Fig. 6

A-MDI-QKD system for codirectional modes. Charlie uses his quantum state generator to send a pair of qubits to each Alice and Bob. After the delays, photons reach the local loops; in them, autocompensating devices and phase modulators are located. Afterwards, light returns to Charlie’s six-port OCs, which direct the light towards a optical integrated circuit working as a Bell-state measurement apparatus. We made apparent the fact that we are using codirectional modes by explicitly showing the two paths of the photons, to be understood as two cores of a MCF as shown, although the representation of the system is somewhat schematic