Just to give a concept of how difficult this is:

> “The challenge was to keep the photons’ arrival time synchronized to within 10 pico-seconds,”

> “Since these detectors only work at temperatures less than one degree above absolute zero the equipment also included a compact cryostat,” said Tittel.

The dark fiber seems like it was important for synchronizing the clocks. And while they claim this could be used for encryption keys, that is really a roundabout way of saying that very little information was actually transmitted/received, although the article doesn't say exactly how little.

If this technology was refined, you'd just use this system to send secure messages without the need for an encryption key.