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Quantum key distribution is supposed to provide a high degree of certainty in the security of secret keys.

That certainty is based on the laws of physics, and all attacks against quantum keys have exploited implementation weaknesses, rather than the underlying physics. Unlike mathematical methods of encryption, quantum key distribution does not provide a key that is difficult to figure out.
Instead, the nature of the key generation process allows any interloper to be detected—you know if your key is secret or not.
At present, quantum key distribution is limited to about 70km to 100km between the two nodes, because fiber optical cables tend to absorb the photons used to carry the key.

To provide end-to-end distribution between, say, Paris and Berlin, you either need to trust the third parties in between (so each node has its own pair of keys that it has generated with neighboring nodes) or have a quantum memory at each node.

A quantum memory allows you to store each qubit and then teleport its state to the next node.

At the end of the key-generation process, teleporting assures that a single shared key is generated between the end-points, and the intermediate nodes have no knowledge of the key.
Therein lies a weakness: maybe the memories at the intermediate can be exploited, allowing a secret key to be intercepted without detection.

To close that particular exploit, a group of researchers has proposed a clever way to scramble a quantum memory.
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