Ultrastable optical frequency transfer and attosecond timing in deployed multicore fiber

The telecommunications industry’s deployment of billions of kilometers of optical fiber has created a vast global network that can be exploited for additional applications such as environmental sensing, quantum networking, and international clock comparisons. However, for reasons such as the unidirectionality of long-haul fiber links, telecom fiber networks cannot always be adapted for important applications beyond data transmission. Fortunately, new multicore optical fibers create the opportunity for application coexistence with data traffic, creating multifunctional networks. Toward that end, we propose and demonstrate the faithful transfer of ultrastable optical signals through deployed multicore fiber in a way that is compatible with the unidirectionality of long-haul fiber optic systems, demonstrating fractional frequency instability of 3 × 10−19 at 10,000 s. By supporting state-of-the-art optical atomic clocks, subsea multicore fibers can break the distance barrier for ultrastable optical frequency transfer, opening the door to intercontinental optical clock comparisons at the highest level, with applications in fundamental physics, relativistic geodesy, and the redefinition of the second.