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Quantum 2.0 Conference and Exhibition

Fiber Entanglement Distribution, and Quantum-Classical Coexistence in C Band

Bhaskar Kanseri, Nishant Kumar Pathak, Abhay Singh Dulta

Published Jan 1, 2025• 1 citation
Publisher

Abstract

We experimentally demonstrate polarization-entangled photon distribution over 50km telecom fiber, achieving robust Bell violation and secure QKD with copropagation of classical data at C-band, enabling cost-effective quantum networks using existing telecom infrastructure.
DOI
10.1364/quantum.2025.qw3a.8

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The authors study the coexistence of quantum entanglement with fiber optical communication at the telecom CCC band. The performance of quantum key distribution based on distributed energy-time entanglement coexisting with classical light is characterized, which paves the way for developing a cost-effective quantum entanglement network compatible with fiber communications.
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The coexistence of quantum information and classical signals within the same fiber link is essential for realizing future quantum networks that utilize the well-established optical fiber infrastructure. In this work, we demonstrate the coexistence of CCC-band energy-time entanglement and LLL-band classical data. By employing wavelength-division multiplexing techniques and polarization management, we achieve quantum entanglement distribution over a 40-km-long fiber carrying $4\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{W}$ of bidirectional received power. Under the highest noise conditions, we obtain a visibility of $84.59\ifmmode\pm\else\textpm\fi{}0.68%$ from Franson interference and calculate a secret key rate of 1.89 kbps using the Bennett-Brassard-Mermin 1992 (BBM92) protocol, showing improved performance compared to an all-CCC-band setup. These results pave the way for large-scale quantum-classical coexistence within existing optical fiber infrastructure.
Compatibility with existing classical network infrastructure offers a scalable path towards deploying large-scale quantum networks. Here, we demonstrate O-band polarization-encoded quantum entanglement distribution over an installed 24.4-km fiber while coexisting with a state-of-the-art fully-loaded C-band classical communications line system and a picosecond-level precision L-band synchronization signal. The classical system carries two 800-Gbps channels while the remainder of the C-band is filled with amplified spontaneous emission, as is standard for such state-of-the-art communications systems. We examine the spontaneous Raman scattering spectrum generated from this broadband C-band light and offer insights into wavelength allocation for O-band quantum channels. Optimal wavelength selection and narrow filtering enable well-preserved Bell state fidelity when coexisting with 21.4-dBm aggregate launch power across the C-band suitable for 36-Tbps transmission. To the best of our knowledge, this is the first implementation of entanglement-based quantum communications between two remote nodes coexisting with independent classical communications traffic. We demonstrate coexistence of quantum entanglement with ultra-high power levels and record classical bandwidth, offering promise for real-world entanglement-based networking integrated within high-capacity communications infrastructure.