Fig. 1

A-MDI-QKD system for collinear modes. Light comes from Charlie along PM-FMF optical fibers. States are delayed by OFDs. They arrive at Alice’s and Bob’s laboratories, where they travel along local loops equipped with autocompensating devices (AD) and phase modulators. Local loop structure is symbolized by a loop with AD inside. Light returns to Charlie, and optical circulators (OC) direct the photons towards a Bell-state measurement apparatus composed by a BS and two MZIs working as mode sorters, together with the corresponding detectors