Design of a polarization-based quantum key distribution link in a turbulent atmospheric channel

Abstract

We present a design for polarization-encoded decoy-state quantum key distribution (QKD) using the BB84 protocol over a turbulent atmospheric channel. The transmitter uses a single 785nm laser, an intensity modulator for decoy-state preparation, and the iPOGNAC, a polarization encoder based on a fiber Sagnac interferometer, for stable polarization encoding. A 660nm beacon co-propagates with the quantum signal and provides pointing feedback for beam stabilization and a timing reference. At the receiver, a dichroic mirror directs the beacon to a sensing and synchronization subsystem, while the quantum signal enters a second iPOGNAC for active measurement-basis selection and detection by two single-photon avalanche detectors. A subsystem containing two fast-steering mirrors stabilizes fiber coupling. This architecture is intended to mitigate turbulence-induced pointing losses and reduce manual alignment. Relative to common four-laser transmitters and four-detector receivers, it also reduces component count and the aggregate detector dark-count rate. Its target repetition rate is on the order of 100MHz, limited primarily by detector dead time.