Time transfer and clock synchronization with ghost frequency comb.

This article presents an experimental demonstration of a "ghost frequency comb" (GFC), a novel frequency-time comb observed via nonlocal intensity correlation measurements of a continuous-wave (CW) multimode fiber ring laser beam. Unlike conventional frequency combs generated by mode-locked lasers that produce pulse trains, the GFC arises from a CW laser with approximately 500,000 longitudinal cavity modes lacking pulse structure, and is revealed only through second-order correlation between two distant photodetectors. The study details the theoretical basis, experimental setup, and measurements confirming the GFC's comb-like ultra-narrow peaks, with applications proposed in nonlocal time transfer and distant clock synchronization. The GFC offers potential advantages in precision spectroscopy and timing-positioning, particularly where CW lasers or nonlocal measurements are preferred, though further optimization is needed to match state-of-the-art clock accuracies.