Physicist Renato Renner from the Institute of Theoretical Physics in Zurich said the problem was that systems were not being built correctly. In 2010, for instance, that a hacker could blind a detector with a strong pulse, rendering it unable to see the secret-keeping photons.
Renner also said that there are many other problems. Photons are generated using a laser tuned to such a low intensity that it’s producing one single photon at a time. There is a certain probability that the laser will make a photon encoded with your secret information and then a second photon with that same information. All an enemy has to do is steal that second photon and they could gain access to your data.
He told Wired that if there were better control over quantum systems than we have with today’s technology then perhaps quantum cryptography could be less susceptible to problems, but such advances are at least 10 years away.