Solar-Blind Free-Space QKD Using Large-Core Fibers for Alignment-Tolerant Link Operation

While fiber-optic quantum key distribution (QKD) systems have reached a technological maturity that permits field deployments, the complexity of free-space optical (FSO) QKD systems introduced to ensure minimal coupling loss over air hinders the use of QKD in fiber-scarce environments. As a response to this challenge, we present a simplified approach that leverages large-core fibers at the receiving air interface of short-range terrestrial FSO links. We show that the increased sensitivity to solar irradiance, as it arises from the adoption of alignment-tolerant multi-mode fibers, can be effectively mitigated through a spectral E-band allocation of the QKD channel. We accomplish a secure-key rate of 869 b/s/detector for a 63-m long outdoor free-space link during day-light operation at 42.9 klux, despite employing a 25-μm core fiber at Bob's QKD receiver. We further demonstrate the co-existence of a classical data stream over the solar-blind FSO QKD link.