If true, does it assume the link between the sending and receiving sides or servers acting on their behalf to be fully optical fiber physical layer or what practical approach is that heading towards?
Do you mean quantum key distribution(QKD)? If so, Then its security is guaranteed by quantum mechanics. As you can see, currently all security proof is based on quantum entanglement.
However, not all QKD protocols are unconditionally secure, since proofs of some protocols require the devices need to be trustable. Yet there are some protocols that are unconditionally secure.