Fig. 1
From: Experimental measurement-device-independent quantum digital signatures
Fibre-based MDI/QKD network. The key elements are coloured red. The rotator (R) in Charlie’s station sets each detector to measure a different polarisation state, H horizontal, V vertical, D diagonal and A anti-diagonal. The intensity modulators (IMs) can be set to high attenuation, to nearly stop the light passing through them (“Stop” in the figure). This can enable Alice-Bob MDI-QKD, when no IM is set to Stop, or Alice-Charlie QKD, when Bob’s IM is set to Stop, or Bob-Charlie QKD, when Alice’s IM is set to Stop. At the same time, the IMs can be used to prepare decoy states21,22,23,24. PM, polarisation modulation; BS, beam splitter; PBS, polarising beam splitter
