limit. However, achieving the asymptotic regime on a 10 km link requires transmitting approximately 1×10 13 quantum symbols. CC power significantly impacts CV-QKD performance. For example, in a DWDM setup with 37 dummy CCs on a 100 GHz grid, CV-QKD can support up to 11 dBm total launch power over a 10 km link when the QC is positioned at 193.2 THz and a 200 GHz channel spacing between the QC and CCs is maintained. Strategic placement of the QC within the C-band also enhances performance. Placing the QC at higher frequency channels relative to CCs improves both the secret key fraction and the maximum supported power of CCs. For instance, placing the QC at 195.9 THz provides better performance than positioning it at 193.2 or 192.2 THz. However, when the QC is placed at 192.2 THz, the maximum supported CC power reduces to 9.8 dBm. Additionally, the relationship between power distribution and power concentration is complex and requires careful consideration. It depends on the number of active CCs, the power of CCs, and their spectral placement relative to the QC. We also analyzed the case of 8×200G modulated CCs and found that placing them 100 GHz from the QC introduces nonlinear noise but still allows a positive secret key fraction. Increasing the spacing to 200 GHz further improves CV-QKD performance. Joint performance analysis revealed that, at a 100 GHz spacing and a 30 dB optical signal-to-noise ratio for CCs, a launch power range of −16.96 to 9.03 dBm supports both error-free CC recovery and positive key rate generation over a 10 km link. This comprehensive analysis highlights the interplay between CV-QKD and CCs, offering practical strategies for optimizing their coexistence in DWDM systems through an SDN control system .
Continuous-variable quantum key distribution (CV-QKD) is a promising protocol that can be easily integrated with classical optical communication systems. However, in the case of quantum-classical co-transmissions, such as dense wavelength division multiplexing with classical channels and time division multiplexing with large-power classical signal, a quantum signal is more susceptible to crosstalk caused by a classical signal, leading to signal distortion and key distribution performance reduction. To address this issue, we propose a noise-suppression scheme based on carrier frequency switching (CFS) that can effectively mitigate the influence of large-power random noise on the weak coherent state. In this noise-suppression scheme, a minimum-value window of the channel's noise power spectrum is searched for and the transmission signal frequency spectrum shifts to the corresponding frequency to avoid large-power channel noise. A digital filter is also utilized to filter out most of the channel noise. Simulation results show that compared to the traditional fixed carrier frequency scheme, the proposed noise-suppression scheme can reduce the excess noise to 1.8%, and the secret key rate can be increased by 1.43 to 2.86 times at different distances. This noise-suppression scheme is expected to be applied in scenarios like quantum-classical co-transmission and multi-QKD co-transmission to provide noise-suppression solutions.
Integrating quantum key distribution (QKD) with classical data transmission over the same fiber is crucial for scalable quantum-secured communication. However, noise from classical channels limits QKD distance. We demonstrate the longest-distance continuous-variable QKD (CV-QKD) over 120 km (20 dB loss) in the asymptotic regime, and over 100 km (17 dB loss) in the finite-size regime, both coexisting with a fully populated coarse wavelength division multiplexing system. Natural mode filtering of the local oscillator and phase noise mitigation enabled this without additional filtering or wavelength reallocation. Benchmarking against a commercial discrete-variable QKD system and considering finite-size effects confirms the feasibility of CV-QKD as a plug-and-play solution for typical 80-100 km long-haul optical networks. Our results set a record fiber distance for CV-QKD, showing its potential for cost-effective, large-scale deployment in existing network infrastructure.