IANAQP
If you intercept the data stream, it gets modified.
(I am not a quantum physicist) but one of the bits that I think I do vaguely understand is that you cannot reconstruct the key without observing what it currently is - and by doing that, you change parts of it. Your reconstruction thus has a percentage of errors and if this percentage of errors is above what might be caused by signal losses etc, the victim should be able to work out that it's been intercepted.
The bit I don't currently understand is how Bob's key sent to Bill can be used by Bill without Bill's observance of the key causing errors. For now, that may just have to stay a mindf*ck.