Coexistence of Optical Frequency Transmission With Quantum Key Distribution Over Multi-Core Media

The quantum-classical coexistence can be obtained using wavelength division multiplexing (WDM), however, the crosstalk between quantum and classical signals due to Raman noise must be handled with extra attention. Multi-core fiber (MCF) provides a good solution for the coexistence based on space division multiplexing (SDM). In this work, we propose and experimentally demonstrate hybrid transmission of ultrastable optical frequency reference and quantum key distribution (QKD) over homogeneous MCF. The scheme achieves optical frequency stabilization at the remote site by beating the loopback frequency signal with a local reference to extract link phase noise, which is then compensated via voltage-controlled oscillator (VCO) feedback. Prior to the experiment, we first analyze the critical challenges in MCFs including inter-core crosstalk and inter-core spontaneous Raman scattering. To minimize the crosstalk between the signals, the quantum signal and the optical frequency propagate through separate fiber cores. Successful hybrid transmission of optical frequency references and QKD is achieved across 20 km seven-core fiber. The frequency instability of$1.2 \times 10^{-17}$at 1 s and$4.2 \times 10^{-19}$at 10000 s, and secure key rate of 317.28 kbps can be realized in this integrated transmission system. The scheme benefits from two merits, ultrastable optical frequency offers high-precision clocks to QKD, and QKD provides one-time-pad keys to encrypt the time data. These results demonstrate the feasibility and tolerance of our QKD system with high-precision frequency reference and the viability of multi-core fibers for next-generation quantum networks.