> This way you can reamplify signals over a large bandwidth (~4 THz) without having to do detection and retransmission (which would be unscalable).

This trick also means the cable doesn't care about the rest of the technology. If it was a retransmitter then we'd need to replace the entire cable if we change from 100Gbps over Protocol #39 to 200 Gbps over Protocol #40 because every retransmitter needs to be equipped for the new protocol, but the optical amplifier doesn't care why these photons turned up, what they mean - when provided with power it just ensures proportionately more photons like them come out of the amplifier.

Because they're not actually the same photons weird quantum tricks that would work on bench scale, where it was literally the same photon at the receiver as when you transmitted, will not work, but any conventional signalling within quite broad limits is OK. Researchers at the University where I studied as an undergraduate developed EDFA.