I barely understand this stuff at all. However, this is a "metro" network and no "exchange" is involved, presumably for precisely the reason you state.
As far as I can make out, the security depends, in theory, on single photons being used to transmit individual bits of the encryption key. Reliably sending and detecting a single photon is quite hard even in perfect conditions: in practical fibres, the noise and attenuation grow as the fibre gets longer which means that there is a practical limit to the size of the network and indeed to the security (you may need to send more than a single photon to guarantee reception, raising the risk of undetected interception).
It will be interesting to see how it pans out in real world tests.
Biting the hand that feeds IT
