Fiber optic coupling Updated +Created
This section is about stuff efficiently getting light into or out of optical fibers, or joining two optical fibers together end to end so that light goes through.
Historically this has been an important development, as it is much harder than with wires since optical fiber has to be very narrow to work properly, e.g. this is mentioned a lot in City of Light: The Story of Fiber Optics.
Quantum key distribution Updated +Created
Man-in-the-middle attack
BB84 is a good first algorithm to look into.
Long story short:
QKD uses quantum mechanics stuff to allow sharing unsnoopable keys: you can detect any snooping and abort communication. Unsnoopability is guaranteed by the known laws of physics, up only to engineering imperfections.
Furthermore, it allows this key distribution without having to physically take a box by car somewhere: once the channel is established, e.g. optical fiber, you can just keep generating perfect keys from it. Otherwise it would be pointless, as you could just drive your one-time pad key every time.
However, the keys likely have a limited rate of generation, so you can't just one-time pad the entire message, except for small text messages. What you would then do is to use the shared key with symmetric encryption.
Therefore, this setup usually ultimately relies on the idea that we believe that symmetric encryption is safer than , even though there aren't mathematical safety proofs of either as of 2020.