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Experimental demonstration of the coexistence of continuous-variable quantum key distribution with an intense DWDM classical channel

Rupesh Kumar, Hao Qin, Romain Alléaume

Published Jan 1, 2014• 7 citations
Publisher

Abstract

We have experimentally performed continuous variable quantum key distribution over up to 50 km, in coexistence with a dense-wavelength-multiplexed channel of several dBms. This opens new perspectives for QKD integration over existing optical network architectures.
DOI
10.1364/cleo_qels.2014.fm4a.1

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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.
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.
Integrating quantum key distribution (QKD) on the existing optical fiber network through dense wavelength-division multiplexing (DWDM) can greatly reduce the costs of quantum channels and improve the scalability and flexibility of the quantum communication network. In the scenarios of an access network or local area network with a large number of users, the connected optical fibers are relatively short and the quantum signal can be adjacent to the classical channels. In this case, the four-wave-mixing noise generated by the classical lights will have a severe impact on the performance of the QKD. We establish a theoretical model to characterize the excess noise in continuous-variable QKD induced by the four-wave mixing, and experimentally demonstrate the validity of the theoretical predictions. In addition, the influence of cross-phase-modulation noise between the carrier of the quantum signal and classical data channels on the QKD is investigated. To suppress the effect of the four-wave-mixing noise, an unequal channel spacing technique is employed. We realize coexistence of entangled state-based continuous-variable QKD and five adjacent DWDM classical channels with 10 dBm launch power and nonreturn-to-zero on-off-keying modulation at 2.5 Gb/s (10 Gb/s).
This paper presents a quantum key distribution (QKD) system that combines classical and quantum signals in a single optical channel for quantum key distribution application. We carefully optimize the communication link by placing the bright classical channel wavelength longer than the weak quantum one, number and specification of wavelength division multiplexers, and maximizing on-off extinction in the intensity modulator to minimize crosstalk of bright classical photons onto the quantum channel. Our experiments demonstrate a secret key rate (SKR) of 4.39 kbps for an emulated channel length of 65.5 km with less than 3% quantum bit error rate (QBER). The experimental study discussed here is relevant for utilization of existing optical fiber networks for QKD, making the system more robust for practical usage.
Continuous-variable quantum key distribution (CV-QKD) has reached a maturity that requires to consider its integrability into optical communication networks. As a key exchange, QKD cannot take the central role in optical network design but has to adapt to the requirements of the network.In this article compact CV-QKD prototypes, which have been demonstrated in operational environments, and several of their key features for facilitating network integration are presented. Among these features are the coverage of typical metro link losses with automatic adaptation to the channel conditions as well as in-channel synchronization, which reduces the spectral needs to a single dense-wavelength-division-multiplexing (DWDM) channel. In many situations, this fully C-band tunable channel can simply be connected to a free multiplexer port of an existing DWDM infrastructure. Any-to-any connectivity between many QKD transmitters and receivers can reduce the number of devices, while dynamic protocol switching allows to address different security requirements and cryptographic tasks.