For really secure communications, you then need to share a new key each time you want to communicate information that is supposed to be secret. The use of a new key each time is considered to be equivalent to using what cryptographers call a one-time pad. A one-time pad harkens back to the days when an actual encryption key was distributed on paper and a unique sheet of paper was used each time. Because each key is used only once, cracking such encryption is difficult.

Wayne Rash, eWeek article, "Why Quantum-Resistant Encryption Needs Quantum Key Distribution for Real Security"