Quantum crypto too easy
So they worked out a complex way of getting partial key, great. But this becomes a man in the middle attach and prone to the issue that as soon as you leave or make a single mistake, you are noticed.
You still have valuable data, but now they know and will be after you.
From the depths of history (say about 5 years ago) there was a much better attack.
Quantum key transport relies on groups or orthogonal detectors at each end of the link, such that as each entangled photon is received one measurement (e.g. polarization) is randomly made and the result stored. Later the other party, making random measures on the entangled pair of each photon you got passes their result table (in the clear) to you. (no risk here as the table is just random bits)
By Xor'ing the tables together you get the crypto key to be used on subsequent comms.
Here is the weakness: the random choice of measure is just that random, it is the comparison of the two tables that results in a key. If you could fix the random choice of one party to a known string then you can know what measure they will make and then be able to listen in, do what they would do and pass on the result.
The easiest way to do that is simple, just send a burst of polarized light bright enough to blind one sensor of one party, before they get to exchanging keys. Then you know that from then on whatever random measurement choice they make the crypto bits will always come from the other channel.
Hard to explain, but easy to do hence looking for my calculator.