Figure 1
An illustration of the proposed OAM-based MDI-QKD. Alice and Bob prepare two mutually unbiased basis states (\(\textbf{V}\), \(\textbf{S}\)) and send them to Charlie through the unsecure channel. The \(\lambda /2\) plate and the q-plate are used to generate a set of vector and scalar modes, which are then attenuated to intensity \(\gamma \in \{\mu ,\nu ,0\}\) using intensity modulator IM. Next, the telescope collimates the quantum states with a finite aperture. They are then sent through a free atmospheric space link to the measurement site controlled by Charlie. The optical states are then collected by the telescope and allowed to interfere with the symmetric beam splitter (BS). Next, the photons are passed through the polarization grating that separates the left and right circularly polarized photons, then guided by mirrors (M1, M2, M3, M4) towards a beam-splitter (BS). As a result, the photons are then measured using the mode sorters (MS) that map OAM to position and then detected by the detectors (L0, R0, L1, R1). This illustration was generated using Inkscape 1.1 software.
