We demonstrate the coexistence of polarization-entangled photons and classical signals in the C band at a received power within most receivers’ sensitivity, enabling direct-to-home quantum key distribution.
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 C-band energy-time entanglement and L-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-C-band setup. These results pave the way for large-scale quantum-classical coexistence within existing optical fiber infrastructure.
Entanglement distribution between spatially distant nodes is crucial for advancing the development of quantum networks. Achieving large-scale, practical quantum networks by leveraging existing fiber infrastructures and commercial technologies can greatly enhance cost efficiency in real-world applications. As such, the simultaneous transmission of quantum entanglement and classical signals within a single optical fiber is becoming increasingly desirable, yet it remains challenging due to the substantial intensity disparities between single-photon-level quantum light and classical traffic. In this study, we theoretically describe and experimentally implement polarization entanglement distribution using a continuous wave pumped integrated AlGaAs Bragg reflection waveguide quantum source, coexisting with classical light in optical fiber at the telecom C-band. The characteristics of noise photons are analyzed and evaluated under different levels of classical light power. Using the BBM92 protocol for quantum key distribution, we demonstrate that temporal and spectral filtering during coincidence detection, combined with optimized source brightness, effectively influences the secure key rate and achievable distance of the system. Polarization drift is measured across various fiber paths, with 35-hour stability tests on fibers ranging up to 55 km in length. Our results can guide the initial design of optimal entanglement distribution networks that are compatible with fiber-optic communication systems.
Incorporating single photon links used for quantum communication applications like quantum key distribution is a challenging task. Direct contamination from the strong classical signal and induced Raman scattering easily obscures the weak quantum signal. Generating entangled photons in the O-band might allow the coexistence of classical and quantum signals. We present results demonstrating the feasibility of transmitting entangled photons and strong classical communication signals over the same fiber.