https://phys.org/news/2018-07-faster-photons-enable-total.html

Transferring data using light passed along fibre optic cables has become increasingly common over the past decades, but each pulse currently contains millions of photons. That means that, in principle, a portion of these could be intercepted without detection.

Secure data is already encrypted, but if an 'eavesdropper' was able to intercept the signals containing details of the code then—in theory—they could access and decode the rest of the message.