Frequency-comb-assisted, broadband optical phase stabilization for quantum communications

To study channel-to-channel phase drift in broadband links, we implement a dense wavelength-division multiplexing (DWDM)-based phase stabilization testbed. We systematically study phase stabilization in adjacent DWDM channels of metropolitan-scale fiber links. A 25GHz frequency comb with a sub-Hz laser generated two DWDM-separable frequency components at 1550.12 nm and 1549.32 nm as signal and reference channels. One of these components is stabilized using a phase-locked-loop circuit, while the phase noise in the other channel is monitored. We demonstrate a significantly reduced phase noise in the monitored channel over a 100 km link. This approach mitigates phase noise in wavelength-detuned long-distance network links while wavelength-separating the classical probing signal and reducing its impact on the quantum channel through Raman scattering in a long fiber.