challenges in quantum key distribution Quantum key distribution (QKD) promises unconditional security in data communication and is currently being deployed in commercial applications. Nonetheless, before QKD can be widely adopted, it faces a number of important challenges such as secret key rate, distance, size, cost, and practical security. nature.com article


  What are challenges in quantum key distribution ?
The article cited in the question (Eleni Diamanti, Hoi-Kwong Lo, Bing Qi, and Zhiliang Yuan: Practical challenges in quantum key distribution, in npj Quantum Information 2016, a Nature Partner Journal) gives a list of challenges facing Quantum Key Distribution, but forgets several important ones:


Not fulfilling the market demand for simplified key distribution. When Man in the Middle attack is a threat, QKD only extends a pre-existing initial secret key, which still must be distributed by trusted means such a courier. QKD is thus not easier to use than classical key distribution and offers no clear advantage compared to distribution of a USB key with a One Time Pad by that same courier.
QKD's claimed security covers keys distribution, not data transmission at commercial speed. The later still relies on classical symmetric cryptography (lacking mathematical proof), with QKD protecting only the low trickle of data that constitutes the keys for classical crypto. Therefore, use of QKD does not unconditionally prevent interception of the bulk data for later decryption by hypothetical improved attacks.
Incompatibility with deployed infrastructure: existing active electronics at optical fiber ends, including in repeaters (often underwater), are incompatible with QKD. Some of the existing fibers might be re-purposable, but at extremely low QKD rate, which worsens the above issue.
QKD does not scale to a global network. What's currently experimented is end-to-end only, or (marginally) with special optical routers that further reduce the usable distance. Blueprints for large networks require trust in the routers, when classical crypto does not.
Complexity of the indispensable information-theoretic (mathematically provable) paraphernalia necessary for QKD to work both reliably and safely. A most cited method for that appeared only 9 years after the birth of QKD, and despite its complexity leaves it to the practitioner to deal with power outages other than by using another courier for a fresh initial secret key.
Lack of security certification: there was no security-certified Quantum Key Distribution system last time I checked, nor any planned reference framework for such certification. Market and laws demand certification (rightly so: several QKD systems have been hacked).