Fig. 3
Autocompensating devices for collinear modes and state generation by phase modulation. At Alice’s and Bob’s sites local loops equipped with autocompensating devices and phase modulators are located. The combination of two CLCs and a Dove prism, each one with a determinate orientation, implements the matrix −σy, which suffices to autocompensate for perturbations due to modal coupling. The phase modulator introduces a phase θ by which to send photons back to Charlie in one of the two MUBs \(\mathcal {B}_{1}\) and \(\mathcal {B}_{2}\) described above